Cytoskeletal reorganization evoked by Rho-associated kinase- and protein kinase C-catalyzed phosphorylation of cofilin and heat shock protein 27, respectively, contributes to myogenic constriction of rat cerebral arteries.

Our understanding of the molecular events contributing to myogenic control of diameter in cerebral resistance arteries in response to changes in intravascular pressure, a fundamental mechanism regulating blood flow to the brain, is incomplete. Myosin light chain kinase and phosphatase activities are known to be increased and decreased, respectively, to augment phosphorylation of the 20-kDa regulatory light chain subunits (LC20) of myosin II, which permits cross-bridge cycling and force development. Here, we assessed the contribution of dynamic reorganization of the actin cytoskeleton and thin filament regulation to the myogenic response and serotonin-evoked constriction of pressurized rat middle cerebral arteries. Arterial diameter and the levels of phosphorylated LC(20), calponin, caldesmon, cofilin, and HSP27, as well as G-actin content, were determined. A decline in G-actin content was observed following pressurization from 10 mm Hg to between 40 and 120 mm Hg and in three conditions in which myogenic or agonist-evoked constriction occurred in the absence of a detectable change in LC20 phosphorylation. No changes in thin filament protein phosphorylation were evident. Pressurization reduced G-actin content and elevated the levels of cofilin and HSP27 phosphorylation. Inhibitors of Rho-associated kinase and PKC prevented the decline in G-actin; reduced cofilin and HSP27 phosphoprotein content, respectively; and blocked the myogenic response. Furthermore, phosphorylation modulators of HSP27 and cofilin induced significant changes in arterial diameter and G-actin content of myogenically active arteries. Taken together, our findings suggest that dynamic reorganization of the cytoskeleton involving increased actin polymerization in response to Rho-associated kinase and PKC signaling contributes significantly to force generation in myogenic constriction of cerebral resistance arteries.

Phospholamban phosphorylation increases the passive calcium leak from cardiac sarcoplasmic reticulum.

Phospholamban (PLN) is a 52 amino acid integral membrane protein of the sarcoplasmic reticulum (SR) that exists in both monomeric and pentameric forms. In its unphosphorylated state, PLN inhibits the SR Ca(2+) ATPase (SERCA). This inhibition is relieved when PLN is phosphorylated as a result of ß-adrenergic stimulation of the heart. Consistent with some predictions from molecular models and from functional studies of PLN incorporated into planar lipid bilayers, it has also been postulated that pentameric PLN can also form ion-selective channels. Other molecular models contradict this hypothesis, however. In the work reported here, we used the Ca(2+)-sensitive fluorescent dye Fura-2, to examine the passive Ca(2+) permeability of the SR membrane in vesicles derived from cardiac ventricle. We have found that phosphorylation of PLN by protein kinase A (PKA) leads to an increase in the rate of Ca(2+) leak from Ca(2+)-loaded SR vesicles. This enhanced rate of Ca(2+) leak from the SR is also observed when SR vesicles are incubated with a PLN specific antibody (A1) that mimics phosphorylation of PLN. The ryanodine receptor blocker ruthenium red does not affect the increased rate of Ca(2+) leak from the SR after PLN phosphorylation with PKA or after exposure to A1 antibody, arguing against a possible role of ryanodine receptors in mediating the enhanced leak. Our results are consistent with the hypothesis that phosphorylated PLN forms or regulates a Ca(2+) leak pathway in cardiac SR membranes in situ.

Epigallocatechin-3-gallate has dual, independent effects on the cardiac sarcoplasmic reticulum/endoplasmic reticulum Ca2+ ATPase.

