Atrial fibrillation promotion by endurance exercise: demonstration and mechanistic exploration in an animal model.

OBJECTIVES: The goal of this study was to assess mechanisms underlying atrial fibrillation (AF) promotion by exercise training in an animal model. BACKGROUND: High-level exercise training promotes AF, but the underlying mechanisms are unclear. METHODS: AF susceptibility was assessed by programmed stimulation in rats after 8 (Ex8) and 16 (Ex16) weeks of daily 1-h treadmill training, along with 4 and 8 weeks after exercise cessation and time-matched sedentary (Sed) controls. Structural remodeling was evaluated by using serial echocardiography and histopathology, autonomic nervous system with pharmacological tools, acetylcholine-regulated potassium current (IKACh) with patch clamp recording, messenger ribonucleic acid expression with quantitative polymerase chain reaction, and regulators of G protein-signaling (RGS) 4 function in knockout mice. RESULTS: AF inducibility increased after 16 weeks of training (e.g., AF >30 s in 64% of Ex16 rats vs 15% of Sed rats; p < 0.01) and rapidly returned to baseline levels with detraining. Atropine restored sinus rhythm in 5 of 5 Ex rats with AF sustained >15 min. Atrial dilation and fibrosis developed after 16 weeks of training and failed to fully recover with exercise cessation. Parasympathetic tone was increased in Ex16 rats and normalized within 4 weeks of detraining. Baroreflex heart rate responses to phenylephrine-induced blood pressure elevation and IKACh sensitivity to carbachol were enhanced in Ex16 rats, implicating both central and end-organ mechanisms in vagal enhancement. Ex rats showed unchanged cardiac adrenergic and cholinergic receptor and IKACh-subunit gene expression, but significant messenger ribonucleic acid downregulation of IKACh-inhibiting RGS proteins was present at 16 weeks. RGS4 knockout mice showed significantly enhanced sensitivity to AF induction in the presence of carbachol. CONCLUSIONS: Chronic endurance exercise increased AF susceptibility in rats, with autonomic changes, atrial dilation, and fibrosis identified as potential mechanistic contributors. Vagal promotion is particularly important and occurs via augmented baroreflex responsiveness and increased cardiomyocyte sensitivity to cholinergic stimulation, possibly due to RGS protein downregulation.

Origin-specific epigenetic program correlates with vascular bed-specific differences in Rgs5 expression.

Cells from multiple origins contribute to vascular smooth muscle cell (VSMC) development. Phenotypic heterogeneity of VSMCs is associated with their point of developmental origin; however, the mechanisms driving such differences are unknown. We here examined the mechanisms controlling vascular bed-specific differences in Rgs5 expression during development. Rgs5 levels were similar across different regions of the vasculature in neonatal animals but were >15-fold higher in descending aortas compared with carotid arteries of adult mice. Thus, vessel bed-specific changes in regulation of Rgs5 expression occurred during vessel maturation. Examination of adult Rgs5-LacZ reporter mice revealed lower Rgs5 expression in VSMCs originating from the third (carotid artery) branchial arch compared with those originating in the fourth and sixth (aortic B segment, right subclavian, and ductus arteriosus) branchial arches. Indeed, a mosaic Rgs5 expression pattern, with discreet LacZ boundaries between VSMCs derived from different developmental origins, was observed. Furthermore, Rgs5-LacZ expression was correlated with the site of VSMC origin (splanchic mesoderm ≈ local mesenchyme > somites > proepicardium > mesothelium). Surprisingly, Rgs5 reporter activity in cultured carotid artery- and descending aorta-derived cells did not recapitulate the differences observed in vivo. Consistent with a developmental origin-specific epigenetic mechanism driving the observed expression differences in vivo, the Rgs5 promoter showed increased methylation on CpG dinucleotides in carotid arteries compared with that in descending aortas in adult but not in neonatal mice. In vitro methylation of the Rgs5 promoter confirmed that its activity is sensitive to transcriptional down-regulation by CpG methylation. These data suggest that an origin-dependent epigenetic program regulates vascular bed- and maturation state-dependent regulation of VSMC-specific gene transcription.

Iron overload decreases CaV1.3-dependent L-type Ca2+ currents leading to bradycardia, altered electrical conduction, and atrial fibrillation.

