[Ca2+]i transients in hypertensive and postinfarction myocytes

Enhanced NCLX-dependent mitochondrial Ca2+ efflux attenuates pathological remodeling in heart failure

Mitochondrial calcium (mCa2+) uptake couples changes in cardiomyocyte energetic demand to mitochondrial ATP production. However, excessive mCa2+ uptake triggers permeability transition and necrosis. Despite these established roles during acute stress, the involvement of mCa2+ signaling in cardiac adaptations to chronic stress remains poorly defined. Changes in NCLX expression are reported in heart failure (HF) patients and models of cardiac hypertrophy. Therefore, we hypothesized that altered mCa2+ homeostasis contributes to the hypertrophic remodeling of the myocardium that occurs upon a sustained increase in cardiac workload. The impact of mCa2+ flux on cardiac function and remodeling was examined by subjecting mice with cardiomyocyte-specific overexpression (OE) of the mitochondrial Na+/Ca2+ exchanger (NCLX), the primary mediator of mCa2+ efflux, to several well-established models of hypertrophic and non-ischemic HF. Cardiomyocyte NCLX-OE preserved contractile function, prevented hypertrophy and fibrosis, and attenuated maladaptive gene programs in mice subjected to chronic pressure overload. Hypertrophy was attenuated in NCLX-OE mice, prior to any decline in cardiac contractility. NCLX-OE similarly attenuated deleterious cardiac remodeling in mice subjected to chronic neurohormonal stimulation. However, cardiomyocyte NCLX-OE unexpectedly reduced overall survival in mice subjected to severe neurohormonal stress with angiotensin II + phenylephrine. Adenoviral NCLX expression limited mCa2+ accumulation, oxidative metabolism, and de novo protein synthesis during hypertrophic stimulation of cardiomyocytes in vitro. Our findings provide genetic evidence for the contribution of mCa2+ to early pathological remodeling in non-ischemic heart disease, but also highlight a deleterious consequence of increasing mCa2+ efflux when the heart is subjected to extreme, sustained neurohormonal stress.

Hierarchical statistical techniques are necessary to draw reliable conclusions from analysis of isolated cardiomyocyte studies

AIMS

It is generally accepted that post-MI heart failure (HF) changes a variety of aspects of sarcoplasmic reticular Ca2+ fluxes but for some aspects there is disagreement over whether there is an increase or decrease. The commonest statistical approach is to treat data collected from each cell as independent, even though they are really clustered with multiple likely similar cells from each heart. In this study, we test whether this statistical assumption of independence can lead the investigator to draw conclusions that would be considered erroneous if the analysis handled clustering with specific statistical techniques (hierarchical tests).

METHODS AND RESULTS

Ca2+ transients were recorded in cells loaded with Fura-2AM and sparks were recorded in cells loaded with Fluo-4AM. Data were analysed twice, once with the common statistical approach (assumption of independence) and once with hierarchical statistical methodologies designed to allow for any clustering. The statistical tests found that there was significant hierarchical clustering. This caused the common statistical approach to underestimate the standard error and report artificially small P values. For example, this would have led to the erroneous conclusion that time to 50% peak transient amplitude was significantly prolonged in HF. Spark analysis showed clustering, both within each cell and also within each rat, for morphological variables. This means that a three-level hierarchical model is sometimes required for such measures. Standard statistical methodologies, if used instead, erroneously suggest that spark amplitude is significantly greater in HF and spark duration is reduced in HF.

CONCLUSION

Ca2+ fluxes in isolated cardiomyocytes show so much clustering that the common statistical approach that assumes independence of each data point will frequently give the false appearance of statistically significant changes. Hierarchical statistical methodologies need a little more effort, but are necessary for reliable conclusions. We present cost-free simple tools for performing these analyses.

Altered Na/Ca exchange distribution in ventricular myocytes from failing hearts

In mammalian cardiac ventricular myocytes, Ca efflux via Na/Ca exchange (NCX) occurs predominantly at T tubules. Heart failure is associated with disrupted t-tubular structure, but its effect on t-tubular function is less clear. We therefore investigated t-tubular NCX activity in ventricular myocytes isolated from rat hearts ∼18 wk after coronary artery ligation (CAL) or corresponding sham operation (Sham). NCX current (INCX) and l-type Ca current (ICa) were recorded using the whole cell, voltage-clamp technique in intact and detubulated (DT) myocytes; intracellular free Ca concentration ([Ca]i) was monitored simultaneously using fluo-4. INCX was activated and measured during application of caffeine to release Ca from sarcoplasmic reticulum (SR). Whole cell INCX was not significantly different in Sham and CAL myocytes and occurred predominantly in the T tubules in Sham myocytes. CAL was associated with redistribution of INCX and ICa away from the T tubules to the cell surface and an increase in t-tubular INCX/ICa density from 0.12 in Sham to 0.30 in CAL myocytes. The decrease in t-tubular INCX in CAL myocytes was accompanied by an increase in the fraction of Ca sequestered by SR. However, SR Ca content was not significantly different in Sham, Sham DT, and CAL myocytes but was significantly increased by DT of CAL myocytes. In Sham myocytes, there was hysteresis between INCX and [Ca]i, which was absent in DT Sham but present in CAL and DT CAL myocytes. These data suggest altered distribution of NCX in CAL myocytes.