We determined the effects of epigallocatechin-3-gallate (EGCG) and epicatechin (EC), on pump turnover and Ca2+ transport by the cardiac form of the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA). Fluorescence spectroscopy was used to directly measure SERCA ATPase activity and to measure Ca2+ uptake into cardiac sarcoplasmic reticulum (SR) vesicles and microsomes derived from human embryonic kidney (HEK) cells expressing human cardiac SERCA2a. We found that EGCG reduces the maximum velocity of Ca2+ uptake into cardiac SR vesicles and increases the Ca2+-sensitivity of uptake in a concentration-dependent manner. EC is less potent than EGCG in increasing the Ca2+-sensitivity of uptake and does not affect maximum uptake velocity. The EGCG-dependent reduction in Ca2+ uptake velocity is well correlated with direct inhibition of SERCA. The effect of EGCG on the Ca2+-sensitivity of Ca2+ uptake into cardiac SR vesicles is affected by the phosphorylation status of phospholamban (PLB). When cardiac SERCA2a is expressed in HEK cells without PLB, EGCG reduces the maximum velocity of Ca2+ uptake but does not affect the Ca2+-sensitivity of uptake into microsomes derived from these cells indicating that the effect of EGCG on Ca2+-sensitivity requires the presence of PLB. Our results show that EGCG has dual effects on SERCA function in cardiac SR vesicles: it directly affects SERCA by reducing maximum uptake velocity; it increases the Ca2+-sensitivity of Ca2+ uptake in a manner that appears to depend on the interaction between SERCA and PLB.

Effects of monovalent cations on Ca2+ uptake by skeletal and cardiac muscle sarcoplasmic reticulum.

Ca(2+) transport by the sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase (SERCA) is sensitive to monovalent cations. Possible K(+) binding sites have been identified in both the cytoplasmic P-domain and the transmembrane transport-domain of the protein. We measured Ca(2+) transport into SR vesicles and SERCA ATPase activity in the presence of different monovalent cations. We found that the effects of monovalent cations on Ca(2+) transport correlated in most cases with their direct effects on SERCA. Choline(+), however, inhibited uptake to a greater extent than could be accounted for by its direct effect on SERCA suggesting a possible effect of choline on compensatory charge movement during Ca(2+) transport. Of the monovalent cations tested, only Cs(+) significantly affected the Hill coefficient of Ca(2+) transport (n(H)). An increase in n(H) from approximately 2 in K(+) to approximately 3 in Cs(+) was seen in all of the forms of SERCA examined. The effects of Cs(+) on the maximum velocity of Ca(2+) uptake were also different for different forms of SERCA but these differences could not be attributed to differences in the putative K(+) binding sites of the different forms of the protein.

Inward rectifier K(+) currents and Kir2.1 expression in renal afferent and efferent arterioles.

The afferent and efferent arterioles regulate the inflow and outflow resistance of the glomerulus, acting in concert to control the glomerular capillary pressure and glomerular filtration rate. The myocytes of these two vessels are remarkably different, especially regarding electromechanical coupling. This study investigated the expression and function of inward rectifier K(+) channels in these two vessels using perfused hydronephrotic rat kidneys and arterioles and myocytes isolated from normal rat kidneys. In afferent arterioles pre-constricted with angiotensin II, elevating [K(+)](0) from 5 to 15 mmol/L induced hyperpolarization (-27 +/- 2 to 41 +/- 3 mV) and vasodilation (6.6 +/- 0.9 to 13.1 +/- 0.6 microm). This manipulation also attenuated angiotensin II-induced Ca(2+) signaling, an effect blocked by 100 micromol/LBa(2+). By contrast, elevating [K(+)](o) did not alter angiotensin II-induced Ca2(+) signaling or vasoconstriction in efferent arterioles, even though a significant hyperpolarization was observed (from -30 +/- 1 to 37 +/- 3 mV, P = 0.003). Both vessels expressed mRNA for Kir2.1 and exhibited anti-Kir2.1 antibody labeling.Patch-clamp measurements revealed prominent inwardly rectifying and Ba(2+)-sensitive currents in afferent and efferent arteriolar myocytes. Our findings indicate that both arterioles express an inward rectifier K(+) current, but that modulation of this current alters responsiveness of only the a different arteriole. The expression of Kir in the efferent arteriole, a resistance vessel whose tone is not affected by membrane potential, is intriguing and may suggest a novel function of this channel in the renal microcirculation.

