CaMKII-dependent phosphorylation regulates basal cardiac pacemaker function via modulation of local Ca2+ releases.

Spontaneous beating of the heart pacemaker, the sinoatrial node, is generated by sinoatrial node cells (SANC) due to gradual change of the membrane potential called diastolic depolarization (DD). Spontaneous, submembrane local Ca(2+) releases (LCR) from ryanodine receptors (RyR) occur during late DD and activate an inward Na(+)/Ca(2+)exchange current to boost the DD rate and fire an action potential (AP). Here we studied the extent of basal Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activation and the role of basal CaMKII-dependent protein phosphorylation in generation of LCRs and regulation of normal automaticity of intact rabbit SANC. The basal level of activated (autophosphorylated) CaMKII in rabbit SANC surpassed that in ventricular myocytes (VM) by approximately twofold, and this was accompanied by high basal level of protein phosphorylation. Specifically, phosphorylation of phospholamban (PLB) at the CaMKII-dependent Thr(17) site was approximately threefold greater in SANC compared with VM, and RyR phosphorylation at CaMKII-dependent Ser(2815) site was ∼10-fold greater in the SA node, compared with that in ventricle. CaMKII inhibition reduced phosphorylation of PLB and RyR, decreased LCR size, increased LCR periods (time from AP-induced Ca(2+) transient to subsequent LCR), and suppressed spontaneous SANC firing. Graded changes in CaMKII-dependent phosphorylation (indexed by PLB phosphorylation at the Thr(17)site) produced by CaMKII inhibition, ß-AR stimulation or phosphodiesterase inhibition were highly correlated with changes in SR Ca(2+) replenishment times and LCR periods and concomitant changes in spontaneous SANC cycle lengths (R(2) = 0.96). Thus high basal CaMKII activation modifies the phosphorylation state of Ca(2+) cycling proteins PLB, RyR, L-type Ca(2+) channels (and likely others), adjusting LCR period and characteristics, and ultimately regulates both normal and reserve cardiac pacemaker function.

Electrochemical Na+ and Ca2+ gradients drive coupled-clock regulation of automaticity of isolated rabbit sinoatrial nodal pacemaker cells.

Coupling of an intracellular Ca(2+) clock to surface membrane ion channels, i.e., a "membrane clock, " via coupling of electrochemical Na(+) and Ca(2+) gradients (ENa and ECa, respectively) has been theorized to regulate sinoatrial nodal cell (SANC) normal automaticity. To test this hypothesis, we measured responses of [Na(+)]i, [Ca(2+)]i, membrane potential, action potential cycle length (APCL), and rhythm in rabbit SANCs to Na(+)/K(+) pump inhibition by the digitalis glycoside, digoxigenin (DG, 10-20 µmol/l). Initial small but significant increases in [Na(+)]i and [Ca(2+)]i and reductions in ENa and ECa in response to DG led to a small reduction in maximum diastolic potential (MDP), significantly enhanced local diastolic Ca(2+) releases (LCRs), and reduced the average APCL. As [Na(+)]i and [Ca(2+)]i continued to increase at longer times following DG exposure, further significant reductions in MDP, ENa, and ECa occurred; LCRs became significantly reduced, and APCL became progressively and significantly prolonged. This was accompanied by increased APCL variability. We also employed a coupled-clock numerical model to simulate changes in ENa and ECa simultaneously with ion currents not measured experimentally. Numerical modeling predicted that, as the ENa and ECa monotonically reduced over time in response to DG, ion currents (ICaL, ICaT, If, IKr, and IbNa) monotonically decreased. In parallel with the biphasic APCL, diastolic INCX manifested biphasic changes; initial INCX increase attributable to enhanced LCR ensemble Ca(2+) signal was followed by INCX reduction as ENCX (ENCX = 3ENa - 2ECa) decreased. Thus SANC automaticity is tightly regulated by ENa, ECa, and ENCX via a complex interplay of numerous key clock components that regulate SANC clock coupling.

New evidence for coupled clock regulation of the normal automaticity of sinoatrial nodal pacemaker cells: bradycardic effects of ivabradine are linked to suppression of intracellular Ca²âº cycling.