Effects of sarcoplasmic reticulum Ca2+-ATPase overexpression in postinfarction rat myocytes

Previous studies in adult myocytes isolated from rat hearts 3 wk after myocardial infarction (MI) demonstrated abnormal contractility and intracellular Ca(2+) concentration ([Ca(2+)](i)) homeostasis and decreased sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2) expression and activity, but sarcoplasmic reticulum Ca(2+) leak was unchanged. In the present study, we investigated whether SERCA2 overexpression in MI myocytes would restore contraction and [Ca(2+)](i) transients to normal. Compared with sham-operated hearts, 3-wk MI hearts exhibited significantly higher left ventricular end-diastolic and end-systolic volumes but lower fractional shortening and ejection fraction, as measured by M-mode echocardiography. Seventy-two hours after adenovirus-mediated gene transfer, SERCA2 overexpression in 3-wk MI myocytes did not affect Na(+)-Ca(2+) exchanger expression but restored the depressed SERCA2 levels toward those measured in sham myocytes. In addition, the reduced sarcoplasmic reticulum Ca(2+) uptake in MI myocytes was improved to normal levels by SERCA2 overexpression. At extracellular Ca(2+) concentration of 5 mM, the subnormal contraction and [Ca(2+)](i) transient amplitudes in MI myocytes (compared with sham myocytes) were restored to normal by SERCA2 overexpression. However, at 0.6 mM extracellular Ca(2+) concentration, the supernormal contraction and [Ca(2+)](i) transient amplitudes in MI myocytes (compared with sham myocytes) were exacerbated by SERCA2 overexpression. We conclude that SERCA2 overexpression was only partially effective in ameliorating contraction and [Ca(2+)](i) transient abnormalities in our rat model of ischemic cardiomyopathy. We suggest that other Ca(2+) transport pathways, e.g., Na(+)-Ca(2+) exchanger, may also play an important role in contractile and [Ca(2+)](i) homeostatic abnormalities in MI myocytes.

Impaired beta-adrenergic response and decreased L-type calcium current of hypertrophied left ventricular myocytes in postinfarction heart failure

Infarct-induced heart failure is usually associated with cardiac hypertrophy and decreased -adrenergic responsiveness. However, conflicting results have been reported concerning the density of L-type calcium current (I Ca(L)), and the mechanisms underlying the decreased -adrenergic inotropic response. We determined I Ca(L) density, cytoplasmic calcium ([Ca2+]i) transients, and the effects of -adrenergic stimulation (isoproterenol) in a model of postinfarction heart failure in rats. Left ventricular myocytes were obtained by enzymatic digestion 8-10 weeks after infarction. Electrophysiological recordings were obtained using the patch-clamp technique. [Ca2+]i transients were investigated via fura-2 fluorescence. -Adrenergic receptor density was determined by [ H]-dihydroalprenolol binding to left ventricle homogenates. Postinfarction myocytes showed a significant 25% reduction in mean I Ca(L) density (5.7 0.28 vs 7.6 0.32 pA/pF) and a 19% reduction in mean peak [Ca2+]i transients (0.13 0.007 vs 0.16 0.009) compared to sham myocytes. The isoproterenol-stimulated increase in I Ca(L) was significantly smaller in postinfarction myocytes (Emax: 63.6 4.3 vs 123.3 0.9% in sham myocytes), but EC50 was not altered. The isoproterenol-stimulated peak amplitude of [Ca2+]i transients was also blunted in postinfarction myocytes. Adenylate cyclase activation through forskolin produced similar I Ca(L) increases in both groups. -Adrenergic receptor density was significantly reduced in homogenates from infarcted hearts (Bmax: 93.89 20.22 vs 271.5 31.43 fmol/mg protein in sham myocytes), while Kd values were similar. We conclude that postinfarction myocytes from large infarcts display reduced I Ca(L) density and peak [Ca2+]i transients. The response to -adrenergic stimulation was also reduced and was probably related to -adrenergic receptor down-regulation and not to changes in adenylate cyclase activity.

Impaired Ca2+ Handling in Perfused Hypertrophic Hearts from Dahl Salt-Sensitive Rats

To clarify the correlation between intracellular Ca2+ dynamics and level of Ca2+-regulatory proteins, changes in Ca2+ handling and these proteins were investigated in a whole-heart experimental model of pressure-overload hypertrophy. We used 17-18-week-old male Dahl salt-sensitive rats (DS) and Dahl salt-resistant rats (DR) fed a high-salt diet. We monitored the fura-2 fluorescence ratio, an index of cytoplasmic Ca2+ concentration ([Ca2+]i), using a Ca2+ analyzer in a retrograde perfused heart. Left ventricular pressure (LVP) and an electrocardiogram were simultaneously recorded. Ca2+ handling was assessed by exposing the hearts to 2 min of low-Na+ (70 mmol/l) perfusion to produce an increase in [Ca2+]i (n = 6), which was sensitive to Ni2+, a blocker of the Na+/Ca2+ exchanger (NCX). In another series, the hearts were stimulated at 2.5 to 5 Hz to determine the Ca2+-force-frequency relationship (n = 6). DS rats showed marked cardiac hypertrophy without any signs of failure. The time-to-peak Ca2+ transient was prolonged in DS compared with that in DR during normal beating. During low-Na+ exposure, the time-to-peak diastolic [Ca2+]i (TTP) and the decay-time from peak [Ca2+]i (DT) were prolonged in DS compared with DR (TTP, 43.3 +/- 4.0 vs. 32.5 +/- 2.5 s, p < 0.05; DT, 70.0 +/- 8.8 vs. 29.2 +/- 2.7 s, p < 0.005). Following pretreatment with 10 mmol/l caffeine to inhibit sarcoplasmic reticulum (SR) function, TTP and DT were still prolonged in DS compared with DR (TTP, 64.2 +/- 9.7 vs. 37.0 +/- 5.8 s, p < 0.05; DT, 55.8 +/- 12.6 vs. 26.0 +/- 5.7 s, p < 0.05). The force (LVP)-frequency relationship was initially positive in DR but was negative at all times in DS (%LVP/2.5 Hz: DS, 90.3 +/- 2.0%; DR, 112.2 +/- 4.5%; p < 0.05). Elevation of diastolic [Ca2+]i (percent increase of baseline) was greater in DS than in DR with increased stimulation (5 Hz: DS, 80.7 +/- 6.7%; DR, 52.1 +/- 5.9%; p < 0.05). In Western blot analysis, the protein level of NCX was equivalent, whereas that of SR Ca2+ ATPase (SERCA2) was significantly decreased in DS compared with DR. These results suggest that slowing of cellular Ca2+ mobilization and removal is related to impaired Ca2+ handling in late-phase cardiac hypertrophy. Both the activity of the NCX and that of the SR may be affected. The SR dysfunction may be associated with change in protein level of SERCA2.

