Sarcolemmal Ca2+ transport activities in cardiac hypertrophy caused by pressure overload

Increased Na⁺/Ca²⁺ exchanger expression/activity constitutes a point of inflection in the progression to heart failure of hypertensive rats

Spontaneously hypertensive rat (SHR) constitutes a genetic model widely used to study the natural evolution of hypertensive heart disease. Ca²⁺-handling alterations are known to occur in SHR. However, the putative modifications of Ca²⁺-handling proteins during the progression to heart failure (HF) are not well established. Moreover, the role of apoptosis in SHR is controversial. We investigated intracellular Ca²⁺, Ca²⁺-handling proteins and apoptosis in SHR vs. control Wistar rats (W) from 3 to 15 months (mo). Changes associated with the transition to HF (i.e. lung edema and decrease in midwall fractional shortening), occurred at 15 mo in 38% of SHR (SHRF). In SHRF, twitch and caffeine-induced Ca²⁺ transients, significantly decreased relative to 6/9 mo and 15 mo without HF signs. This decrease occurred in association with a decrease in the time constant of caffeine-Ca²⁺ transient decay and an increase in Na⁺/Ca²⁺ exchanger (NCX) abundance (p<0.05) with no changes in SERCA2a expression/activity. An increased Ca²⁺-calmodulin-kinase II activity, associated with an enhancement of apoptosis (TUNEL and Bax/Bcl2) was observed in SHR relative to W from 3 to 15 mo.

CONCLUSIONS

1. Apoptosis is an early and persistent event that may contribute to hypertrophic remodeling but would not participate in the contractile impairment of SHRF. 2. The increase in NCX expression/activity, associated with an increase in Ca²⁺ efflux from the cell, constitutes a primary alteration of Ca²⁺-handling proteins in the evolution to HF. 3. No changes in SERCA2a expression/activity are observed when HF signs become evident.

Electrical and structural remodeling in left ventricular hypertrophy-a substrate for a decrease in QRS voltage?

Electrical remodeling in advanced stages of cardiovascular diseases creates a substrate for triggering and maintenance of arrhythmias. The electrical remodeling is a continuous process initiated already in the early stages of cardiological pathology. The aim of this opinion article was to discuss the changes in electrical properties of myocardium in left ventricular hypertrophy (LVH), with special focus on its early stage, as well as their possible reflection in the QRS amplitude of the electrocardiogram. It critically appraises the classical hypothesis related to the QRS voltage changes in LVH. The hypothesis of the relative voltage deficit is discussed in the context of supporting evidence from clinical studies, animal experiments, and simulation studies. The underlying determinants of electrical impulse propagation which may explain discrepancies between "normal" ECG findings and increased left ventricular size/mass in LVH are reviewed.

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.

Na-dependent changes in intracellular Ca in spontaneously hypertensive rat hearts

To determine whether Na/Ca exchange is altered in primary hypertension, Na-dependent changes in intracellular Ca, ([Ca]i), were measured in isolated perfused hearts from Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Intracellular Na, (Nai, mEq/kg dry wt), and [Ca]i were measured by NMR spectroscopy. Control [Ca]i was less in WKY than SHR (176 +/- 18 vs 253 +/- 21 nmol/l; mean +/- S.E., P < 0.05), whereas Nai was not significantly different. One explanation for this is that net Na/Ca exchange flux is decreased in SHR. If this hypothesis is correct, the rate of Ca uptake in SHR should be less than WKY when Na/Ca exchange is reversed by decreasing the transmembrane Na gradient. The Na gradient was reduced by decreasing extracellular Na, ([Na]o) and/or by increasing [Na]i. To increase [Na]i, Na uptake was stimulated by acidification while Na extrusion by Na/K ATPase was inhibited by K-free perfusion. Seventeen minutes after acidification, Nai had increased but was not significantly different in SHR and WKY (18.0 +/- 2.3 to 57.4 +/- 7.6 vs 20.3 +/- 0.6 to 66.5 +/- 4.8 mEq/kg dry wt, respectively). Yet [Ca]i was greater in WKY than SHR (1768 +/- 142 vs 1201 +/- 90 nmol/l; P < 0.05). [Ca]i was also measured after decreasing [Na]o from 141 to 30 mmol/l. Fifteen minutes after reducing [Na]o, [Ca]i was greater in WKY than SHR (833 +/- 119 vs 425 +/- 94 nmol/l; P < 0.05). Thus for both protocols, decreasing the transmembrane Na gradient led to increased [Ca]i in both SHR and WKY, but less increase in SHR. The results are consistent with the hypothesis that Na/Ca exchange activity is less in SHR than WKY myocardium.