Beneficial clinical bradycardic effects of ivabradine (IVA) have been interpreted solely on the basis of If inhibition, because IVA specifically inhibits If in sinoatrial nodal pacemaker cells (SANC). However, it has been recently hypothesized that SANC normal automaticity is regulated by crosstalk between an "M clock," the ensemble of surface membrane ion channels, and a "Ca(2+) clock," the sarcoplasmic reticulum (SR). We tested the hypothesis that crosstalk between the two clocks regulates SANC automaticity, and that indirect suppression of the Ca(2+) clock further contributes to IVA-induced bradycardia. IVA (3 µM) not only reduced If amplitude by 45 ± 6% in isolated rabbit SANC, but the IVA-induced slowing of the action potential (AP) firing rate was accompanied by reduced SR Ca(2+) load, slowed intracellular Ca(2+) cycling kinetics, and prolonged the period of spontaneous local Ca(2+) releases (LCRs) occurring during diastolic depolarization. Direct and specific inhibition of SERCA2 by cyclopiazonic acid (CPA) had effects similar to IVA on LCR period and AP cycle length. Specifically, the LCR period and AP cycle length shift toward longer times almost equally by either direct perturbations of the M clock (IVA) or the Ca(2+) clock (CPA), indicating that the LCR period reports the crosstalk between the clocks. Our numerical model simulations predict that entrainment between the two clocks that involves a reduction in INCX during diastolic depolarization is required to explain the experimentally AP firing rate reduction by IVA. In summary, our study provides new evidence that a coupled-clock system regulates normal cardiac pacemaker cell automaticity. Thus, IVA-induced bradycardia includes a suppression of both clocks within this system.

Mechanisms that match ATP supply to demand in cardiac pacemaker cells during high ATP demand.

The spontaneous action potential (AP) firing rate of sinoatrial node cells (SANCs) involves high-throughput signaling via Ca(2+)-calmodulin activated adenylyl cyclases (AC), cAMP-mediated protein kinase A (PKA), and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII)-dependent phosphorylation of SR Ca(2+) cycling and surface membrane ion channel proteins. When the throughput of this signaling increases, e.g., in response to ß-adrenergic receptor activation, the resultant increase in spontaneous AP firing rate increases the demand for ATP. We hypothesized that an increase of ATP production to match the increased ATP demand is achieved via a direct effect of increased mitochondrial Ca(2+) (Ca(2+)m) and an indirect effect via enhanced Ca(2+)-cAMP/PKA-CaMKII signaling to mitochondria. To increase ATP demand, single isolated rabbit SANCs were superfused by physiological saline at 35 ± 0.5°C with isoproterenol, or by phosphodiesterase or protein phosphatase inhibition. We measured cytosolic and mitochondrial Ca(2+) and flavoprotein fluorescence in single SANC, and we measured cAMP, ATP, and O2 consumption in SANC suspensions. Although the increase in spontaneous AP firing rate was accompanied by an increase in O2 consumption, the ATP level and flavoprotein fluorescence remained constant, indicating that ATP production had increased. Both Ca(2+)m and cAMP increased concurrently with the increase in AP firing rate. When Ca(2+)m was reduced by Ru360, the increase in spontaneous AP firing rate in response to isoproterenol was reduced by 25%. Thus, both an increase in Ca(2+)m and an increase in Ca(2+) activated cAMP-PKA-CaMKII signaling regulate the increase in ATP supply to meet ATP demand above the basal level.

Sarcoplasmic reticulum Ca2+ pumping kinetics regulates timing of local Ca2+ releases and spontaneous beating rate of rabbit sinoatrial node pacemaker cells.

RATIONALE: Sinoatrial node cells (SANCs) generate local, subsarcolemmal Ca(2+) releases (LCRs) from sarcoplasmic reticulum (SR) during late diastolic depolarization. LCRs activate an inward Na(+)-Ca(2+) exchange current (I(NCX)), which accelerates diastolic depolarization rate, prompting the next action potential (AP). The LCR period, ie, a delay between AP-induced Ca(2+) transient and LCR appearance, defines the time of late diastolic depolarization I(NCX) activation. Mechanisms that control the LCR period, however, are still unidentified. OBJECTIVE: To determine dependence of the LCR period on SR Ca(2+) refilling kinetics and establish links between regulation of SR Ca(2+) replenishment, LCR period, and spontaneous cycle length. METHODS AND RESULTS: Spontaneous APs and SR luminal or cytosolic Ca(2+) were recorded using perforated patch and confocal microscopy, respectively. Time to 90% replenishment of SR Ca(2+) following AP-induced Ca(2+) transient was highly correlated with the time to 90% decay of cytosolic Ca(2+) transient (T-90(C)). Local SR Ca(2+) depletions mirror their cytosolic counterparts, LCRs, and occur following SR Ca(2+) refilling. Inhibition of SR Ca(2+) pump by cyclopiazonic acid dose-dependently suppressed spontaneous SANCs firing up to ≈50%. Cyclopiazonic acid and graded changes in phospholamban phosphorylation produced by ß-adrenergic receptor stimulation, phosphodiesterase or protein kinase A inhibition shifted T-90(C) and proportionally shifted the LCR period and spontaneous cycle length (R(2)=0.98). CONCLUSIONS: The LCR period, a critical determinant of the spontaneous SANC cycle length, is defined by the rate of SR Ca(2+) replenishment, which is critically dependent on SR pumping rate, Ca(2+) available for pumping, supplied by L-type Ca(2+) channel, and ryanodine receptor Ca(2+) release flux, each of which is modulated by cAMP-mediated protein kinase A-dependent phosphorylation.