Rescue of contractile abnormalities by Na+/Ca2+ exchanger overexpression in postinfarction rat myocytes

Previous studies on myocytes isolated from rat hearts 3 wk after myocardial infarction (MI) demonstrated increased cell length, reduced Na(+)/Ca(2+) exchange (NCX1) activity, altered contractility, and intracellular Ca(2+) concentration ([Ca(2+)](i)) transients. In the present study, we investigated whether NCX1 overexpression in MI myocytes would restore contraction and [Ca(2+)](i) transients to normal. When myocytes were placed in culture under continued electrical-field stimulation conditions, differences in contraction amplitudes and cell lengths between sham and MI myocytes were preserved for at least 48 h. Infection of both sham and MI myocytes by adenovirus expressing green fluorescent protein resulted in >95% infection, as evidenced by green fluorescent protein fluorescence, but contraction amplitudes at 6-, 24-, and 48-h postinfection were not affected. NCX1 overexpression in MI myocytes resulted in lower diastolic [Ca(2+)](i) levels at all extracellular Ca(2+) concentrations ([Ca(2+)](o)) examined, suggesting enhanced forward NCX1 activity. At 5 mM [Ca(2+)](o), subnormal contraction and [Ca(2+)](i) transient amplitudes in MI myocytes (compared with sham myocytes) were restored toward normal levels by overexpressing NCX1. At 0.6 mM [Ca(2+)](o), supranormal contraction and [Ca(2+)](i) transient amplitudes in MI myocytes (compared with sham myocytes) were lowered by NCX1 overexpression. We conclude that overexpression of NCX1 in MI myocytes was effective in improving contractile dysfunction, most likely because of enhancement of both Ca(2+) efflux and influx during a cardiac cycle. We suggest that decreased NCX1 activity may play an important role in contractile abnormalities in postinfarction myocytes.

Effects of cariporide and losartan on hypertrophy, calcium transients, contractility, and gene expression in congestive heart failure

BACKGROUND

The purpose of this study was to compare long-term effects of cariporide with those of losartan in postinfarction heart failure.

METHODS AND RESULTS

Female Sprague-Dawley rats with large myocardial infarctions and sham controls were randomized to losartan, cariporide, or placebo after 7 days and treated for 49 days. Cardiac function was assessed by echocardiography and measurement of left ventricular pressures, and gene expression was assessed by competitive reverse transcription-polymerase chain reaction. Cell dimensions, shortening, and relaxation were determined by videomicroscopy and calcium transients by fura 2. Losartan reduced postinfarction systolic and diastolic left ventricular dilation (by 24% and 31%, respectively), left and right ventricular weight (by 22% and 26%, respectively), and cardiomyocyte hypertrophy length and width (by 62% and 54%, respectively). Induction of myocardial atrial natriuretic peptide decreased 66%. Cariporide did not affect postinfarction hypertrophy or atrial natriuretic peptide. Losartan and cariporide respectively improved reduced cellular contractility (55% and 30%) and reduced elevated systolic (86% and 27%) and diastolic (49% and 43%) calcium. Losartan and cariporide respectively reduced prolonged time to 50% relaxation (66% and 25%) and time to 50% calcium reduction (55% and 53%).

CONCLUSIONS

Losartan and cariporide improve cardiomyocyte contractility and calcium regulation in chronic heart failure. Losartan has salutary effects on postinfarction remodeling and gene expression, whereas cariporide is neutral.

Altered excitation-contraction coupling in myocytes from remodeled myocardium after chronic myocardial infarction

Following myocardial infarction (MI), the left ventricle undergoes progressive dilatation and eccentric hypertrophy, i.e., remodeling, which is greater in the adjacent than the remote region. The cellular mechanisms underlying these regional differences were studied. One (n=5) and 8 weeks (n=8) after anteroapical MI in sheep, cardiac myocytes were isolated from the adjacent and remote regions. At 8 weeks after MI, myocyte function in the remote region was not different from values either in sham controls (n=3) or animals 1 week after MI. At 8 weeks after MI, myocyte contractile function (% contraction) was decreased, P<0.01, in the adjacent region (6.4+/-0.4%), as compared with the remote region (8.8+/-0.5%) and was associated with decreased amplitude of Ca(2+)transients (adjacent, 0.69+/-0.09 v remote, 1.08+/-0.20, P<0.05) and L-type Ca(2+)current density (adjacent, 3.6+/-0.2 v remote, 4.8+/-0.2 pA/pF, P<0.05). Relaxation was also impaired significantly in myocytes from the adjacent region, associated with decreased protein levels of SERCA2a. The myocytes were hypertrophied more in the adjacent region than the remote region. Furthermore, focal areas of central myofibrillar lysis and increased glycogen deposition were observed in the adjacent region. These results indicate that impaired excitation-contraction coupling underlies dysfunction of myocytes from the adjacent non-infarcted myocardium after chronic MI, even in the absence of heart failure. Hypertrophy is implicated as the mechanism, since these changes were noted at 8 weeks, but not at 1 week after MI.

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.

Sprint training shortens prolonged action potential duration in postinfarction rat myocyte: mechanisms

Two electrophysiological manifestations of myocardial infarction (MI)-induced myocyte hypertrophy are prolongation of action potential duration (APD) and reduction of transient outward current (I(to)) density. Because high-intensity sprint training (HIST) ameliorated myocyte hypertrophy and improved myocyte Ca(2+) homeostasis and contractility after MI, the present study evaluated whether 6-8 wk of HIST would shorten the prolonged APD and improve the depressed I(to) in post-MI myocytes. There were no differences in resting membrane potential and action potential amplitude (APA) measured in myocytes isolated from sham-sedentary (Sed), MI-Sed, and MI-HIST groups. Times required for repolarization to 50 and 90% APA were significantly (P < 0.001) prolonged in MI-Sed myocytes. HIST reduced times required for repolarization to 50 and 90% APA to values observed in Sham-Sed myocytes. The fast and slow components of I(to) were significantly (P < 0.0001) reduced in MI-Sed myocytes. HIST significantly (P < 0.001) enhanced the fast and slow components of I(to) in MI myocytes, although not to levels observed in Sham-Sed myocytes. There were no significant differences in steady-state I(to) inactivation and activation parameters among Sham-Sed, MI-Sed, and MI-HIST myocytes. Likewise, recovery from time-dependent inactivation was also similar among the three groups. We suggest that normalization of APD after MI by HIST may be mediated by restoration of I(to) toward normal levels.