BACKGROUND: Chronic iron overload (CIO) is associated with blood disorders such as thalassemias and hemochromatosis. A major prognostic indicator of survival in patients with CIO is iron-mediated cardiomyopathy characterized by contractile dysfunction and electrical disturbances, including slow heart rate (bradycardia) and heart block. METHODS AND RESULTS: We used a mouse model of CIO to investigate the effects of iron on sinoatrial node (SAN) function. As in humans, CIO reduced heart rate (≈20%) in conscious mice as well as in anesthetized mice with autonomic nervous system blockade and in isolated Langendorff-perfused mouse hearts, suggesting that bradycardia originates from altered intrinsic SAN pacemaker function. Indeed, spontaneous action potential frequencies in SAN myocytes with CIO were reduced in association with decreased L-type Ca(2+) current (I(Ca,L)) densities and positive (rightward) voltage shifts in I(Ca,L) activation. Pacemaker current (I(f)) was not affected by CIO. Because I(Ca,L) in SAN myocytes (as well as in atrial and conducting system myocytes) activates at relatively negative potentials due to the presence of Ca(V)1.3 channels (in addition to Ca(V)1.2 channels), our data suggest that elevated iron preferentially suppresses Ca(V)1.3 channel function. Consistent with this suggestion, CIO reduced Ca(V)1.3 mRNA levels by ≈40% in atrial tissue (containing SAN) and did not lower heart rate in Ca(V)1.3 knockout mice. CIO also induced PR-interval prolongation, heart block, and atrial fibrillation, conditions also seen in Ca(V)1.3 knockout mice. CONCLUSIONS: Our results demonstrate that CIO selectively reduces Ca(V)1.3-mediated I(Ca,L), leading to bradycardia, slowing of electrical conduction, and atrial fibrillation as seen in patients with iron overload.

The primary benefits of angiotensin-converting enzyme inhibition on cardiac remodeling occur during sleep time in murine pressure overload hypertrophy.

OBJECTIVES: Our objective was to test the hypothesis that there is a significant diurnal variation for the therapeutic benefit of angiotensin-converting enzyme (ACE) inhibitors on pressure-overload cardiovascular hypertrophy. BACKGROUND: Physiological and molecular processes exhibit diurnal rhythms that may affect efficacy of disease treatment (chronotherapy). Evidence suggests that the heart primarily remodels during sleep. Although a growing body of clinical and epidemiological evidence suggests that the timing of therapy, such as ACE inhibition, alters diurnal blood pressure patterns in patients with hypertension, the benefits of chronotherapy on myocardial and vascular remodeling have not been studied. METHODS: We examined the effects of the short-acting ACE inhibitor, captopril, on the structure and function of cardiovascular tissue subjected to pressure overload by transverse aortic constriction (TAC) in mice. Captopril (15 mg/kg intraperitoneally) or placebo was administered at either murine sleep time or wake time for 8 weeks starting 1 week after surgery. RESULTS: TAC mice given captopril at sleep time had improved cardiac function and significantly decreased heart: body weight ratios, myocyte cross-sectional areas, intramyocardial vascular medial wall thickness, and perivascular collagen versus TAC mice given captopril or placebo during wake time. Captopril induced similar drops in blood pressure at sleep or wake time, suggesting that time-of-day differences were not attributable to blood pressure changes. These beneficial effects of captopril were correlated with diurnal changes in ACE mRNA expression in the heart. CONCLUSIONS: The ACE inhibitor captopril benefited cardiovascular remodeling only when administered during sleep; wake-time captopril ACE inhibition was identical to that of placebo. These studies support the hypothesis that the heart (and vessels) remodel during sleep time and also illustrate the importance of diurnal timing for some cardiovascular therapies.

Genetic deletion of regulator of G-protein signaling 4 (RGS4) rescues a subset of fragile X related phenotypes in the FMR1 knockout mouse.

Fragile X syndrome (FXS), the most common cause of inherited mental retardation, is caused by the loss of the mRNA binding protein, FMRP. Persons with FXS also display epileptic seizures, social anxiety, hyperactivity, and autistic behaviors. The metabotropic glutamate receptor theory of FXS postulates that in the absence of FMRP, enhanced signaling though G-protein coupled group I metabotropic glutamate receptors in the brain contributes to many of the abnormalities observed in the disorder. However, recent evidence suggests that alterations in cellular signaling through additional G-protein coupled receptors may also be involved in the pathogenesis of FXS, thus providing impetus for examining downstream molecules. One group of signaling molecules situated downstream of the receptors is the regulator of G-protein signaling (RGS) proteins. Notably, RGS4 is highly expressed in brain and has been shown to negatively regulate signaling through Group I mGluRs and GABA(B) receptors. To examine the potential role for RGS4 in the pathogenesis of FXS, we generated FXS/RGS4 double knockout mice. Characterization of these mice revealed that a subset of FXS related phenotypes, including increased body weight, altered synaptic protein expression, and abnormal social behaviors, were rescued in the double knockout mice. Other phenotypes, such as hyperactivity and macroorchidism, were not affected by the loss of RGS4. These findings suggest that tissue and cell-type specific differences in GPCR signaling and RGS function may contribute to the spectrum of phenotypic differences observed in FXS.

