Effects of chronic run training on Na+-dependent Ca2+ efflux from rat left ventricular myocytes

Effects of aerobic training, resistance training, or combined resistance-aerobic training on the left ventricular myocardium in a rat model

This study follows the left ventricular (LV) hypertrophy in rats undergoing aerobic training alone (A), resistance training alone (R), or combined resistance and aerobic training (RA) (usually referred as concurrent training) program. A sedentary control group (C) was included. LV remodeling was evaluated using electron and light microscopy. The LV weight to body weight (LVW: BW) increased 11.4% in A group, 35% in the R group, and 18% in the RA group compared to the C group. The LV thickness increased 6% in the A group, 17% in the R group, and 10% in the RA group. The LV internal diameter increased 19% in the A group, 3% in the R group, and 8% in the RA group compared with the C group. The cross-sectional area of cardiomyocyte increased by 1% with the A group, 27% with R group, and 12% with RA training. The capillary density increased by 5.4% with A training, 11.0% with R training, and 7.7% with RA training compared with the C group. The volume fraction of interstitial collagen increased by 0.4% with training A, increased by 2.8% with R training, and 0.9% with RA training. In conclusion, except for the LV internal diameter, which increased more in the A group, the cardiac parameters increased more in the R group than in the other groups and in RA group than in A group. Collagen density increased from 5.4 ± 0.8% in the C group to 5.8 ± 0.6% in the A group (n. s.) (P > 0.05), to 8.2 ± 0.7% in the R group (P < 0.05), and to 6.3 ± 0.4% in the RA group (P < 0.05). These results demonstrate a significant increase for collagen content in the LV with R and RA exercise, but the increase was higher with R training alone than with RA training.

Endurance exercise training reduces cardiac sodium/calcium exchanger expression in animals susceptible to ventricular fibrillation

Aim: Increased sodium/calcium exchanger activity (NCX1, an important regulator of cardiomyocyte cystolic calcium) may provoke arrhythmias. Exercise training can decrease NCX1 expression in animals with heart failure improving cytosolic calcium regulation, and could thereby reduce the risk for ventricular fibrillation (VF). Methods: To test this hypothesis, a 2-min coronary occlusion was made during the last minute of exercise in dogs with healed myocardial infarctions; 23 had VF (S, susceptible) and 13 did not (R, resistant). The animals were randomly assigned to either 10-week exercise training (progressively increasing treadmill running; S n = 9; R n = 8) or 10-week sedentary (S n = 14; R n = 5) groups. At the end of the 10-week period, the exercise + ischemia test provoked VF in sedentary but not trained susceptible dogs. On a subsequent day, cardiac tissue was harvested and NCX1 protein expression was determined by Western blot. Results: In the sedentary group, NCX1 expression was significantly (ANOVA, P < 0.05) higher in susceptible compared to resistant dogs. In contrast, NCX1 levels were similar in the exercise trained resistant and susceptible animals. Conclusion: These data suggest that exercise training can restore a more normal NCX1 level in dogs susceptible to VF, improving cystolic calcium regulation and could thereby reduce the risk for sudden death following myocardial infarction.

Cardiac autonomic neural remodeling and susceptibility to sudden cardiac death: effect of endurance exercise training