Apoptosis and the systolic dysfunction in congestive heart failure. Story of apoptosis interruptus and zombie myocytes

Although previously it was believed that apoptosis could not occur in the terminally differentiated tissue, such as adult heart muscle cells, recent studies in endomyocardial biopsies from patients with dilated cardiomyopathy and in explanted hearts from patients with end-stage heart failure undergoing cardiac transplantation have demonstrated histologic evidence of apoptosis. Whereas neurohormonal activation during heart failure leads to compensatory hemodynamic alterations, coupled with ventricular dilatation, it induces transcription factors and myocyte hypertrophy. Persistent growth stimulation in terminally differentiated cells may lead paradoxically to apoptotic cell death. The apoptosis in cardiomyopathic hearts is associated with cytochrome c release from mitochondria to cytoplasm and activation of proteolytic caspase-8 and -3. Although the caspases are duly processed, the fragmentation of the nuclear proteins (including DNA) is completed less frequently, and only a variable degree of fragmentation of cytoplasmic proteins (including contractile proteins) is observed. It is hypothesized that release of cytochrome c from mitochondria should interfere with energy production and lead to functional impairment and variable loss of contractile proteins in a living heart muscle cell should contribute to systolic dysfunction. Because a nuclear blueprint is retained, however, the dysfunctional cell may continue to exist and in favorable conditions, such as with LVAD support, the apoptotic process may subside. Potential feasibility of reversal of heart failure should renew efforts to develop more targeted pharmaceutical intervention within the apoptotic cascade and allow newer paradigm for the management of heart failure.

Effects of intracellular calcium elevation on action potential and L-type calcium current of normal and chronically infarcted rat ventricles

The present work investigated the effects of raising [Ca+2]i levels on action potential (AP) and L-type calcium current (I(Ca.L)) of normal and chronically infarcted rat ventricles. Experiments were performed by conventional electrophysiology and whole-cell patch-clamp techniques. In the former, APs were recorded in ventricular strips subjected to different pacing rates or elevation of [Ca+2]o levels. In the latter, I(Ca.L) was studied in isolated myocytes in the absence of an intracellular Ca+2 chelator. The acceleration of heart rate (6 to 240 beats/min) reduced AP duration measured at 20%, 50%, and 90% repolarization (APD20, APD50, and APD90) in the infarcted group, and increased APD20 and APD50 in the control group. Rising [Ca+]o (1.25 to 5.0 mmol/L) induced a decrease of APD20 and APD50 in both groups. Voltage clamp revealed a smaller I(Ca.L) density at approximately -17 mV in myocytes from infarcted ventricles (-1.86 +/- 0.37 vs -3.98 +/- 0.65 pA/pF, P < .05), and the appearance of a non-K+ outward current coupled to I(Ca.L). The results suggest the participation of a Ca+2-activated outward current in the repolarization of normal and infarcted rat ventricles.

Sprint training restores normal contractility in postinfarction rat myocytes

The significance of 6-8 wk of high-intensity sprint training (HIST) on contractile abnormalities of myocytes isolated from rat hearts with prior myocardial infarction (MI) was investigated. Compared with the sedentary (Sed) condition, HIST attenuated myocyte hypertrophy observed post-MI primarily by reducing cell lengths but not cell widths. At high extracellular Ca(2+) concentration (5 mM) and low pacing frequency (0.1 Hz), conditions that preferentially favored Ca(2+) influx over efflux, MI-Sed myocytes shortened less than Sham-Sed myocytes did. HIST significantly improved contraction amplitudes in MI myocytes. Under conditions that favored Ca(2+) efflux, i.e., low extracellular Ca(2+) concentration (0.6 mM) and high pacing frequency (2 Hz), MI-Sed myocytes contracted more than Sham-Sed myocytes. HIST did not appreciably affect contraction amplitudes of MI myocytes under these conditions. Compared with MI-Sed myocytes, HIST myocytes showed significant improvement in time required to reach one-half maximal contraction amplitude shortening, maximal myocyte shortening and relengthening velocities, and half time of relaxation. Our results indicate that HIST instituted shortly after MI improved cellular contraction in surviving myocytes. Because our previous studies demonstrated that, in post-MI myocytes, HIST improved intracellular Ca(2+) dynamics, enhanced sarcoplasmic reticulum Ca(2+) uptake and Ca(2+) content, and restored Na(+)/Ca(2+) exchange current toward normal, we hypothesized that improvement in MI myocyte contractile function by HIST was likely mediated by normalization of cellular Ca(2+) homeostatic mechanisms.

Sprint training normalizes Ca(2+) transients and SR function in postinfarction rat myocytes

Previous studies have shown that myocytes isolated from sedentary (Sed) rat hearts 3 wk after myocardial infarction (MI) undergo hypertrophy, exhibit altered intracellular Ca(2+) concentration ([Ca(2+)](i)) dynamics and abnormal contraction, and impaired sarcoplasmic reticulum (SR) function manifested as prolonged half-time of [Ca(2+)](i) decline. Because exercise training elicits positive adaptations in cardiac contractile function and myocardial Ca(2+) regulation, the present study examined whether 6-8 wk of high-intensity sprint training (HIST) would restore [Ca(2+)](i) dynamics and SR function in MI myocytes toward normal. In MI rats, HIST ameliorated myocyte hypertrophy as indicated by significant (P </= 0.05) decreases in whole cell capacitances [Sham-Sed 179 +/-12 (n = 20); MI-Sed 226 +/- 7 (n = 20); MI-HIST 183 +/- 11 pF (n = 19)]. HIST significantly (P < 0.0001) restored both systolic [Ca(2+)](i) [Sham-Sed 421 +/- 9 (n = 79); MI-Sed 350 +/- 6 (n = 70); MI-HIST 399 +/- 9 nM (n = 70)] and half-time of [Ca(2+)](i) decline (Sham-Sed 0. 197 +/- 0.005; MI-Sed 0.247 +/- 0.006; MI-HIST 0.195 +/- 0.006 s) toward normal. Compared with Sham-Sed myocytes, SR Ca(2+)-ATPase expression significantly (P < 0.001) decreased by 44% in MI-Sed myocytes. Surprisingly, expression of SR Ca(2+)-ATPase was further reduced in MI-HIST myocytes to 26% of that measured in Sham-Sed myocytes. There were no differences in calsequestrin expression among the three groups. Expression of phospholamban was not different between Sham-Sed and MI-Sed myocytes but was significantly (P < 0.01) reduced in MI-HIST myocytes by 25%. Our results indicate that HIST instituted shortly after MI improves [Ca(2+)](i) dynamics in surviving myocytes. Improvement in SR function by HIST is mediated not by increased SR Ca(2+)-ATPase expression, but by modulating phospholamban regulation of SR Ca(2+)-ATPase activity.