Myosin heavy chain expression in renal afferent and efferent arterioles: relationship to contractile kinetics and function.

The physiological role of smooth muscle myosin heavy chain (MHC) isoform diversity is poorly understood. The expression of MHC-B, which contains an insert at the ATP binding pocket, has been linked to enhanced contractile kinetics. We recently reported that the renal afferent arteriole exhibits an unusually rapid myogenic response and that its kinetic features allow this vessel to modulate tone in response to alterations in systolic blood pressure. In the present study, we examined MHC expression patterns in renal afferent and efferent arterioles. These two vessels regulate glomerular inflow and outflow resistances and control the pressure within the intervening glomerular capillaries (PGC). Whereas the afferent arteriole must respond rapidly to increases in blood pressure, the efferent arteriole plays a distinctly different role, maintaining a tonic elevation in outflow resistance to preserve function when renal perfusion is compromised. Using RT-PCR, Western analysis, and immunofluorescence imaging of intact isolated arterioles, we found that the afferent arteriole predominantly expresses the MHC-B isoform, whereas the efferent arteriole expresses only the slower-cycling MHC-A isoform. We examined the kinetics of angiotensin II- and norepinephrine-induced vasoconstriction and found that the afferent arteriole responds approximately 3-fold faster than the efferent arteriole. Our findings thus point to the renal microcirculation as a unique and important example of smooth muscle adaptation in regard to MHC isoform expression and physiological function.

Sex-dependent impairment of cardiac action potential conduction in type 1 diabetic rats.

The incidence of diabetes mellitus is increasing. Cardiac dysfunction often develops, resulting in diverse arrhythmias. These arise from ion channel remodeling or from altered speed and pattern of impulse propagation. Few studies have investigated impulse propagation in the diabetic heart. We previously showed a reduced conduction reserve in the diabetic heart, with associated changes in intercellular gap junctions. The present study investigated whether these effects are sex specific. Hearts from control and streptozotocin-diabetic male and female rats were used. Optical mapping was performed with the voltage-sensitive dye di-4-ANEPPS, using Langendorff-perfused hearts. Isolated ventricular cells and tissue sections were used for immunofluorescent labeling of the gap junction protein connexin43 (Cx43). The gap junction uncoupler heptanol (0.75 mM) or elevated K(+) (9 mM, to reduce cell excitability) produced significantly greater slowing of propagation in diabetic males than females. In ovariectomized diabetic females, 9 mM K(+) slowed conduction significantly more than in nonovariectomized females. The subcellular redistribution (lateralization) of the gap junction protein Cx43 was smaller in diabetic females. Pretreatment of diabetic males with the angiotensin-converting enzyme inhibitor quinapril reduced Cx43 lateralization and the effects of 9 mM K(+) on propagation. In conclusion, the slowing of cardiac impulse propagation in type 1 diabetes is smaller in female rats, partly due to the presence of female sex hormones. This difference is (partly) mediated by sex differences in activation of the cardiac renin-angiotensin system.

Inhibition of a cardiac sarcoplasmic reticulum chloride channel by tamoxifen.

Anion and cation channels present in the sarcoplasmic reticulum (SR) are believed to be necessary to maintain the electroneutrality of SR membrane during Ca(2+) uptake by the SR Ca(2+) pump (SERCA). Here we incorporated canine cardiac SR ion channels into lipid bilayers and studied the effects of tamoxifen and other antiestrogens on these channels. A Cl(-) channel was identified exhibiting multiple subconductance levels which could be divided into two primary conductance bands. Tamoxifen decreases the time the channel spends in its higher, voltage-sensitive band and the mean channel current. The lower, voltage-insensitive, conductance band is not affected by tamoxifen, nor is a K(+) channel present in the cardiac SR preparation. By examining SR Ca(2+) uptake, SERCA ATPase activity, and SR ion channels in the same preparation, we also estimated SERCA transport current, SR Cl(-) and K(+) currents, and the density of SERCA, Cl(-), and K(+) channels in cardiac SR membranes.