Sudden cardiac death resulting from ventricular tachyarrhythmias remains the leading cause of death in industrially developed countries, accounting for between 300,000 and 500,000 deaths each year in the United States. Yet, despite the enormity of this problem, both the identification of factors contributing to ventricular fibrillation as well as the development of safe and effective antiarrhythmic agents remain elusive. Subnormal cardiac parasympathetic regulation coupled with an elevated cardiac sympathetic activation may allow for the formation of malignant ventricular arrhythmias. In particular, myocardial infarction can reduce cardiac parasympathetic regulation and alter beta-adrenoceptor subtype expression enhancing beta(2)-adrenoceptor sensitivity that can lead to intracellular calcium dysregulation and arrhythmias. As such, myocardial infarction can induce a remodeling of cardiac autonomic regulation that may be required to maintain cardiac pump function. If alterations in cardiac autonomic regulation play an important role in the genesis of life-threatening arrhythmias, then one would predict that interventions designed to either augment parasympathetic activity and/or reduce cardiac adrenergic activity would also protect against ventricular fibrillation. Recently, studies using a canine model of sudden death demonstrate that endurance exercise training (treadmill running) enhanced cardiac parasympathetic regulation (increased heart rate variability), restored a more normal beta-adrenoceptor balance (i.e., reduced beta(2)-adrenoceptor sensitivity and expression), and protected against ventricular fibrillation induced by acute myocardial ischemia. Thus exercise training may reverse the autonomic neural remodeling induced by myocardial infarction and thereby enhance the electrical stability of the heart in individuals shown to be at an increased risk for sudden cardiac death.

Functional expression of the Na+/Ca2+ exchanger in the embryonic mouse heart

The Na(+)/Ca(2+) exchanger (NCX) is one of the earliest functional genes and is currently assumed to compensate at least in part for the rudimentary sarcoplasmic reticulum in the developing mouse heart. However, to date little is known about the functional expression of NCX during development. This prompted us to investigate the NCX current (I(NCX)) in very early (embryonic day E8.5-E9.5 post coitum), early (E10.5-E11.5), middle (E13.5) and late (E16.5) stage mouse embryonic cardiomyocytes. For standard I(NCX) measurements, [Ca(2+)](i) was buffered to 150 nmol/l and voltage ramps were applied from +60 mV to -120 mV. At very early stages of development, we observed a prominent role of the I(NCX) Ca(2+) inward mode in elevating the cytosolic Ca(2+) concentration ([Ca(2+)](i)). Accordingly, a high I(NCX) density was observed (+60 mV: 4.6+/-0.7 pA/pF, n=14). Likewise, we found a strong Ca(2+) outward mode of I(NCX) (-120 mV: -3.9+/-0.7 pA/pF, n=14). At later stages, however, I(NCX) Ca(2+) inward mode was reduced by 54+/-6% (n=15, p<0.0001) in ventricular and 68+/-10% (n=9, p<0.0006) in atrial cells. For the outward mode, a reduction by 43+/-10% (n=15, p<0.01) in ventricular and 62+/-11% (n=9, p<0.004) in atrial cardiomyocytes was observed. By contrast, NCX isoform expression and the reversal potential did not significantly change during development. Thus, NCX displays a prominent Ca(2+) inward and outward mode during early embryonic heart development pointing to its important contribution to maintain [Ca(2+)](i) homeostasis. The functional and protein expression of NCX declines during further development.

Exercise Training Improves Cardiac Function Postinfarction: Special Emphasis on Recent Controversies on Na+/Ca2+ Exchanger

Exercise training instituted after myocardial infarction improves many steps involved in cardiac excitation-contraction coupling. Focusing on Na/Caexchange, current controversies regarding whether it mediates Cainflux during an action potential, whether it is increased or decreased in disease models, whether protein kinase A alters its activity, and whether exercise training affects its function are reviewed. Finally, a novel target for exercise training in the heart is suggested.