The "funny" current (I(f)) inhibition by ivabradine at membrane potentials encompassing spontaneous depolarization in pacemaker cells.

Recent clinical trials have shown that ivabradine (IVA), a drug that inhibits the funny current (I(f)) in isolated sinoatrial nodal cells (SANC), decreases heart rate and reduces morbidity and mortality in patients with cardiovascular diseases. While IVA inhibits I(f), this effect has been reported at essentially unphysiological voltages, i.e., those more negative than the spontaneous diastolic depolarization (DD) between action potentials (APs). We tested the relative potency of IVA to block I(f) over a wide range of membrane potentials, including those that encompass DD governing to the SANC spontaneous firing rate. A clinically relevant IVA concentration of 3 µM to single, isolated rabbit SANC slowed the spontaneous AP firing rate by 15%. During voltage clamp the maximal I(f) was 18 ± 3 pA/pF (at -120 mV) and the maximal I(f) reduction by IVA was 60 ± 8% observed at -92 ± 4 mV. At the maximal diastolic depolarization (~-60 mV) I(f) amplitude was only -2.9 ± 0.4 pA/pF, and was reduced by only 41 ± 6% by IVA. Thus, I(f) amplitude and its inhibition by IVA at physiologically relevant membrane potentials are substantially less than that at unphysiological (hyperpolarized) membrane potentials. This novel finding more accurately describes how IVA affects SANC function and is of direct relevance to numerical modeling of SANC automaticity.

Beat-to-beat Ca(2+)-dependent regulation of sinoatrial nodal pacemaker cell rate and rhythm.

Whether intracellular Ca(2+) regulates sinoatrial node cell (SANC) action potential (AP) firing rate on a beat-to-beat basis is controversial. To directly test the hypothesis of beat-to-beat intracellular Ca(2+) regulation of the rate and rhythm of SANC we loaded single isolated SANC with a caged Ca(2+) buffer, NP-EGTA, and simultaneously recorded membrane potential and intracellular Ca(2+). Prior to introduction of the caged Ca(2+) buffer, spontaneous local Ca(2+) releases (LCRs) during diastolic depolarization were tightly coupled to rhythmic APs (r²=0.9). The buffer markedly prolonged the decay time (T50) and moderately reduced the amplitude of the AP-induced Ca(2+) transient and partially depleted the SR load, suppressed spontaneous diastolic LCRs and uncoupled them from AP generation, and caused AP firing to become markedly slower and dysrhythmic. When Ca(2+) was acutely released from the caged compound by flash photolysis, intracellular Ca(2+) dynamics were acutely restored and rhythmic APs resumed immediately at a normal rate. After a few rhythmic cycles, however, these effects of the flash waned as interference with Ca(2+) dynamics by the caged buffer was reestablished. Our results directly support the hypothesis that intracellular Ca(2+) regulates normal SANC automaticity on a beat-to-beat basis.

Ca2+-regulated-cAMP/PKA signaling in cardiac pacemaker cells links ATP supply to demand.

RATIONALE: In sinoatrial node cells (SANC), Ca(2+) activates adenylate cyclase (AC) to generate a high basal level of cAMP-mediated/protein kinase A (PKA)-dependent phosphorylation of Ca(2+) cycling proteins. These result in spontaneous sarcoplasmic-reticulum (SR) generated rhythmic Ca(2+) oscillations during diastolic depolarization, that not only trigger the surface membrane to generate rhythmic action potentials (APs), but, in a feed-forward manner, also activate AC/PKA signaling. ATP is consumed to pump Ca(2+) to the SR, to produce cAMP, to support contraction and to maintain cell ionic homeostasis. OBJECTIVE: Since feedback mechanisms link ATP-demand to ATP production, we hypothesized that (1) both basal ATP supply and demand in SANC would be Ca(2+)-cAMP/PKA dependent; and (2) due to its feed-forward nature, a decrease in flux through the Ca(2+)-cAMP/PKA signaling axis will reduce the basal ATP production rate. METHODS AND RESULTS: O(2) consumption in spontaneous beating SANC was comparable to ventricular myocytes (VM) stimulated at 3 Hz. Graded reduction of basal Ca(2+)-cAMP/PKA signaling to reduce ATP demand in rabbit SANC produced graded ATP depletion (r(2)=0.96), and reduced O(2) consumption and flavoprotein fluorescence. Neither inhibition of glycolysis, selectively blocking contraction nor specific inhibition of mitochondrial Ca(2+) flux reduced the ATP level. CONCLUSIONS: Feed-forward basal Ca(2+)-cAMP/PKA signaling both consumes ATP to drive spontaneous APs in SANC and is tightly linked to mitochondrial ATP production. Interfering with Ca(2+)-cAMP/PKA signaling not only slows the firing rate and reduces ATP consumption, but also appears to reduce ATP production so that ATP levels fall. This distinctly differs from VM, which lack this feed-forward basal cAMP/PKA signaling, and in which ATP level remains constant when the demand changes.