Fatigue preconditioning increases fatigue resistance in mouse flexor digitorum brevis muscles with non-functioning K(ATP) channels.

The objective of this study was to determine how an initial fatigue bout (FAT1 at 37°C) affects free myoplasmic Ca(2+) concentration and force ([Ca(2+)](i)/force) during a subsequent fatigue bout (FAT2) in mouse flexor digitorum brevis (FDB). During FAT1, both tetanic [Ca(2+)](i)/force decreased; however, they decreased to significantly lower levels when FAT1 was carried out in the presence of glibenclamide, a sarcolemmal K(ATP) (sK(ATP)) channel blocker. Glibenclamide also elicited greater increases in unstimulated [Ca(2+)](i)/force, which occurred when fibres failed to fully relax between contractions during FAT1. Finally, glibenclamide impaired force recovery after FAT1. The decreases in tetanic [Ca(2+)](i)/force and increases in unstimulated [Ca(2+)](i)/force were slower during FAT2 elicited 60 min after FAT1. Under control conditions, the effects were small with very few significant differences. In the presence of glibenclamide, on the other hand, the differences between FAT1 and FAT2 were very large. Unexpectedly, the differences in unstimulated and tetanic [Ca(2+)](i)/force between control and glibenclamide conditions observed during FAT1 were no longer observed during FAT2. The lack of differences was not related to a failure of glibenclamide to block K(ATP) channels during FAT2 because the effects of FAT1 on FAT2 were also observed using Kir6.2(-/-) mouse FDB, which lack sK(ATP) channel activity. The differences in [Ca(2+)](i)/force between FAT1 and FAT2 could be observed with FAT1 duration of just 30 s and a FAT1-FAT2 interval of at least 30 min. A modulation of factors involved in ischaemic pre-conditioning, i.e. A1-adenosine receptors, sK(ATP) and mitochondrial K(ATP) (mK(ATP)) channels, PKC and reactive oxygen species, during FAT1 had no effect on FAT2 fatigue kinetics. It is concluded that a preceding fatigue bout triggers an acute physiological process that prevents the contractile dysfunction induced by non-functioning K(ATP) channels.

RGS proteins: identifying new GAPs in the understanding of blood pressure regulation and cardiovascular function.

Understanding the mechanisms that underlie BP (blood pressure) variation in humans and animal models may provide important clues for reducing the burden of uncontrolled hypertension in industrialized societies. High BP is often associated with increased signalling via G-protein-coupled receptors. Three members of the RGS (regulator of G-protein signalling) superfamily RGS2, RGS4 and RGS5 have been implicated in the attenuation of G-protein signalling pathways in vascular and cardiac myocytes, as well as cells of the kidney and autonomic nervous system. In the present review, we discuss the current state of knowledge regarding their differential expression and function in cardiovascular tissues, and the likelihood that one or more of these alleles are candidate hypertension genes. Together, findings from the studies described herein suggest that development of methods to modulate the expression and function of RGS proteins may be a possible strategy for the treatment and prevention of hypertension and cardiovascular disease.

RGS4 regulates parasympathetic signaling and heart rate control in the sinoatrial node.

Heart rate is controlled by the opposing activities of sympathetic and parasympathetic inputs to pacemaker myocytes in the sinoatrial node (SAN). Parasympathetic activity on nodal myocytes is mediated by acetylcholine-dependent stimulation of M(2) muscarinic receptors and activation of Galpha(i/o) signaling. Although regulators of G protein signaling (RGS) proteins are potent inhibitors of Galpha(i/o) signaling in many tissues, the RGS protein(s) that regulate parasympathetic tone in the SAN are unknown. Our results demonstrate that RGS4 mRNA levels are higher in the SAN compared to right atrium. Conscious freely moving RGS4-null mice showed increased bradycardic responses to parasympathetic agonists compared to wild-type animals. Moreover, anesthetized RGS4-null mice had lower baseline heart rates and greater heart rate increases following atropine administration. Retrograde-perfused hearts from RGS4-null mice showed enhanced negative chronotropic responses to carbachol, whereas SAN myocytes showed greater sensitivity to carbachol-mediated reduction in the action potential firing rate. Finally, RGS4-null SAN cells showed decreased levels of G protein-coupled inward rectifying potassium (GIRK) channel desensitization and altered modulation of acetylcholine-sensitive potassium current (I(KACh)) kinetics following carbachol stimulation. Taken together, our studies establish that RGS4 plays an important role in regulating sinus rhythm by inhibiting parasympathetic signaling and I(KACh) activity.

Contractile dysfunctions in ATP-dependent K+ channel-deficient mouse muscle during fatigue involve excessive depolarization and Ca2+ influx through L-type Ca2+ channels.