Influence of age and run training on cardiac Na+/Ca2+ exchange

Effects of age and training on myocardial Na+/Ca2+ exchange were examined in young sedentary (YS; 14-15 mo), aged sedentary (AS; 27-31 mo), and aged trained (AT; 8- to 11-wk treadmill run training) male Fischer Brown Norway rats. Whole heart performance and isolated cardiocyte Na+/Ca2+ exchange characteristics were measured. At the whole heart level, a small but significant slowing of late isovolumic left ventricular (LV) relaxation, which may be indicative of altered Na+/Ca2+ exchange activity, was seen in hearts from AS rats. This subtle impairment in relaxation was not observed in hearts from AT rats. At the single-cardiocyte level, late action potential duration was prolonged, resting membrane potential was more positive, and overshoot potential was greater in cardiocytes from AS rats than from YS rats (P < 0.05). Training did not influence any of these age-related action potential characteristics. In electrically paced cardiocytes, neither shortening nor intracellular Ca2+ concentration ([Ca2+]i) dynamics was influenced by age or training. Similarly, neither age nor training influenced the rate of [Ca2+]i clearance via forward (Nain+ /Caout2+) Na+/Ca2+ exchange after caffeine-induced Ca2+ release from the sarcoplasmic reticulum or cardiac Na+/Ca2+ exchanger protein (NCX1) expression. However, when whole cell patch-clamp techniques combined with fluorescence microscopy were used to evaluate the ability of Na+/Ca2+ exchange to alter cytosolic [Ca2+] ([Ca2+]c) under conditions where membrane potential (Vm) and internal and external [Na+] and [Ca2+] could be controlled, we observed age-associated increases in forward Na+/Ca2+ exchange-mediated [Ca2+]c clearance (P < 0.05) that were not influenced by training. The age-related increase in forward Na+/Ca2+ exchange activity provides a hypothetical explanation for the late action potential prolongation observed in this study.

Effects of sprint training on contractility and [Ca(2+)](i) transients in adult rat myocytes

The effects of 6-8 wk of high-intensity sprint training (HIST) on rat myocyte contractility and intracellular Ca(2+) concentration ([Ca(2+)](i)) transients were investigated. Compared with sedentary (Sed) myocytes, HIST induced a modest (5%) but significant (P < 0.0005) increase in cell length with no changes in cell width. In addition, the percentage of myosin heavy chain alpha-isoenzyme increased significantly (P < 0.02) from 0.566 +/- 0.077% in Sed rats to 0.871 +/- 0.006% in HIST rats. At all three (0.6, 1.8, and 5 mM) extracellular Ca(2+) concentrations ([Ca(2+)](o)) examined, maximal shortening amplitudes and maximal shortening velocities were significantly (P < 0.0001) lower and half-times of relaxation were significantly (P < 0.005) longer in HIST myocytes. HIST myocytes had significantly (P < 0.0001) higher diastolic [Ca(2+)](i) levels. Compared with Sed myocytes, systolic [Ca(2+)](i) levels in HIST myocytes were higher at 0.6 mM [Ca(2+)](o), similar at 1.8 mM [Ca(2+)](o), and lower at 5 mM [Ca(2+)](o). The amplitudes of [Ca(2+)](i) transients were significantly (P < 0.0001) lower in HIST myocytes. Half-times of [Ca(2+)](i) transient decline, an estimate of sarcoplasmic reticulum (SR) Ca(2+) uptake activity, were not different between Sed and HIST myocytes. Compared with Sed hearts, Western blots demonstrated a significant (P < 0.03) threefold decrease in Na(+)/Ca(2+) exchanger, but SR Ca(2+)-ATPase and calsequestrin protein levels were unchanged in HIST hearts. We conclude that HIST effected diminished myocyte contractile function and [Ca(2+)](i) transient amplitudes under the conditions studied. We speculate that downregulation of Na(+)/Ca(2+) exchanger may partly account for the decreased contractility in HIST myocytes.

Exercise training alters an anoxia-induced, glibenclamide-sensitive current in rat ventricular cardiocytes

The effect of training on properties of a sarcolemmal ATP-sensitive K+ current (I(K(ATP))) was examined in left ventricular cardiocytes isolated from sedentary (Sed) and trained (Tr) female Sprague-Dawley rats. Whole cell patch-clamp techniques were used to characterize I(K(ATP)), an anoxia-inducible, glibencamide-sensitive current. An anoxic condition was induced by superfusing cells with a buffer that was equilibrated with 100% N(2), maintained under a layer of argon, and that contained 2-deoxy-D-glucose. Over a 1-h period of anoxia, 59% of Tr cells and 85% Sed cells expressed I(K(ATP)). In those cells that did express I(K(ATP)), the time to expression of the current during the anoxic period occurred significantly later in cells from the Tr group compared with the Sed. Peak I(K(ATP)) density was significantly lower in the Tr cells compared with the Sed cells. These results indicate that the onset and magnitude of I(K(ATP)) were altered by training. These alterations in I(K(ATP)) may be reflective of processes that contribute to training-induced cardioprotection against ischemia-reperfusion damage.