High sensitivity, quantitative measurements of polyphosphate using a new DAPI-based approach.

Polyphosphate (poly-P) is an important metabolite and signaling molecule in prokaryotes and eukaryotes. DAPI (4',6-diamidino-2-phenylindole), a widely used fluorescent label for DNA, also interacts with polyphosphate. Binding of poly-P to DAPI, shifts its peak emission wavelength from 475 to 525 nm (excitation at 360 nm), allowing use of DAPI for detection of poly-P in vitro, and in live poly-P accumulating organisms. This approach, which relies on detection of a shift in fluorescence emission, allows use of DAPI only for qualitative detection of relatively high concentrations of poly-P, in the microg/ml range. Here, we report that long-wavelength excitation (> or = 400 nm) of the DAPI-poly-P complex provides a dramatic increase in the sensitivity of poly-P detection. Using excitation at 415 nm, fluorescence of the DAPI-poly-P complex can be detected at a higher wavelength (550 nm) for as little as 25 ng/ml of poly-P. Fluorescence emission from free DAPI and DAPI-DNA are minimal at this wavelength, making the DAPI-poly-P signal highly specific and essentially independent of the presence of DNA. In addition, we demonstrate the use of this protocol to measure the activity of poly-P hydrolyzing enzyme, polyphosphatase and demonstrate a similar signal from the mitochondrial region of cultured neurons.

Effects of phloretin and phloridzin on Ca2+ handling, the action potential, and ion currents in rat ventricular myocytes.

The effects of the phytoestrogens phloretin and phloridzin on Ca(2+) handling, cell shortening, the action potential, and Ca(2+) and K(+) currents in freshly isolated cardiac myocytes from rat ventricle were examined. Phloretin increased the amplitude and area and decreased the rate of decline of electrically evoked Ca(2+) transients in the myocytes. These effects were accompanied by an increase in the Ca(2+) load of the sarcoplasmic reticulum, as determined by the area of caffeine-evoked Ca(2+) transients. An increase in the extent of shortening of the myocytes in response to electrically evoked action potentials was also observed in the presence of phloretin. To further examine possible mechanisms contributing to the observed changes in Ca(2+) handling and contractility, the effects of phloretin on the cardiac action potential and plasma membrane Ca(2+) and K(+) currents were examined. Phloretin markedly increased the action potential duration in the myocytes, and it inhibited the Ca(2+)-independent transient outward K(+) current (I(to)). The inwardly rectifying K(+) current, the sustained outward delayed rectifier K(+) current, and L-type Ca(2+) currents were not significantly different in the presence and absence of phloretin, nor was there any evidence that the Na(+)/Ca(2+) exchanger was affected. The effects of phloretin on Ca(2+) handling in the myocytes are consistent with its effects on I(to). Phloridzin did not significantly alter the amplitude or area of electrically evoked Ca(2+) transients in the myocytes, nor did it have detectable effects on the sarcoplasmic reticulum Ca(2+) load, cell shortening, or the action potential.

Differential autocrine modulation of atrial and ventricular potassium currents and of oxidative stress in diabetic rats.