The thyroid hormone analog DITPA restores I(to) in rats after myocardial infarction

Previous studies have established that reductions in repolarizing currents occur in heart disease and can contribute to life-threatening arrhythmias in myocardium. In this study, we investigated whether the thyroid hormone analog 3, 5-diiodothyropropionic acid (DITPA) could restore repolarizing transient outward K(+) current (I(to)) density and gene expression in rat myocardium after myocardial infarction (MI). Our findings show that I(to) density was reduced after MI (14.0 +/- 1.0 vs. 10.2 +/- 0.9 pA/pF, sham vs. post-MI at +40 mV). mRNA levels of Kv4.2 and Kv4.3 genes were decreased but Kv1.4 mRNA levels were increased post-MI. Corresponding changes in Kv4.2 and Kv1.4 protein were also observed. Chronic treatment of post-MI rats with 10 mg/kg DITPA restored I(to) density (to 15.2 +/- 1.1 pA/pF at +40 mV) as well as Kv4.2 and Kv1.4 expression to levels observed in sham-operated controls. Other membrane currents (Na(+), L-type Ca(2+), sustained, and inward rectifier K(+) currents) were unaffected by DITPA treatment. Associated with the changes in I(to) expression, action potential durations (current-clamp recordings in isolated single right ventricular myocytes and monophasic action potential recordings from the right free wall in situ) were prolonged after MI and restored with DITPA treatment. Our results demonstrate that DITPA restores I(to) density in the setting of MI, which may be useful in preventing complications associated with I(to) downregulation.

In situ SR function in postinfarction myocytes

Previous studies have shown lower systolic intracellular Ca(2+) concentrations ([Ca(2+)](i)) and reduced sarcoplasmic reticulum (SR)-releasable Ca(2+) contents in myocytes isolated from rat hearts 3 wk after moderate myocardial infarction (MI). Ca(2+) entry via L-type Ca(2+) channels was normal, but that via reverse Na(+)/Ca(2+) exchange was depressed in 3-wk MI myocytes. To elucidate mechanisms of reduced SR Ca(2+) contents in MI myocytes, we measured SR Ca(2+) uptake and SR Ca(2+) leak in situ, i.e., in intact cardiac myocytes. For sham and MI myocytes, we first demonstrated that caffeine application to release SR Ca(2+) and inhibit SR Ca(2+) uptake resulted in a 10-fold prolongation of half-time (t(1/2)) of [Ca(2+)](i) transient decline compared with that measured during a normal twitch. These observations indicate that early decline of the [Ca(2+)](i) transient during a twitch in rat myocytes was primarily mediated by SR Ca(2+)-ATPase and that the t(1/2) of [Ca(2+)](i) decline is a measure of SR Ca(2+) uptake in situ. At 5.0 mM extracellular Ca(2+), systolic [Ca(2+)](i) was significantly (P </= 0.05) lower (337 +/- 11 and 416 +/- 18 nM in MI and sham, respectively) and t(1/2) of [Ca(2+)](i) decline was significantly longer (0.306 +/- 0.014 and 0.258 +/- 0.014 s in MI and sham, respectively) in MI myocytes. The 19% prolongation of t(1/2) of [Ca(2+) ](i) decline was associated with a 23% reduction in SR Ca(2+)-ATPase expression (detected by immunoblotting) in MI myocytes. SR Ca(2+) leak was measured by a novel electrophysiological technique that did not require assigning empirical constants for intracellular Ca(2+) buffering. SR Ca(2+) leak rate was not different between sham and MI myocytes: the time constants of SR Ca(2+) loss after thapsigargin were 290 and 268 s, respectively. We conclude that, independent of decreased SR filling by Ca(2+) influx, the lower SR Ca(2+) content in MI myocytes was due to reduced SR Ca(2+) uptake and SR Ca(2+)-ATPase expression, but not to enhanced SR Ca(2+) leak.

Diminished basal phosphorylation level of phospholamban in the postinfarction remodeled rat ventricle: role of beta-adrenergic pathway, G(i) protein, phosphodiesterase, and phosphatases

Three weeks after myocardial infarction (MI) in the rat, remodeled hypertrophy of noninfarcted myocardium is at its maximum and the heart is in a compensated stage with no evidence of heart failure. Our hemodynamic measurements at this stage showed a slight but insignificant decrease of +dP/dt but a significantly higher left ventricular end-diastolic pressure. To investigate the basis of the diastolic dysfunction, we explored possible defects in the beta-adrenergic receptor-G(s/i) protein-adenylyl cyclase-cAMP-protein kinase A-phosphatase pathway, as well as molecular or functional alterations of sarcoplasmic reticulum Ca(2+)-ATPase and phospholamban (PLB). We found no significant difference in both mRNA and protein levels of sarcoplasmic reticulum Ca(2+)-ATPase and PLB in post-MI left ventricle compared with control. However, the basal levels of both the protein kinase A-phosphorylated site (Ser16) of PLB (p16-PLB) and the calcium/calmodulin-dependent protein kinase-phosphorylated site (Thr17) of PLB (p17-PLB) were decreased by 76% and 51% in post-MI myocytes (P<0.05), respectively. No change was found in the beta-adrenoceptor density, G(salpha) protein level, or adenylyl cyclase activity. Inhibition of phosphodiesterase and G(i) protein by Ro-20-1724 and pertussis toxin, respectively, did not correct the decreased p16-PLB or p17-PLB levels. Stimulation of beta-adrenoceptor or adenylyl cyclase increased both p16-PLB and p17-PLB in post-MI myocytes to the same levels as in sham myocytes, suggesting that decreased p16-PLB and p17-PLB in post-MI myocytes is not due to a decrease in the generation of p16-PLB or p17-PLB. We found that type 1 phosphatase activity was increased by 32% (P<0.05) with no change in phosphatase 2A activity. Okadaic acid, a protein phosphatase inhibitor, significantly increased p16-PLB and p17-PLB levels in post-MI myocytes and partially corrected the prolonged relaxation of the [Ca(2+)](i) transient. In summary, prolonged relaxation of post-MI remodeled myocardium could be explained, in part, by altered basal levels of p16-PLB and p17-PLB caused by increased protein phosphatase 1 activity.