Glutathione supplementation and training increases myocardial resistance to ischemia-reperfusion in vivo

The present study examined the effects of oral reduced glutathione (GSH) supplementation in conjunction with endurance training on contractile function, antioxidant defense, and oxidative damage in response to ischemia-reperfusion (I/R) in rat hearts. Female Sprague-Dawley rats (age 4 mo, n = 72) were randomly assigned to a treadmill-trained (T; 25 m/min, 15% grade, for 75 min/day, 5 days/wk, for 10 wk) or untrained (U) group. Each group was further divided into rats receiving 5 g GSH/kg diet during the final 17 days of training (GSH-S) and control (C) groups. One-half of each group of rats was subjected to I/R by surgical occlusion of the main coronary artery for 45 min, followed by 30-min reperfusion or sham operation. Left ventriclar (LV) peak systolic pressure (LVSP) and contractility (+dP/dt), measured with a catheter inserted into the LV via the carotid artery, decreased with I/R in all groups (P < 0.05). However, LVSP with I/R in the T/GSH-S group was 9.5%, 17%, and 18% higher (P < 0.05) than that in the U/GSH-S, T/C, and U/C groups, respectively. +dP/dt with I/R was 19%, 27%, and 29% (P < 0.05) greater in the T/GSH-S group versus the T/C, U/GSH-S, and U/C groups, respectively. I/R decreased heart GSH content by 12-17% (P < 0.05) and increased oxidized glutathione (GSSG) by 20-27% (P < 0.05). T/GSH-S hearts showed 15% higher GSH (P < 0.05) and a 32% higher GSH-to-GSSG ratio (P < 0.05) than the U/C group at the end of I/R. Myocardial superoxide dismutase, GSH peroxidase, glutathione reductase, and gamma-glutamyl transpeptidase activities were increased with treadmill training in both GSH-S and C rats. I/R induced myocardial lipid peroxidation and lactate dehydrogenase release were attenuated with T/GSH-S treatment. The present data indicate that training in conjunction with dietary GSH supplementation can increase myocardial GSH content and antioxidant defense capacity, thereby protecting the intact heart against oxidative damage and functional retardation caused by I/R.

Renal hypertension prevents run training modification of cardiomyocyte diastolic Ca2+ regulation in male rats

The combined effects of endurance run training and renal hypertension on cytosolic Ca2+ concentration ([Ca2+]c) dynamics and Na+-dependent Ca2+ regulation in rat left ventricular cardiomyocytes were examined. Male Fischer 344 rats underwent stenosis of the left renal artery [hypertensive (Ht), n = 18] or a sham operation [normotensive (Nt), n = 20]. One-half of the rats from each group were treadmill trained for >16 wk. Cardiomyocyte fura 2 fluorescence ratio transients were recorded for 7 min during electrical pacing at 0.5 Hz, 2 mM extracellular Ca2+ concentration, and 29°C. The rate of [Ca2+]c decline was not changed by run training in the Nt group but was reduced in the Ht group. At 7 min, cardiomyocytes were exposed to 10 mM caffeine in the absence of Na+ and Ca2+, which triggered sarcoplasmic reticular Ca2+ release and suppressed Ca2+efflux via Na+/Ca2+ exchanger. External Na+ was then added, and Na+-dependent Ca2+ efflux rate was recorded. Treadmill training significantly enhanced Na+-dependent Ca2+efflux rate under these conditions in the Nt group but not in the Ht group. These data provide evidence that renal hypertension prevents the normal run training-induced modifications in diastolic [Ca2+]c regulation mechanisms, including Na+/Ca2+ exchanger.