Rhythmic beating of stem cell-derived cardiac cells requires dynamic coupling of electrophysiology and Ca cycling.

There is an intense interest in differentiating embryonic stem cells to engineer biological pacemakers as an alternative to electronic pacemakers for patients with cardiac pacemaker function deficiency. Embryonic stem cell-derived cardiocytes (ESCs), however, often exhibit dysrhythmic excitations. Using Ca(2+) imaging and patch-clamp techniques, we studied requirements for generation of spontaneous rhythmic action potentials (APs) in late-stage mouse ESCs. Sarcoplasmic reticulum (SR) of ESCs generates spontaneous, rhythmic, wavelet-like Local Ca(2+)Releases (LCRs) (inhibited by ryanodine, tetracaine, or thapsigargin). L-type Ca(2+)current (I(CaL)) induces a global Ca(2+) release (CICR), depleting the Ca(2+) content SR which resets the phases of LCR oscillators. Following a delay, SR then generates a highly synchronized spontaneous Ca(2+)release of multiple LCRs throughout the cell. The LCRs generate an inward Na(+)/Ca(2+)exchanger (NCX) current (absent in Na(+)-free solution) that ignites the next AP. Interfering with SR Ca(2+) cycling (ryanodine, caffeine, thapsigargin, cyclopiazonic acid, BAPTA-AM), NCX (Na(+)-free solution), or I(CaL) (nifedipine) results in dysrhythmic excitations or cessation of automaticity. Inhibition of cAMP/PKA signaling by a specific PKA inhibitor, PKI, decreases SR Ca(2+) loading, substantially reducing both spontaneous LCRs (number, size, and amplitude) and rhythmic AP firing. In contrast, enhancing PKA signaling by cAMP increases the LCRs (number, size, duration) and converts irregularly beating ESCs to rhythmic "pacemaker-like" cells. SR Ca(2+) loading and LCR activity could be also increased with a selective activation of SR Ca(2+) pumping by a phospholamban antibody. We conclude that SR Ca(2+) loading and spontaneous rhythmic LCRs are driven by inherent cAMP/PKA activity. I(CaL) synchronizes multiple LCR oscillators resulting in strong, partially synchronized diastolic Ca(2+) release and NCX current. Rhythmic ESC automaticity can be achieved by boosting "coupling" factors, such as cAMP/PKA signaling, that enhance interactions between SR and sarcolemma.

A full range of mouse sinoatrial node AP firing rates requires protein kinase A-dependent calcium signaling.

Recent perspectives on sinoatrial nodal cell (SANC)(*) function indicate that spontaneous sarcoplasmic reticulum (SR) Ca(2+) cycling, i.e. an intracellular "Ca(2+) clock," driven by cAMP-mediated, PKA-dependent phosphorylation, interacts with an ensemble of surface membrane electrogenic molecules ("surface membrane clock") to drive SANC normal automaticity. The role of AC-cAMP-PKA-Ca(2+) signaling cascade in mouse, the species most often utilized for genetic manipulations, however, has not been systematically tested. Here we show that Ca(2+) cycling proteins (e.g. RyR2, NCX1, and SERCA2) are abundantly expressed in mouse SAN and that spontaneous, rhythmic SR generated local Ca(2+) releases (LCRs) occur in skinned mouse SANC, clamped at constant physiologic [Ca(2+)]. Mouse SANC also exhibits a high basal level of phospholamban (PLB) phosphorylation at the PKA-dependent site, Serine16. Inhibition of intrinsic PKA activity or inhibition of PDE in SANC, respectively: reduces or increases PLB phosphorylation, and markedly prolongs or reduces the LCR period; and markedly reduces or accelerates SAN spontaneous firing rate. Additionally, the increase in AP firing rate by PKA-dependent phosphorylation by ß-adrenergic receptor (ß-AR) stimulation requires normal intracellular Ca(2+) cycling, because the ß-AR chronotropic effect is markedly blunted when SR Ca(2+) cycling is disrupted. Thus, AC-cAMP-PKA-Ca(2+) signaling cascade is a major mechanism of normal automaticity in mouse SANC.