Effects of stimulation frequency on calcium transients in noninfarcted myocardium: modulation by chronic captopril treatment

BACKGROUND

Angiotensin-converting enzyme (ACE) inhibition produces beneficial effects in patients and experimental animals after myocardial infarction (MI). However, the mechanisms accounting for these effects are incompletely understood.

METHODS AND RESULTS

We recorded contractile force and intracellular calcium (Ca2+i) transients in papillary muscles from sham-operated rats (n = 8), untreated rats with heart failure after MI (MI; n = 7), and MI rats receiving captopril treatment for 5 weeks (n = 4). All studies were performed 6 weeks after MI or sham surgery. In muscles from sham-operated rats, increasing stimulation frequency from 0.33 to 3.0 Hz was associated with no change in the peak amplitude or the time to the peak of the Ca2+i transients. In contrast, in muscles from MI rats, stimulation at 3.0 Hz caused a marked increase in the amplitude of the Ca2+i transients (170% of baseline), prolongation of the time to the peak of the Ca2+i transient (54 +/- 2 to 84 +/- 8 * ms), and a prominent alternans pattern. Tissue hypoxia did not appear to be responsible for the abnormal response to rapid stimulation in the myocardium from infarcted hearts because bubbling the bath solution with 95% N2/5% CO2 resulted in no change in the amplitude of the Ca2+i transients in muscles from both groups. Muscles from captopril-treated MI rats responded like sham-operated controls, with no change in the amplitude or time course of the Ca2+i transients during rapid stimulation.

CONCLUSION

In myocardium isolated from rats with postinfarction heart failure, increasing stimulation frequency causes marked increases in peak Ca2+i , prolongation of the time course of the Ca2+i transient, and Ca2+i alternans. Despite the increased Ca2+i transients, contractility declined during rapid pacing. We hypothesize that these changes could be explained by a frequency-related decline in intracellular pH and/or a decrease in sarcolemmal Ca2+ extrusion. The frequency-dependent abnormalities of cellular Ca2+ regulation in the infarcted heart are prevented by long-term treatment with an ACE inhibitor.

Hypertension augments ethanol-induced depression of cell shortening and intracellular Ca(2+) transients in adult rat ventricular myocytes

Ethanol, a risk factor for myocardial dysfunction, depresses myocardial contraction. This study was to determine whether ethanol-induced myocardial depression is affected by hypertension. Mechanical properties of ventricular myocytes isolated from both normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats were evaluated using a video edge-detection system. Myocytes were electrically stimulated to contract at 0.5 Hz. Contractile properties analyzed include peak twitch amplitude (PTA), time-to-PTA (TPS), time-to-90% relengthening (TR(90)), and maximal velocities of shortening/relengthening (+/-dL/dt). Intracellular Ca(2+) transients were measured as fura-2 fluorescence intensity (DeltaFFI) changes. Acute ethanol exposure (80-640 mg/dl) caused a concentration-dependent inhibition of PTA and DeltaFFI in both WKY and SHR myocytes. The extent of maximal inhibition of PTA and FFI was significantly greater in SHRs (53.7 and 38.9%) compared to the WKY group (21.0 and 25.4%). Ethanol did not affect TPS but shortened TR(90) and slowed +/-dL/dt at high concentration ranges. Interestingly, the augmented ethanol-induced inhibition of cell shortening in hypertension was greatly attenuated by Ca(2+) channel opener BayK 8644 (1 microM). These results suggest that ethanol-induced myocardial depression may be augmented in hypertension, possibly due to mechanism(s) involving sarcolemmal Ca(2+) channels.

Relationship between K+ channel down-regulation and [Ca2+]i in rat ventricular myocytes following myocardial infarction

1. Cardiac hypertrophy and prolongation of the cardiac action potential are hallmark features of heart disease. We examined the molecular mechanisms and the functional consequences of this action potential prolongation on calcium handling in right ventricular myocytes obtained from rats 8 weeks following ligation of the left anterior descending coronary artery (post-myocardial infarction (MI) myocytes). 2. Compared with myocytes from sham-operated rats (sham myocytes), post-MI myocytes showed significant reductions in transient outward K+ current (Ito) density (sham 19.7 +/- 1.1 pA pF-1 versus post-MI 11.0 +/- 1.3 pA pF-1; means +/- s.e.m.), inward rectifier K+ current density (sham -13.7 +/- 0.6 pA pF-1 versus post-MI -10.3 +/- 0.9 pA pF-1) and resting membrane potential (sham -84.4 +/- 1.3 mV versus post-MI -74.1 +/- 2.6 mV). Depressed Ito amplitude correlated with significant reductions in Kv4.2 and Kv4.3 mRNA and Kv4.2 protein levels. Kv1.4 mRNA and protein levels were increased and coincided with the appearance of a slow component of recovery from inactivation for Ito. 3. In current-clamp recordings, post-MI myocytes showed a significant increase in [Ca2+]i transient amplitude compared with sham myocytes. Using voltage-clamp depolarizations, no intrinsic differences in Ca2+ handling by the sarcoplasmic reticulum or in L-type Ca2+ channel density (ICa,L) were detected between the groups. 4. Stimulation of post-MI myocytes with an action potential derived from a sham myocyte reduced the [Ca2+] transient amplitude to the sham level and vice versa. 5. The net Ca2+ influx per beat via ICa,L was increased about 2-fold in myocytes stimulated with post-MI action potentials compared with sham action potentials. 6. Our findings demonstrate that reductions in K+ channel expression in post-MI myocytes prolong action potential duration resulting in elevated Ca2+ influx and [Ca2+]i transients.