The autocrine modulation of cardiac K(+) currents was compared in ventricular and atrial cells (V and A cells, respectively) from Type 1 diabetic rats. K(+) currents were measured by using whole cell voltage clamp. ANG II was measured by ELISA and immunofluorescent labeling. Oxidative stress was assessed by immunofluorescent labeling with dihydroethidium, a measure of superoxide ions. In V cells, K(+) currents are attenuated after activation of the renin-angiotensin system (RAS) and the resulting ANG II-mediated oxidative stress. In striking contrast, these currents are not attenuated in A cells. Inhibition of the angiotensin-converting enzyme (ACE) also has no effect, in contrast to current augmentation in V cells. ANG II levels are enhanced in V, but not in A, cells. However, the high basal ANG II levels in A cells suggest that in these cells, ANG II-mediated pathways are suppressed, rather than ANG II formation. Concordantly, superoxide ion levels are lower in diabetic A than in V cells. Several findings indicate that high atrial natriuretic peptide (ANP) levels in A cells inhibit RAS activation. In male diabetic V cells, in vitro ANP (300 nM-1 muM, >5 h) decreases oxidative stress and augments K(+) currents, but not when excess ANG II is present. ANP has no effect on ventricular K(+) currents when the RAS is not activated, as in control males, in diabetic males treated with ACE inhibitor and in diabetic females. In conclusion, the modulation of K(+) currents and oxidative stress is significantly different in A and V cells in diabetic rat hearts. The evidence suggests that this is largely due to inhibition of RAS activation and/or action by ANP in A cells. These results may underlie chamber-specific arrhythmogenic mechanisms.

Inhibition of cAMP-dependent protein kinase under conditions occurring in the cardiac dyad during a Ca2+ transient.

The space between the t-tubule invagination and the sarcoplasmic reticulum (SR) membrane, the dyad, in ventricular myocytes has been predicted to experience very high [Ca2+] for short periods of time during a Ca2+ transient. The dyadic space accommodates many protein kinases responsible for the regulation of Ca2+ handling proteins of the cell. We show in vitro that cAMP-dependent protein kinase (PKA) is inhibited by high [Ca2+] through a shift in the ratio of CaATP/MgATP toward CaATP. We further generate a three-dimensional mathematical model of Ca2+ and ATP diffusion within dyad. We use this model to predict the extent to which PKA would be inhibited by an increased CaATP/MgATP ratio during a Ca2+ transient in the dyad in vivo. Our results suggest that under normal physiological conditions a myocyte paced at 1 Hz would experience up to 55% inhibition of PKA within the cardiac dyad, with inhibition averaging 5% throughout the transient, an effect which becomes more pronounced as the myocyte contractile frequency increases (at 7 Hz, PKA inhibition averages 28% across the dyad throughout the duration of a Ca2+ transient).

Effects of phytoestrogens on sarcoplasmic/endoplasmic reticulum calcium ATPase 2a and Ca2+ uptake into cardiac sarcoplasmic reticulum.

Phytoestrogens are naturally occurring estrogenic compounds found in plants and plant products. These compounds are also known to exert cellular effects independent of their interactions with estrogen receptors. We studied the effects of the phytoestrogens phloretin, phloridzin, genistein, and biochanin A on Ca(2+) uptake into the cardiac muscle sarcoplasmic reticulum (SR). Genistein and biochanin A did not affect SR Ca(2+) uptake. On the other hand, phloretin and phloridzin decreased the maximum velocity of SR Ca(2+) uptake but did not affect the Hill coefficient or the Ca(2+) sensitivity of uptake. Measurements of the ATPase activity of the cardiac SR Ca(2+) pump (SERCA2a) revealed direct inhibitory effects of phloretin and phloridzin on SERCA2a. Neither compound induced a detectable change in the permeability of the SR membrane to Ca(2+). These results indicate that phloretin and phloridzin inhibit cardiac SR Ca(2+) uptake by directly inhibiting SERCA2a.

Localization of telokin at the intercalated discs of cardiac myocytes.