Milk fat globule protein epidermal growth factor-8: a pivotal relay element within the angiotensin II and monocyte chemoattractant protein-1 signaling cascade mediating vascular smooth muscle cells invasion.

Advancing age induces aortic wall thickening that results from the concerted effects of numerous signaling proteins, many of which have yet to be identified. To search for novel proteins associated with aortic wall thickening, we have performed a comprehensive quantitative proteomic study to analyze aortic proteins from young (8 months) and old (30 months) rats and identified 50 proteins that significantly change in abundance with aging. One novel protein, the milk fat globule protein epidermal growth factor 8 (MFG-E8), increases 2.3-fold in abundance in old aorta. Transcription and translation analysis demonstrated that aortic MFG-E8 mRNA and protein levels increase with aging in several mammalian species including humans. Dual immunolabeling shows that MFG-E8 colocalizes with both angiotensin II and monocyte chemoattractant protein (MCP)-1 within vascular smooth muscle cells (VSMCs) of the thickened aged aortic wall. Exposure of early passage VSMCs from young aorta to angiotensin II markedly increases MFG-E8 and enhances invasive capacity to levels observed in VSMCs from old rats. Treatment of VSMCs with MFG-E8 increases MCP-1 expression and VSMCs invasion that are inhibited by the MCP-1 receptor blocker vCCI. Silencing MFG-E8 RNA substantially reduces MFG-E8 expression and VSMCs invasion capacity. The data indicate that arterial MFG-E8 significantly increases with aging and is a pivotal relay element within the angiotensin II/MCP-1/VSMC invasion signaling cascade. Thus, targeting of MFG-E8 within this signaling axis pathway is a potential novel therapy for the prevention and treatment of the age-associated vascular diseases such as atherosclerosis.

Constitutive phosphodiesterase activity restricts spontaneous beating rate of cardiac pacemaker cells by suppressing local Ca2+ releases.

Spontaneous beating of rabbit sinoatrial node cells (SANCs) is controlled by cAMP-mediated, protein kinase A-dependent local subsarcolemmal ryanodine receptor Ca(2+) releases (LCRs). LCRs activated an inward Na(+)/Ca(2+) exchange current that increases the terminal diastolic depolarization rate and, therefore, the spontaneous SANC beating rate. Basal cAMP in SANCs is elevated, suggesting that cAMP degradation by phosphodiesterases (PDEs) may be low. Surprisingly, total suppression of PDE activity with a broad-spectrum PDE inhibitor, 3'-isobutylmethylxanthine (IBMX), produced a 9-fold increase in the cAMP level, doubled cAMP-mediated, protein kinase A-dependent phospholamban phosphorylation, and increased SANC firing rate by approximately 55%, indicating a high basal activity of PDEs in SANCs. A comparison of specific PDE1 to -5 inhibitors revealed that the specific PDE3 inhibitor, milrinone, accelerated spontaneous firing by approximately 47% (effects of others were minor) and increased amplitude of L-type Ca(2+) current (I(Ca,L)) by approximately 46%, indicating that PDE3 was the major constitutively active PDE in the basal state. PDE-dependent control of the spontaneous SANC firing was critically dependent on subsarcolemmal LCRs, ie, PDE inhibition increased LCR amplitude and size and decreased LCR period, leading to earlier and augmented LCR Ca(2+) release, Na(+)/Ca(2+) exchange current, and an increase in the firing rate. When ryanodine receptors were disabled by ryanodine, neither IBMX nor milrinone was able to amplify LCRs, accelerate diastolic depolarization rate, or increase the SANC firing rate, despite preserved PDE inhibition-induced augmentation of I(Ca,L) amplitude. Thus, basal constitutive PDE activation provides a novel and powerful mechanism to decrease cAMP, limit cAMP-mediated, protein kinase A-dependent increase of diastolic ryanodine receptor Ca(2+) release, and restrict the spontaneous SANC beating rate.

Cholinergic receptor signaling modulates spontaneous firing of sinoatrial nodal cells via integrated effects on PKA-dependent Ca(2+) cycling and I(KACh).