Effects of impaired Ca2+ homeostasis on contraction in postinfarction myocytes

The significance of altered Ca2+ influx and efflux pathways on contractile abnormalities of myocytes isolated from rat hearts 3 wk after myocardial infarction (MI) was investigated by varying extracellular Ca2+ concentration ([Ca2+]o, 0.6-5.0 mM) and pacing frequency (0.1-5.0 Hz). Myocytes isolated from 3-wk MI hearts were significantly longer than those from sham-treated (Sham) hearts (125 +/- 1 vs. 114 +/- 1 micrometer, P < 0.0001). At high [Ca2+]o and low pacing frequency, conditions that preferentially favored Ca2+ influx over efflux, Sham myocytes shortened to a greater extent than 3-wk MI myocytes. Conversely, under conditions that favored Ca2+ efflux (low [Ca2+]o and high pacing frequency), MI myocytes shortened more than Sham myocytes. At intermediate [Ca2+]o and pacing frequencies, differences in steady-state contraction amplitudes between Sham and MI myocytes were no longer significant. Collectively, the interpretation of these data was that Ca2+ influx and efflux pathways were subnormal in MI myocytes and that they contributed to abnormal cellular contractile behavior. Because Na+/Ca2+ exchange activity, but not whole cell Ca2+ current, was depressed in 3-wk MI rat myocytes, our results on steady-state contraction are consistent with, but not proof of, the hypothesis that depressed Na+/Ca2+ exchange accounted for abnormal contractility in MI myocytes. The effects of depressed Na+/Ca2+ exchange on MI myocyte mechanical activity were further evaluated in relaxation from caffeine-induced contractures. Because Ca2+ uptake by sarcoplasmic reticulum was inhibited by caffeine and with the assumption that intracellular Na+ and membrane potential were similar between Sham and MI myocytes, myocyte relaxation from caffeine-induced contracture can be taken as an estimate of Ca2+ extrusion by Na+/Ca2+ exchange. In MI myocytes, in which Na+/Ca2+ exchange activity was depressed, the half time of relaxation (1.54 +/- 0.14 s) was significantly (P < 0.02) prolonged compared with that measured in Sham myocytes (1.10 +/- 0.10 s).

Shortening and [Ca2+] dynamics of left ventricular myocytes isolated from exercise-trained rats

The effects of run endurance training and fura 2 loading on the contractile function and Ca2+ regulation of rat left ventricular myocytes were examined. In myocytes not loaded with fura 2, the maximal extent of myocyte shortening was reduced with training under our pacing conditions [0.5 Hz at 2.0 and 0.75 mM external Ca2+ concentration ([Ca2+]o)], although training had no effect on the temporal characteristics. The "light" loading of myocytes with fura 2 markedly suppressed (approximately 50%) maximal shortening in the sedentary and trained groups, although the temporal characteristics of myocyte shortening were significantly prolonged in the trained group. No discernible differences in the dynamic characteristics of the intracellular Ca2+ concentration ([Ca2+]) transient were detected at 2.0 mM [Ca2+]o, although peak [Ca2+] and rate of [Ca2+] rise during caffeine contracture were greater in the trained state at 0.75 mM [Ca2+]o. We conclude that training induced a diminished myocyte contractile function under the conditions studied here and a more effective coupling of inward Ca2+ current to sarcoplasmic reticulum Ca2+ release at low [Ca2+]o, and that fura 2 and its loading vehicle DMSO significantly alter the intrinsic characteristics of myocyte contractile function and Ca2+ regulation.

Microtubules modulate cardiomyocyte beta-adrenergic response in cardiac hypertrophy

The role of microtubules in modulating cardiomyocyte beta-adrenergic response was investigated in rats with cardiac hypertrophy. Male Sprague-Dawley rats underwent stenosis of the abdominal aorta (hypertensive, HT) or sham operation (normotensive, NT). Echocardiography and isolated left ventricular cardiomyocyte dimensions demonstrated cardiac hypertrophy in the HT rats after 30 wk. Cardiomyocyte microtubule fraction was assayed by high-speed centrifugation and Western blot. In contrast to previous reports of increased microtubules after acute pressure overload, microtubule fraction for HT was significantly lower than that for NT. Cardiomyocytes were exposed to either 1 microM colchicine, 10 microM taxol, or equivalent volume of vehicle. Colchicine decreased microtubules, and taxol increased microtubules in both groups. Cardiomyocyte cytosolic calcium ([Ca2+]c) and shortening/relaxation dynamics were assessed during exposure to increasing isoproterenol concentrations. The beta-adrenergic response for these variables in the HT group was blunted compared with NT. However, increased microtubule assembly by taxol partially recovered the normal beta-adrenergic response for time to peak [Ca2+]c, time to peak shortening, and mechanical relaxation variables. Microtubule assembly may play a significant role in determining cardiomyocyte beta-adrenergic response in chronic cardiac hypertrophy.

Sprint training attenuates myocyte hypertrophy and improves Ca2+ homeostasis in postinfarction myocytes

Myocytes isolated from rat hearts 3 wk after myocardial infarction (MI) had decreased Na+/Ca2+ exchange currents (I Na/Ca; 3 Na+ out:1 Ca2+ in) and sarcoplasmic reticulum (SR)-releasable Ca2+ contents. These defects in Ca2+ regulation may contribute to abnormal contractility in MI myocytes. Because exercise training elicits positive adaptations in cardiac contractile function and myocardial Ca2+ regulation, the present study examined whether 6-8 wk of high-intensity sprint training (HIST) would ameliorate some of the cellular maladaptations observed in post-MI rats with limited exercise activity (Sed). In MI rats, HIST did not affect citrate synthase activities of plantaris muscles but significantly increased the percentage of cardiac alpha-myosin heavy chain (MHC) isoforms (57.2 +/- 1.9 vs. 49.3 +/- 3.5 in MI-HIST vs. MI-Sed, respectively; P < or = 0.05). At the single myocyte level, HIST attenuated cellular hypertrophy observed post-MI, as evidenced by reductions in cell lengths (112 +/- 4 vs. 130 +/- 5 micrograms in MI-HIST vs. MI-Sed, respectively; P < or = 0.005) and cell capacitances (212 +/- 8 vs. 242 +/- 9 pF in MI-HIST vs. MI-Sed, respectively; P < or = 0.015). Reverse I Na/Ca was significantly lower (P < or = 0.0001) in myocytes from MI-Sed rats compared with those from rats that were sham operated and sedentary. HIST significantly increased reverse I Na/Ca (P < or = 0.05) without affecting the amount of Na+/Ca2+ exchangers (detected by immunoblotting) in MI myocytes. SR-releasable Ca2+ content, as estimated by integrating forward I Na/Ca during caffeine-induced SR Ca2+ release, was also significantly increased (P < or = 0.02) by HIST in MI myocytes. We conclude that the enhanced cardiac output and stroke volume in post-MI rats subjected to HIST are mediated, at least in part, by reversal of cellular maladaptations post-MI.