Endurance training alters outward K+ current characteristics in rat cardiocytes

The effect of endurance run training on outward K+ currents with rapidly inactivating (I(to)) and sustained or slowly inactivating (I(sus)) characteristics was examined in left ventricular (LV) cardiocytes isolated from sedentary (Sed) and treadmill-trained (Tr) female Sprague-Dawley rats. Isolated LV cardiocytes were used in whole cell patch-clamp studies to characterize whole cell I(to) and I(sus). Peak I(to) was greatest in cells isolated from the Tr group. When I(to) was corrected for cell capacitance to yield a current density, most, but not all, of the Sed vs. Tr differences in I(to) magnitude were eliminated. Regardless of how I(to) was expressed (e.g., I(to) or I(to) density), the time required to achieve a peak value was markedly shortened in the cardiocytes isolated from the Tr group. Training elicited a reduction in I(sus) density. Action potential characteristics were determined in Sed and Tr cardiocytes in primary culture. Training did not affect resting membrane potential, whereas peak membrane potential was reduced and time to peak membrane potential was prolonged in the Tr group. In addition, time to 50% repolarization was significantly increased in cells from the Tr group. Collectively, these data indicate that I(to) and I(sus) characteristics are altered by training in isolated LV cardiocytes. These alterations in I(to) and I(sus) may be responsible, at least in part, for the training-induced alterations in action potential configuration in cardiocytes in primary culture.

Regional effects of voluntary exercise on cell size and contraction-frequency responses in rat cardiac myocytes

A model of voluntary exercise, in which rats are given free access to a running wheel over a 14-week period, led to left ventricular hypertrophy. To test whether the hypertrophic response to exercise was uniformly distributed across the ventricular wall, single ventricular myocytes were isolated from the sub-epicardium (EPI) and sub-endocardium (ENDO) of exercised rats and from sedentary rats for comparison. Cellular hypertrophy (approximately 20 % greater cell volume) was seen in ENDO cells from exercised animals, but no significant changes were observed in EPI cells when compared with sedentary controls. This regional effect of exercise may be a response to transmural changes in ventricular wall stress and/or strain. Cell contraction was measured as cell shortening in ENDO and EPI cells at stimulation frequencies between 1 and 9 Hz at 37 degrees C. Exercise training had no effect on cell shortening. Positive and negative contraction-frequency relationships (CFRs) were found in both EPI and ENDO cells between 1 and 5 Hz; at higher frequencies (5–9 Hz), all myocytes displayed a negative CFR. The CFR of a myocyte was, therefore, independent of regional origin and unaffected by exercise. These results suggest that, in vivo, the rat heart displays a negative CFR. We conclude that increased cell size may be a more important adaptive response to exercise than a modification of excitation-contraction coupling.

Treadmill running produces both positive and negative physiological adaptations in Sprague-Dawley rats

Exercise training produces a vast array of physiological adaptations, ranging from changes in metabolism to muscle mitochondrial biogenesis. Researchers studying the physiological effects of exercise often use animal models that employ forced exercise regimens that include aversive motivation, which could activate the stress response. This study examined the effect of forced treadmill running (8 wk) on several physiological systems that are sensitive to training and stress. Forced treadmill running produced both positive and negative physiological adaptations. Indicative of positive training adaptations, exercised male Sprague-Dawley rats had a decrease in body weight gain and an increase in muscle citrate synthase activity compared with sedentary controls. In contrast, treadmill running also resulted in the potentially negative adaptations of adrenal hypertrophy, thymic involution, decreased serum corticosteroid binding globulin, elevated lymphocyte nitrite concentrations, suppressed lymphocyte proliferation, and suppressed antigen-specific IgM. Such alterations in neuroendocrine tissues and immune responses are commonly associated with chronic stress. Thus treadmill running produces both positive training adaptations and potentially negative adaptations that are indicative of chronic stress. Researchers employing forced activity need to be aware that this type of exercise procedure also produces physiological adaptations indicative of chronic stress and that these changes could potentially impact other measures of interest.