Prior studies indicate that cholinergic receptor (ChR) activation is linked to beating rate reduction (BRR) in sinoatrial nodal cells (SANC) via 1) a G(i)-coupled reduction in adenylyl cyclase (AC) activity, leading to a reduction of cAMP or protein kinase A (PKA) modulation of hyperpolarization-activated current (I(f)) or L-type Ca(2+) currents (I(Ca,L)), respectively; and 2) direct G(i)-coupled activation of ACh-activated potassium current (I(KACh)). More recent studies, however, have indicated that Ca(2+) cycling by the sarcoplasmic reticulum within SANC (referred to as a Ca(2+) clock) generates rhythmic, spontaneous local Ca(2+) releases (LCR) that are AC-PKA dependent. LCRs activate Na(+)-Ca(2+) exchange (NCX) current, which ignites the surface membrane ion channels to effect an AP. The purpose of the present study was to determine how ChR signaling initiated by a cholinergic agonist, carbachol (CCh), affects AC, cAMP, and PKA or sarcolemmal ion channels and LCRs and how these effects become integrated to generate the net response to a given intensity of ChR stimulation in single, isolated rabbit SANC. The threshold CCh concentration ([CCh]) for BRR was approximately 10 nM, half maximal inhibition (IC(50)) was achieved at 100 nM, and 1,000 nM stopped spontaneous beating. G(i) inhibition by pertussis toxin blocked all CCh effects on BRR. Using specific ion channel blockers, we established that I(f) blockade did not affect BRR at any [CCh] and that I(KACh) activation, evidenced by hyperpolarization, first became apparent at [CCh] > 30 nM. At IC(50), CCh reduced cAMP and reduced PKA-dependent phospholamban (PLB) phosphorylation by approximately 50%. The dose response of BRR to CCh in the presence of I(KACh) blockade by a specific inhibitor, tertiapin Q, mirrored that of CCh to reduced PLB phosphorylation. At IC(50), CCh caused a time-dependent reduction in the number and size of LCRs and a time dependent increase in LCR period that paralleled coincident BRR. The phosphatase inhibitor calyculin A reversed the effect of IC(50) CCh on SANC LCRs and BRR. Numerical model simulations demonstrated that Ca(2+) cycling is integrated into the cholinergic modulation of BRR via LCR-induced activation of NCX current, providing theoretical support for the experimental findings. Thus ChR stimulation-induced BRR is entirely dependent on G(i) activation and the extent of G(i) coupling to Ca(2+) cycling via PKA signaling or to I(KACh): at low [CCh], I(KACh) activation is not evident and BRR is attributable to a suppression of cAMP-mediated, PKA-dependent Ca(2+) signaling; as [CCh] increases beyond 30 nM, a tight coupling between suppression of PKA-dependent Ca(2+) signaling and I(KACh) activation underlies a more pronounced BRR.

Calcium cycling protein density and functional importance to automaticity of isolated sinoatrial nodal cells are independent of cell size.

Spontaneous, localized, rhythmic ryanodine receptor (RyRs) Ca(2+) releases occur beneath the cell membrane during late diastolic depolarization in cardiac sinoatrial nodal cells (SANCs). These activate the Na(+)/Ca(2+) exchanger (NCX1) to generate inward current and membrane excitation that drives normal spontaneous beating. The morphological background for the proposed functional of RyR and NCX crosstalk, however, has not been demonstrated. Here we show that the average isolated SANC whole cell labeling density of RyRs and SERCA2 is similar to atrial and ventricle myocytes, and is similar among SANCs of all sizes. Labeling of NCX1 is also similar among SANCs of all sizes and exceeds that in atrial and ventricle myocytes. Submembrane colocalization of NCX1 and cardiac RyR (cRyR) in all SANCs exceeds that in the other cell types. Further, the Cx43 negative primary pacemaker area of the intact rabbit sinoatrial node (SAN) exhibits robust positive labeling for cRyR, NCX1, and SERCA2. Functional studies in isolated SANCs show that neither the average action potential (AP) characteristics, nor those of intracellular Ca(2+) releases, nor the spontaneous cycle length vary with cell size. Chelation of intracellular [Ca(2+)], or disabling RyRs or NCX1, markedly attenuates or abolishes spontaneous SANC beating in all SANCs. Thus, there is dense labeling of SERCA2, RyRs, and NCX1 in small-sized SANCs, thought to reside within the SAN center, the site of impulse initiation. Because normal automaticity of these cells requires intact Ca(2+) cycling, interactions of SERCA, RyR2 and NCX molecules are implicated in the initiation of the SAN impulse.

Do hypertensive individuals have enlarged aortic root diameters? Insights from studying the various subtypes of hypertension.