Isolated myocyte contractile function is normal in postinfarct remodeled rat heart with systolic dysfunction

BACKGROUND

Postinfarction ventricular remodeling is associated with lengthening and contractile dysfunction of the remote noninfarcted myocardium. Mechanisms underlying this phenomenon remain unclear.

METHODS AND RESULTS

We studied serial changes in global left ventricular (LV) structure and function in infarcted (1, 2, 4, and 6 weeks after myocardial infarction) and sham-operated rat hearts and correlated them with structural and functional changes in myocytes isolated from the remote LV myocardium in the same hearts. Rats with myocardial infarction developed significant remodeling. The heart weight-to-body weight ratios were increased. LV volumes at filling pressure of 10 mm Hg were higher (305+/-28 versus 215+/-12 microL, P<.01). This was accompanied by global LV dysfunction (in vivo LV end-diastolic pressure, 4+/-1 versus 23+/-1.6 mm Hg; Langendorff LV developed pressure, 105+/-4 versus 62+/-9 mm Hg, P<.001 for both). Myocytes isolated from these hearts showed significant structural remodeling (LV myocytes, 24% longer and 15% wider; right ventricular myocytes, 38% longer and 31% wider, all P<.05). LV myocyte length correlated with changes in LV volume (r=.79) and function (LV developed pressure, r=-.81). However, LV myocytes from the same hearts showed normal contractile function and intracellular Ca2+ transients at baseline and during inotropic stimulation with increasing extracellular Ca2+ (1 to 6 mmol/L). The shortening-frequency relationship was also similar in myocytes from sham and myocardial infarction rats.

CONCLUSIONS

Postinfarct LV remodeling occurs predominantly by myocyte lengthening rather than by myocyte slippage. However, contractile function of the unloaded myocytes from the remote noninfarcted LV myocardium of the remodeled heart is normal. Therefore, myocyte contractile abnormalities may not contribute to global dysfunction of the remodeled heart. Reduced myocyte mass and nonmyocyte factors like increased wall stress, altered LV geometry, and changes in the myocardial interstitium may be more important in the genesis of postinfarct LV dysfunction in this model.

Enhanced Na(+)-Ca2+ exchange in the infarcted heart. Implications for excitation-contraction coupling

Cellular Ca2+ regulation is abnormal in diseased hearts. We designed this study to assess the role of the Na(+)-Ca2+ exchanger in excitation-contraction coupling in surviving myocardium of the infarcted heart. We measured cellular contractions and whole-cell currents in single left ventricular myocytes isolated from the hearts of rabbits with healed myocardial infarction (MI). Eight weeks after MI, rabbits had left ventricular dysfunction without overt heart failure. Myocytes isolated from regions adjacent to the infarcted zone were significantly longer than cells from control hearts. At low stimulation rates (0.5 Hz), the amplitude of field-stimulated contractions was increased (11.6 +/- 0.5% versus 10.2 +/- 0.6% resting cell length), whereas the time to peak shortening and action potential duration were prolonged in the MI cells. When stimulation frequency was increased to 2.0 Hz, cellular shortening did not change or decreased in myocytes from infarcted hearts, whereas control cells had a positive shortening-interval relationship. Cells from infarcted hearts had a significantly decreased (31%) L-type Ca2+ current (ICa) density but no change in the current-voltage relationship or the kinetics of ICa inactivation. Maximal Na(+)-Ca2+ exchange current density was significantly increased (32%) in the cells from infarcted hearts. Sarcoplasmic reticulum (SR) Ca2+ content during a stable train of contractions, as estimated from caffeine-induced inward currents, was slightly increased (P = NS) in the MI myocytes. To determine whether Na(+)-Ca2+ exchange influenced SR Ca2+ content, cells were clamped at potentials between -70 and +90 mV for 400 ms. The amplitude of the contraction during a subsequent clamp step to +10 mV was then measured as an index of SR loading that occurred during the preceding clamp step. Steps to positive potentials produced greater augmentation of the subsequent contraction in MI than in control myocytes. In myocytes from the infarcted heart, increased activity of the Na(+)-Ca2+ exchanger may promote Ca2+ entry or decrease Ca2+ extrusion. This relative augmentation of inward Ca2+ flux by the exchanger may enhance SR Ca2+ loading and thus support contractility that would otherwise be impaired as a result of decreased Ca2+ current. However, Ca2+ influx by the exchanger may contribute to the prolongation of contractions in myocytes from infarcted hearts.

Dietary calcium supplementation and dopamine-beta-hydroxylase in spontaneously hypertensive rats

Spontaneously hypertensive 4-week-old male rats were fed, before and after the onset of hypertension, with either commercial chow (control) or commercial chow combined with different forms of milk proteins with or without calcium supplementation. After 40 weeks, rats were still hypertensive, and dopamine-beta-hydroxylase enzyme activity measured simultaneously in serum and adrenal was found to be higher than in the controls. The enzyme activity in rats fed diets with milk proteins was increased significantly in both serum and adrenal compared with the control, and such enhancement was significantly higher than that observed in animals fed the commercial diet supplemented with calcium (1.2%), suggesting that dietary calcium intake associated with dietary protein of high digestibility, such as casein, potentiates the endogenous mechanisms regulating the homeostasis of calcium more than calcium supplementation itself. Moreover, the selective and additive effect of diets supplemented with milk proteins and calcium on adrenal enzyme activity clearly suggests a relationship between cardiovascular diseases involving the genesis of hypertension and stress mechanisms through the hypothalamo-pituitary adreno-sympathetic axis.