Chronic low-dose treatment with enalapril induced cardiac regression of left ventricular hypertrophy

Numerous studies suggest that the renin angiotensin system (RAS) is involved in the development of cardiac hypertrophy. In the present study we produced cardiac hypertrophy in rats subjected to abdominal aortic banding and also induced cardiac regression by the administration of an angiotensin converting enzyme (ACE) inhibitor, enalapril, at 3, 10 and 30 mg/kg/day. Each drug was administered to the rats for 6 weeks from 6 weeks after aortic banding. The left ventricular weight significantly decreased at 10 and 30 mg/kg/day of enalapril as well as the systolic blood pressure. Using the reverse transcriptase polymerase chain reaction, the increased levels of ACE and AT1 mRNA were significantly inhibited in the aortic banding rats treated with the above concentrations of enalapril. The ACE activity in both the plasma and heart tissue preparations was significantly inhibited by enalapril. Similar observations were also seen after the administration of angiotensin type 1 receptor blockade, E-4177, into the aortic banding rats. The treatment with enalapril at 3 mg/kg/day did not reduce the left ventricular weight or the systolic blood pressure in the aortic banding rats. However, this low-dose treatment did significantly decrease the left ventricle to body weight ratio in the aortic banding rats without a reduction of the systolic blood pressure. Therefore, using the low-dose enalapril, the ACE activity in plasma was in part inhibited and the levels of ACE mRNA also decreased in the heart tissue of aortic banding rats, while the level of AT1 mRNA showed no such decrease. These results thus indicate that chronic ACE inhibitor at low doses has a beneficial effect on the regression in the pressure-induced cardiac hypertrophy. It is thus assumed that this effect may also contribute to the presence of an alternate pathway for the conversion of angiotensin I to angiotensin II which might also act as a possible mechanism for cardiac regression.

Effect of haemodynamic pressure overload of the adult ferret right ventricle on inotropic responsiveness to external calcium and rest periods

The inotropic effects of external calcium concentration ([Ca2+]o] and rest periods have been compared in papillary muscles isolated from control (n = 4) and pressure-overloaded right (n = 5) ventricles of adult ferrets. Hypertrophy was induced by pulmonary artery clipping for 30-45 days. Under control conditions (3 mM [Ca2+]o, 0.1 Hz), the isometric twitch force of hypertrophied muscles was decreased by 75%, time to peak was increased by 30% and time to half-relaxation was increased by 50% compared with non-hypertrophied preparations. The sensitivity of contraction to [Ca2+]o was decreased in hypertrophied muscles compared with control ([Ca2+] required for half-maximal contraction: 4.1 mM vs 1.7 mM) and the maximal contraction reached at high [Ca2+]o was smaller in pressure-overloaded muscles compared with control (8.3 +/- 2.0 mN mm-2 vs 19.0 +/- 2.1 mN mm-2 respectively). In both groups, rest periods longer than the steady-state interval were initially accompanied by a potentiation of the first post-rest contraction compared with steady-state. Peak potentiation occurred after a rest of 120 s in hypertrophied muscles and after a rest of 60 s in control. The maximal relative potentiation, i.e. compared with the steady-state twitch, was higher in hypertrophied muscles (+75%) than in control (+20%). After peak potentiation, the amplitude of the first post-rest contraction progressively decreased with increasing periods of rest, although at a slower rate in hypertrophy compared with control. The time constants of post-rest decay were 1203 +/- 99 s and 528 +/- 24 s respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular Na activity measurements in the control and hypertrophied heart of the ferret: an ion-sensitive micro-electrode study

Because of the role of intracellular Na on cardiac contractility and of the depressed isometric contractile response of the hypertrophied myocardium, the effects of pressure overload on the intracellular Na activity (aiNa) have been investigated in papillary muscles isolated from the ferret right ventricle. In animals subjected to pulmonary artery clipped for 1-2 months, right ventricle-to-body weight ratio was increased by about 39% in comparison with the control group. aiNa was measured in quiescent papillary muscles, by means of Na-sensitive micro-electrodes, at room temperature (19-22 degrees C). aiNa values were, in the control ventricular cells, 7.8 +/- 1.1 mM (mean +/- SD; n = 20) and in the hypertrophied ones, 8.0 +/- 1.2 mM (n = 49). During superfusion by medium with a reduced extracellular Na concentration ([Na]0), aiNa declined in control and pressure-overloaded muscles to similar steady-state levels at a given [Na]0. aiNa fall was mono-exponential and was characterized by a smaller time constant in the hypertrophied group upon total withdrawal of Na0 (control 209 +/- 19 s, n = 4; hypertrophied 128 +/- 42 s, n = 6). In the absence of external K, aiNa increased to levels that were not significantly different between both groups. It was concluded that, in quiescent preparations, steady-state aiNa was not modified by the hypertrophic process. However, pressure overload induced a modification of aiNa regulation by a possible alteration of the sarcolemmal Na/Ca exchange, although other mechanisms, such as mitochondrial Ca transport, could be involved in the differential response to Na0 removal.