Muscles deficient in ATP-dependent potassium (KATP) channels develop contractile dysfunctions during fatigue that may explain their apparently faster rate of fatigue compared with wild-type muscles. The objectives of this study were to determine: (1) whether the contractile dysfunctions, namely unstimulated force and depressed force recovery, result from excessive membrane depolarization and Ca2+ influx through L-type Ca2+ channels; and (2) whether reducing the magnitude of these two contractile dysfunctions reduces the rate of fatigue in KATP channel-deficient muscles. To reduce Ca2+ influx, we lowered the extracellular Ca2+ concentration ([Ca2+]o) from 2.4 to 0.6 mM or added 1 microM verapamil, an L-type Ca2+ channel blocker. Flexor digitorum brevis (FDB) muscles deficient in KATP channels were obtained by exposing wild-type muscles to 10 microM glibenclamide or by using FDB from Kir6.2-/- mice. Fatigue was elicited with one contraction per second for 3 min at 37 degrees C. In wild-type FDB, lowered [Ca2+]o or verapamil did not affect the decrease in peak tetanic force and unstimulated force during fatigue and force recovery following fatigue. In KATP channel-deficient FDB, lowered [Ca2+]o or verapamil slowed down the decrease in peak tetanic force recovery, reduced unstimulated force and improved force recovery. In Kir6.2-/- FDB, the rate of fatigue became slower than in wild-type FDB in the presence of verapamil. The cell membrane depolarized from -83 to -57 mV in normal wild-type FDB. The depolarizations in some glibenclamide-exposed fibres were similar to those of normal FDB, while in other fibres the cell membrane depolarized to -31 mV in 80 s, which was also the time when these fibres supercontracted. It is concluded that: (1) KATP channels are crucial in preventing excessive membrane depolarization and Ca2+ influx through L-type Ca2+ channels; and (2) they contribute to the decrease in force during fatigue.

KATP channel deficiency in mouse flexor digitorum brevis causes fibre damage and impairs Ca2+ release and force development during fatigue in vitro.

Activation of the K(ATP) channels results in faster fatigue rates as the channels depress action potential amplitude, whereas abolishing the channel activity has no effect in whole extensor digitorum longus (EDL) and soleus muscles. In this study, we examined the effects of abolished K(ATP) channel activity during fatigue at 37 degrees C on free intracellular Ca(2+) (Ca(2+)(i)) and tetanic force using single muscle fibres and small muscle bundles from the flexor digitorum brevis (FDB). K(ATP) channel deficient muscle fibres were obtained (i) pharmacologically by exposing wild-type fibres to glibenclamide, and (ii) genetically using null mice for the Kir6.2 gene (Kir6.2(-/-) mice). Fatigue was elicited using 200 ms tetanic contractions every second for 3 min. This study demonstrated for the first time that abolishing K(ATP) channel activity at 37 degrees C resulted in faster fatigue rates, where decreases in peak Ca(2+)(i) and tetanic force were faster in K(ATP) channel deficient fibres than in control wild-type fibres. Furthermore, several contractile dysfunctions were also observed in K(ATP) channel deficient muscle fibre. They included partially or completely supercontracted single muscle fibres, greater increases in unstimulated Ca(2+)(i) and unstimulated force, and lower force recovery. We propose that the observed faster rate of fatigue in K(ATP) channel deficient fibres is because the decreases in peak Ca(2+)(i) and force caused by contractile dysfunctions prevail over the expected slower decreases when the channels do not depress action potential amplitude.

The mouse dystrophin muscle promoter/enhancer drives expression of mini-dystrophin in transgenic mdx mice and rescues the dystrophy in these mice.

Successful gene therapy for Duchenne muscular dystrophy (DMD) requires the restoration of dystrophin protein in skeletal muscles. To achieve this goal, appropriate regulatory elements that impart tissue-specific transgene expression need to be identified. Currently, most muscle-directed gene therapy studies utilize the muscle creatine kinase promoter. We have previously described a muscle enhancer element (mDME-1) derived from the mouse dystrophin gene that increases transcription from the mouse dystrophin muscle promoter. Here, we explore the use of this native mouse dystrophin muscle promoter/enhancer to drive expression of a human dystrophin minigene in transgenic mice. We show that the dystrophin promoter can provide tissue-specific transgene expression and that the mini-dystrophin protein is expressed at the sarcolemma of skeletal muscles from mdx mice, where it restores the dystrophin-associated glycoprotein complex. The level of transgene expression obtained is sufficient to protect mdx muscles from the morphological and physiological symptoms of muscular dystrophy, as well as from exercise-induced damage. Therefore, the dystrophin muscle promoter/enhancer sequence represents an alternative for use in gene therapy vectors for the treatment of DMD.