Telokin is identical in sequence to the C-terminal portion of myosin light chain kinase but is expressed independently. We have used monoclonal antibodies specific to the non-telokin portion of myosin light chain kinase and to telokin, immunofluorescence microscopy and image reconstruction to demonstrate the presence of telokin in cardiac myocytes and to study its subcellular distribution. Antibodies to telokin labeled the intercalated discs of adult cardiac myocytes and similar structures in isolated intercalated disc preparations. Antibodies specific to the non-telokin portion of myosin light chain kinase did not label intercalated discs in either of these preparations. Western blots of isolated intercalated discs with anti-telokin revealed a 23kDa protein that co-migrates with purified telokin on SDS-PAGE. Deconvolution, reconstruction and analysis of fluorescence images of isolated intercalated discs labeled with anti-telokin and anti-beta-catenin, anti-gamma-catenin or anti-connexin43 indicated that telokin is only partially co-localized with these proteins at the discs.

Inhibition of SERCA2 Ca(2+)-ATPases by Cs(+).

Replacement of K(+) with Cs(+) on the cytoplasmic side of the sarcoplasmic reticulum (SR) membrane reduces the maximum velocity (V(max)) of Ca(2+) uptake into the SR of saponin-permeabilized rat ventricular myocytes. To compare the sensitivity of the cardiac and smooth muscle/non-muscle forms of the sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA2a and -2b respectively) to replacement of K(+) with Cs(+), SERCA2a and SERCA2b were expressed in HEK-293 cells. Ca(2+) uptake into HEK cell microsomes was inhibited by replacement of extravesicular K(+) with Cs(+) (V(max) of SERCA2a-mediated Ca(2+) uptake in CsCl was 80% of that in KCl; V(max) of SERCA2b-mediated uptake was 70% of that in KCl). The Ca(2+) sensitivity of uptake was decreased for both SERCA2a- and SERCA2b-mediated uptake and the Hill coefficients were increased in the presence of CsCl. The effects of Cs(+) on uptake were associated with direct inhibition of the ATPase activity of SERCA2a and SERCA2b. Our results indicate that cation binding sites are present in both SERCA2 isoforms, although the extent to which SERCA2b is inhibited by K(+) replacement is greater than that of SERCA2a or SERCA1. Consideration of these results and the recent molecular modeling work of others suggests that monovalent cations could interact with the Ca(2+) binding region of SERCA.

Responses of Ca2+-binding proteins to localized, transient changes in intracellular [Ca2+].

In smooth muscle cells, various transient, localized [Ca(2+)] changes have been observed that are thought to regulate cell function without necessarily inducing contraction. Although a great deal of effort has been put into detecting these transients and elucidating the mechanisms involved in their generation, the extent to which these transient Ca(2+) signals interact with intracellular Ca(2+)-binding molecules remains relatively unknown. To understand how the spatial and temporal characteristics of an intracellular Ca(2+) signal influence its interaction with Ca(2+)-binding proteins, mathematical models of Ca(2+) diffusion and regulation in smooth muscle cells were used to study Ca(2+) binding to prototypical proteins with one or two Ca(2+)-binding sites. Simulations with the models: (1) demonstrate the extent to which the rate constants for Ca(2+)-binding to proteins and the spatial and temporal characteristics of different Ca(2+) transients influence the magnitude and time course of the responses of these proteins to the transients; (2) predict significant differences in the responses of proteins with one or two Ca(2+)-binding sites to individual Ca(2+) transients and to trains of transients; (3) demonstrate how the kinetic characteristics determine the fidelity with which the responses of Ca(2+)-sensitive molecules reflect the magnitude and time course of transient Ca(2+) signals. Overall, this work demonstrates the clear need for complete information about the kinetics of Ca(2+) binding for determining how well Ca(2+)-binding molecules respond to different types of Ca(2+) signals. These results have important implications when considering the possible modulation of Ca(2+)- and Ca(2+)/calmodulin-dependent proteins by localized intracellular Ca(2+) transients in smooth muscle cells and, more generally, in other cell types.

Ca2+ regulation in the near-membrane microenvironment in smooth muscle cells.