BACKGROUND: Aortic root diameter (AoD) increases with aging and is related to body size. AoD is also presumed to increase in hypertension. In prior studies, however, after adjusting for age and body size, AoD did not differ between hypertensive and normotensive (NT) individuals. Hypertension is a heterogeneous condition with various subtypes that differ in pathophysiology and age distribution. We assessed whether AoD differs among subjects with the various subtypes of hypertension and nonhypertensive individuals. METHODS: In 1,256 volunteers aged 30-79 years (48% women, 48% hypertensive; all untreated), AoD was measured at the sinuses of Valsalva with transthoracic echocardiography. Using cutoff values based on the Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure, subjects were identified as NT (23%), or prehypertensive (PH, 29%), or as having isolated diastolic (IDH, 6%), isolated systolic (ISH, 12%), or systolic-diastolic (SDH, 30%) hypertension. Groups were compared using analysis of variance with Bonferroni's correction. RESULTS: AoD increased with age and body surface area (BSA) in both men (r = 0.25 and 0.19, respectively) and women (r = 0.30 and 0.22, respectively) (all P < 0.0001). In men, those identified as having IDH, ISH, and SDH each had a 6% larger AoD than NT individuals (all P < 0.05). In women, those identified with ISH and SDH had a 10 and 8% larger AoD than NT individuals, respectively (all P < 0.05). In both sexes, after indexing to BSA, only ISH individuals exhibited larger AoD compared with NT individuals (both P < 0.05). But, with further adjustment for age, these differences were no longer observed. CONCLUSIONS: Even when the subtypes of hypertension are examined separately, age and BSA, not hypertension status, account for the AoD differences between NT and hypertensive subjects.

Membrane potential fluctuations resulting from submembrane Ca2+ releases in rabbit sinoatrial nodal cells impart an exponential phase to the late diastolic depolarization that controls their chronotropic state.

Stochastic but roughly periodic LCRs (Local subsarcolemmal ryanodine receptor-mediated Ca(2+) Releases) during the late phase of diastolic depolarization (DD) in rabbit sinoatrial nodal pacemaker cells (SANCs) generate an inward current (I(NCX)) via the Na(+)/Ca(2+) exchanger. Although LCR characteristics have been correlated with spontaneous beating, the specific link between LCR characteristics and SANC spontaneous beating rate, ie, impact of LCRs on the fine structure of the DD, have not been explicitly defined. Here we determined how LCRs and resultant I(NCX) impact on the DD fine structure to control the spontaneous SANC firing rate. Membrane potential (V(m)) recordings combined with confocal Ca(2+) measurements showed that LCRs impart a nonlinear, exponentially rising phase to the DD later part, which exhibited beat-to-beat V(m) fluctuations with an amplitude of approximately 2 mV. Maneuvers that altered LCR timing or amplitude of the nonlinear DD (ryanodine, BAPTA, nifedipine or isoproterenol) produced corresponding changes in V(m) fluctuations during the nonlinear DD component, and the V(m) fluctuation response evoked by these maneuvers was tightly correlated with the concurrent changes in spontaneous beating rate induced by these perturbations. Numerical modeling, using measured LCR characteristics under these perturbations, predicted a family of local I(NCX) that reproduced V(m) fluctuations measured experimentally and determined the onset and amplitude of the nonlinear DD component and the beating rate. Thus, beat-to-beat V(m) fluctuations during late DD phase reflect the underlying LCR/I(NCX) events, and the ensemble of these events forms the nonlinear DD component that ultimately controls the SANC chronotropic state in tight cooperation with surface membrane ion channels.

Elevated aortic pulse wave velocity, a marker of arterial stiffness, predicts cardiovascular events in well-functioning older adults.

BACKGROUND: Aging results in vascular stiffening and an increase in the velocity of the pressure wave as it travels down the aorta. Increased aortic pulse wave velocity (aPWV) has been associated with mortality in clinical but not general populations. The objective of this investigation was to determine whether aPWV is associated with total and cardiovascular (CV) mortality and CV events in a community-dwelling sample of older adults. METHODS AND RESULTS: aPWV was measured at baseline in 2488 participants from the Health, Aging and Body Composition (Health ABC) study. Vital status, cause of death and coronary heart disease (CHD), stroke, and congestive heart failure were determined from medical records. Over 4.6 years, 265 deaths occurred, 111 as a result of cardiovascular causes. There were 341 CHD events, 94 stroke events, and 181 cases of congestive heart failure. Results are presented by quartiles because of a threshold effect between the first and second aPWV quartiles. Higher aPWV was associated with both total mortality (relative risk, 1.5, 1.6, and 1.7 for aPWV quartiles 2, 3, and 4 versus 1; P=0.019) and cardiovascular mortality (relative risk, 2.1, 3.0, and 2.3 for quartiles 2, 3, and 4 versus 1; P=0.004). aPWV quartile was also significantly associated with CHD (P=0.007) and stroke (P=0.001). These associations remained after adjustment for age, gender, race, systolic blood pressure, known CV disease, and other variables related to events. CONCLUSIONS: Among generally healthy, community-dwelling older adults, aPWV, a marker of arterial stiffness, is associated with higher CV mortality, CHD, and stroke.