A mathematical model of cardiocyte Ca(2+) dynamics with a novel representation of sarcoplasmic reticular Ca(2+) control

Cardiac contraction and relaxation dynamics result from a set of simultaneously interacting Ca(2+) regulatory mechanisms. In this study, cardiocyte Ca(2+) dynamics were modeled using a set of six differential equations that were based on theories, equations, and parameters described in previous studies. Among the unique features of the model was the inclusion of bidirectional modulatory interplay between the sarcoplasmic reticular Ca(2+) release channel (SRRC) and calsequestrin (CSQ) in the SR lumen, where CSQ acted as a dynamic rather than simple Ca(2+) buffer, and acted as a Ca(2+) sensor in the SR lumen as well. The inclusion of this control mechanism was central in overcoming a number of assumptions that would otherwise have to be made about SRRC kinetics, SR Ca(2+) release rates, and SR Ca(2+) release termination when the SR lumen is assumed to act as a simple, buffered Ca(2+) sink. The model was sufficient to reproduce a graded Ca(2+)-induced Ca(2+) release (CICR) response, CICR with high gain, and a system with reasonable stability. As constructed, the model successfully replicated the results of several previously published experiments that dealt with the Ca(2+) dependence of the SRRC (, J. Gen. Physiol. 85:247-289), the refractoriness of the SRRC (, Am. J. Physiol. 270:C148-C159), the SR Ca(2+) load dependence of SR Ca(2+) release (, Am. J. Physiol. 268:C1313-C1329;, J. Biol. Chem. 267:20850-20856), SR Ca(2+) leak (, J. Physiol. (Lond.). 474:463-471;, Biophys. J. 68:2015-2022), SR Ca(2+) load regulation by leak and uptake (, J. Gen. Physiol. 111:491-504), the effect of Ca(2+) trigger duration on SR Ca(2+) release (, Am. J. Physiol. 258:C944-C954), the apparent relationship that exists between sarcoplasmic and sarcoplasmic reticular calcium concentrations (, Biophys. J. 73:1524-1531), and a variety of contraction frequency-dependent alterations in sarcoplasmic [Ca(2+)] dynamics that are normally observed in the laboratory, including rest potentiation, a negative frequency-[Ca(2+)] relationship, and extrasystolic potentiation. Furthermore, under the condition of a simulated Ca(2+) overload, an alternans-like state was produced. In summary, the current model of cardiocyte Ca(2+) dynamics provides an integrated theoretical framework of fundamental cellular Ca(2+) regulatory processes that is sufficient to predict a broad array of observable experimental outcomes.

Chronic run training suppresses alpha-adrenergic response of rat cardiomyocytes and isovolumic left ventricle

The effects of endurance run training on alpha-adrenergic responsiveness of rat left ventricle (LV) were examined in cardiomyocytes and isovolumic LV. Female Sprague-Dawley rats were sedentary (Sed) or trained (Tr) for >20 wk by treadmill running. Cardiomyocyte shortening and fura 2 fluorescence ratio were recorded before and during 5-min exposure to 5 microM phenylephrine (PE) while paced at 0.5 Hz in 2 mM extracellular Ca2+ concentration at 29 degrees C. Cardiomyocyte shortening and shortening velocity increased with PE, and these effects were more pronounced in the Sed group. The rate of cytosolic Ca2+ concentration removal was reduced by PE in the Sed cardiomyocytes, but was unaffected in the Tr. Isovolumic LV pressure was recorded immediately before and during 5-min perfusion with 5 microM PE during pacing at 280 beats/min and 37 degrees C, and positive inotropy due to PE was more pronounced in the Sed than in the Tr. These data demonstrated that the effects of alpha-adrenergic stimulation on myocardial positive inotropy and calcium regulation were reduced in this rat model of run training at both the cellular and whole organ levels.