The microenvironment between the plasma membrane and the near-membrane sarcoplasmic reticulum (SR) may play an important role in Ca(2+) regulation in smooth muscle cells. We used a three-dimensional mathematical model of Ca(2+) diffusion and regulation and experimental measurements of SR Ca(2+) uptake and the distribution of the SR in isolated smooth muscle cells to predict the extent that the near-membrane SR could load Ca(2+) after the opening of single plasma membrane Ca(2+) channels. We also modeled the effect of SR uptake on 1), single-channel Ca(2+) transients in the near-membrane space; 2), the association of Ca(2+) with Ca(2+) buffers in this space; and 3), the amount of Ca(2+) reaching the central cytoplasm of the cell. Our results indicate that, although single-channel Ca(2+) transients could increase SR Ca(2+) to a certain extent, SR Ca(2+) uptake is not rapid enough to greatly affect the magnitude of these transients or their spread to the central cytoplasm unless the Ca(2+) uptake rate of the peripheral SR is an order-of-magnitude higher than the mean rate derived from our experiments. Immunofluorescence imaging, however, did not reveal obvious differences in the density of SR Ca(2+) pumps or phospholamban between the peripheral and central SR in smooth muscle cells.

Tamoxifen inhibits Na+ and K+ currents in rat ventricular myocytes.

Tamoxifen is an estrogen receptor antagonist used in the treatment of breast cancer. However, tamoxifen has been shown to induce QT prolongation of the electrocardiogram, thereby potentially causing life-threatening polymorphic ventricular arrhythmias. The purpose of the present study was to elucidate the electrophysiological mechanism(s) that underlie the arrhythmogenic effects of tamoxifen. We used standard ruptured whole cell and perforated patch-clamping techniques on rat ventricular myocytes to investigate the effects of tamoxifen on cardiac action potential (AP) waveforms and the underlying K+ currents. Tamoxifen (3 micromol/l) markedly prolonged AP duration, decreased maximal rate of depolarization, and decreased resting membrane potential. At this concentration, tamoxifen significantly depressed the Ca2+-independent transient outward K+ current (Ito), sustained outward delayed rectifier K+ current (Isus), inward rectifier K+ current (IK1), and Na+ current (INa) in the myocytes. Lower concentrations of tamoxifen (1 micromol/l) also decreased the resting membrane potential and significantly depressed IK1 to 79 +/- 5% (n = 5; at -120 mV) of pretreatment values. The results of this study indicate that inhibition of Ito, Isus, and IK1 by tamoxifen may underlie AP prolongation in cardiac myocytes and thereby contribute to prolonged QT interval observed in patients.

Postnatal changes in caldesmon expression and localization in cardiac myocytes.

Caldesmon is a heat-stable protein found in both muscle and non-muscle tissue. It binds to a number of contractile and cytoskeletal proteins and may be involved in regulating acto-myosin interaction in smooth muscle cells and/or the assembly of microfilaments in muscle and non-muscle cells. We have shown previously that caldesmon is localized at the Z-lines in adult cardiac myocytes and that both the low- and high-molecular-weight forms (/-caldesmon and h-caldesmon, respectively) are present in atrial and ventricular myocytes. Here we examined the expression of caldesmon and its localization in freshly isolated cardiac myocytes during postnatal development and when these myocytes were grown in culture. We found that /-caldesmon is expressed in both neonatal and adult rat ventricular myocytes. The expression of h-caldesmon, however, was not detected in myocytes from newborn animals but increased during the first 2 weeks of postnatal development. Caldesmon was generally not co-localized with alpha-actinin at the Z-lines in neonatal myocytes but became increasingly more so during the first 2 weeks of postnatal development. When myocytes from 5- and 10-day-old rats were grown in primary culture, h-caldesmon expression decreased and caldesmon could not be detected at the Z-lines in the cultured cells. These results indicate that caldesmon plays a role at the Z-lines in adult cardiac myocytes; however, its localization at the Z-lines is not necessary for the prenatal development that occurs at these sites or for the establishment of a contractile phenotype in cultured cardiac myocytes.