Walking may be related to less vascular stiffness in the Activity Counseling Trial (ACT).

BACKGROUND: The ACT was a clinical trial of various patient education and counseling interventions to increase physical activity in sedentary primary care populations. It provided the opportunity to measure the effect of increasing physical activity on aortic pulse wave velocity (APWV), a measure of vascular stiffness, in a relatively healthy middle-aged population. The effects of the interventions, as well as the impact of walking and correlates such as older age and maximal oxygen uptake (VO2max), on APWV were assessed. METHODS: The participants in this study were a subset of the 874 persons recruited for the ACT. Information about self-reported physical activity and disease status was collected at baseline (464 persons), 6-month (528 persons), and 24-month (555 persons) intervals. Physiological measures included APWV, systolic blood pressure, and other correlates. RESULTS: In multivariate analyses, the various treatment arms did not have a significant effect on APWV. However, walking in hours per day was associated with slower APWV times or less stiffness (P = .03). This was significant for women and consistent but not significant for men. In addition, age, clinic site, race, systolic blood pressure, and VO2max were independently associated with APWV. CONCLUSIONS: Increased walking frequency over a 24-month period was predictive of reduced vascular stiffness in ACT. The more significant result for walking frequency in women than in men might be caused by the presence of a low Vo2max or physical activity threshold for an effect of walking on APWV, which most women achieved but most men had surpassed at the start of the study. Although needing confirmation because this was a secondary analysis, modest physical activity may have a beneficial effect on large vessel structure.

Correlates of aortic stiffness in elderly individuals: a subgroup of the Cardiovascular Health Study.

BACKGROUND: Arterial stiffness has been associated with aging, hypertension, and diabetes; however, little data has been published examining risk factors associated with arterial stiffness in elderly individuals. METHODS: Longitudinal associations were made between aortic stiffness and risk factors measured approximately 4 years earlier. Aortic pulse wave velocity (PWV), an established index of arterial stiffness, was measured in 356 participants (53.4% women, 25.3% African American), aged 70 to 96 years, from the Pittsburgh site of the Cardiovascular Health Study during 1996 to 1998. RESULTS: Mean aortic pulse wave velocity (850 cm/sec, range 365 to 1863) did not differ by ethnicity or sex. Increased aortic stiffness was positively associated with higher systolic blood pressure (SBP), age, fasting and 2-h postload glucose, fasting and 2-h insulin, triglycerides, waist circumference, body mass index, truncal fat, decreased physical activity, heart rate, and common carotid artery wall thickness (P < .05). After controlling for age and SBP, the strongest predictors of aortic stiffness in men were heart rate (P = .001) and 2-h glucose (P = .063). In women, PWV was positively associated with heart rate (P = .018), use of antihypertensive medication (P = .035), waist circumference (P = .030), and triglycerides (P = .081), and was negatively associated with physical activity (P = .111). Results were similar when the analysis was repeated in nondiabetic individuals and in those free of clinical or subclinical cardiovascular disease in 1992 to 1993. CONCLUSIONS: In these elderly participants, aortic stiffness was positively associated with risk factors associated with the insulin resistance syndrome, increased common carotid intima-media thickness, heart rate, and decreased physical activity measured several years earlier.

Longitudinal perspective on the conundrum of central arterial stiffness, blood pressure, and aging.

The age-associated increase in arterial stiffness has long been considered to parallel or to cause the age-associated increase in blood pressure (BP). Yet, the rates at which pulse wave velocity (PWV), a measure of arterial stiffness, and BP trajectories change over time within individuals who differ by age and sex have not been assessed and compared. This study determined the evolution of BP and aortic PWV trajectories during a 9.4-year follow-up in >4000 community-dwelling men and women of 20 to 100 years of age at entry into the SardiNIA Study. Linear mixed effects model analyses revealed that PWV accelerates with time during the observation period, at about the same rate over the entire age range in both men and women. In men, the longitudinal rate at which BP changed over time, however, did not generally parallel that of PWV acceleration: at ages>40 years the rates of change in systolic BP (SBP) and pulse pressure (PP) increase plateaued and then declined so that SBP, itself, also declined at older ages, whereas PP plateaued. In women, SBP, diastolic BP, and mean BP increased at constant rates across all ages, producing an increasing rate of increase in PP. Therefore, increased aortic stiffness is implicated in the age-associated increase in SBP and PP. These findings indicate that PWV is not a surrogate for BP and that arterial properties other than arterial wall stiffness that vary by age and sex also modulate the BP trajectories during aging and lead to the dissociation of PWV, PP, and SBP trajectories in men.