A possible action of nicardipine on the cardiac sarcolemmal Na+-Ca2+ exchange

Cardiac Na + -Ca 2+ Exchange

Abstract —The Na + -Ca 2+ exchanger (NCX) is one of the essential regulators of Ca 2+ homeostasis in cardiomyocytes and thus an important modulator of the cardiac contractile function. The purpose of this review is to survey recent advances in cardiac NCX research, with particular emphasis on molecular and pharmacological aspects. The NCX function is thought to be regulated by a variety of cellular factors. However, data obtained by use of different experimental systems often appear to be in conflict. Where possible, we endeavor to provide a rational interpretation of such data. We also provide a summary of current work relating to the structure and function of the cardiac NCX. Recent molecular studies of the NCX protein are beginning to shed light on structural features of the ion translocation pathway in the NCX membrane domain, which seems likely to be formed, at least partly, by the phylogenetically conserved α-1 and α-2 repeat structures and their neighboring membrane-spanning segments. Finally, we discuss new classes of NCX inhibitors with improved selectivity. One of these, 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate (KB-R7943), appears to exhibit unique selectivity for Ca 2+ -influx–mode NCX activity. Data obtained with these inhibitors should provide a basis for designing more selective and clinically useful drugs targeting NCX.

Ca2+ antagonists do not protect isolated perfused rat hepatocytes from anoxic injury

Ca2+ antagonists were studied during anoxia in perfused isolated rat hepatocytes. Cytosolic free calcium (Ca2+i) was measured with aequorin. Anoxia was induced for 2 h by saturating the perfusate with 95% N2/5+ CO2. Anoxia increased Ca2+i in two distinct phases reaching a maximum of 1.5 microM. The increase in Ca2+i was caused by Ca2+ influx from the extracellular fluids because the main Ca2+i surge was totally abolished in Ca(2+)-free media. LDH release increased 6-fold during the second hour of anoxia, but when Ca2+ was removed from the perfusate during the anoxic period, LDH rose only 2.7-fold. Ca2+ antagonists (10(-7) to 10(-5) M) did not prevent the increase in Ca2+i and the rise in LDH release. On the contrary, high concentrations (10(-6) and 10(-5) M) of the blockers nifedipine and diltiazem significantly increased anoxic cell injury. The observation that the increase in LDH and the rise in Ca2+i were not suppressed by Ca2+ antagonists suggests that (i) Ca2+ antagonists protect the whole liver from anoxic injury by acting on cells other than parenchymal cells; (ii) the influx of Ca2+ responsible for the massive increase in hepatocyte Ca2+i evoked by anoxia did not take place through voltage-sensitive Ca2+ channels but must have occurred via the Na(+)-Ca2+ antiporter operating in the reverse mode (Ca2+ influx vs. Na+ efflux), and (iii) high concentrations of Ca2+ antagonists may be deleterious to the parenchymal cells of the liver.

Na(+)-Ca2+ antiporter activity of rat hepatocytes. Effect of adrenalectomy on Ca2+ uptake and release from plasma membrane vesicles

The presence and mode of Na(+)-Ca2+ antiporter activity were studied in hepatocytes isolated from sham-operated or adrenalectomized rats and in inside-out plasma membrane vesicles isolated from rat liver. Decreasing extracellular Na+ (Na+o) immediately increased cytosolic free calcium (Ca2+i). The rise in Ca2+i was proportional to the reduction in Na+o and was caused by an increased calcium influx, presumably on the Na(+)-Ca2+ antiporter operating in the reverse mode. Perfusing the cells with Ca(2+)-free media stimulated Ca2+ efflux and decreased Ca2+i, an effect dependent on Na+o. This suggests an activation of the forward mode of Na(+)-Ca2+ exchange. There was little difference in these parameters between sham and adx groups. In contrast, steady-state calcium uptake by inside-out plasma membrane vesicles was inhibited 40% after adrenalectomy. The decreased calcium uptake was not caused by a deficiency in the ATP-dependent Ca2+ pump, whose Km and Vmax were unaffected by adrenalectomy, but by an Na(+)-dependent leak from the vesicles. Ca2+ efflux was proportional to the extravesicular Na+ concentration, suggesting that the calcium leak may take place on a Na(+)-Ca2+ antiporter. This Na(+)-dependent calcium efflux was significantly increased in vesicles prepared from adx rat livers. These results suggest that hepatocytes have functional Na(+)-Ca2+ antiporters that can operate in both forward and reverse modes. Under normal conditions, the Na(+)-Ca2+ antiporter apparently operates in the reverse mode as a Ca2+ influx pathway. The increase in Na(+)-dependent Ca2+ efflux evoked by adrenalectomy in plasma membrane vesicles could explain the recent results we obtained in hepatocytes isolated from adx rats, showing increased calcium influx, increased Ca2+i, increased intracellular calcium sequestration, and increased plasmalemmal calcium cycling.

Alterations in cardiac function and subcellular membrane activities after hypervitaminosis D3

The present study was designed to induce massive accumulation of calcium in the myocardium and to evaluate the effect of calcium overload on myocardial contractile function and biochemical activity of cardiac subcellular membranes. Rats were treated with an oral administration of 500,000 units/kg of vitamin D3 for 3 consecutive days, and their hearts were sampled on the 5th day for biochemical analysis. On the 4th and 5th days, heart rate, mean aortic pressure, left ventricular systolic pressure and left ventricular dP/dt were significantly lowered in vitamin D3-treated rats, demonstrating the existence of appreciable myocardial contractile dysfunction. Marked increases in the myocardial calcium (67-fold increase) and mitochondrial calcium contents (24-fold increase) were observed by hypervitaminosis D3. Mitochondrial oxidative phosphorylation and ATPase activity were significantly reduced by this treatment. A decline in sarcolemmal Na+, K(+)-ATPase activity was also observed, while relatively minor or insignificant changes in calcium uptake and ATPase activities of sarcoplasmic reticulum were detectable. Electron microscopic examination revealed calcium deposits in the mitochondria after vitamin D3 treatment. The results suggest that hypervitaminosis D3 produces massive accumulation of calcium in the myocardium, particularly in the cardiac mitochondrial membrane, which may induce an impairment in the mitochondrial function and eventually may lead to a failure in the cardiac contractile function.

Enantiomer selectivity and the development of tolerance to the behavioral effects of the calcium channel activator BAY K 8644

The putative behavioral effects of the enantiomers of BAY K 8644 and the behavioral responses to (+/-)-BAY K 8644 following chronic injection were assessed on motor function in mice. The interaction of the enantiomers of BAY K 8644 with mouse brain dihydropyridine binding sites was also evaluated. The calcium channel activating enantiomer (-)-S-BAY K 8644 impaired rotarod and motor activity with an ED50 value of 0.5 mg/kg. The calcium channel blocker enantiomer (+)-R-BAY K 8644 neither affected rotarod nor motor activity. (+)-R-BAY K 8644, and the structurally related dihydropyridine calcium channel blockers nifedipine and (-)-202-791 inhibited the impairment of rotarod activity by (-)-S-BAY K 8644 in a dose-dependent manner. (+/-)-BAY K 8644 produced convulsions in mice with a CD50 of 5 mg/kg. Chronic injection of (+/-)-BAY K 8644 (8 mg/kg IP once each day for four days) resulted in a significant tolerance to, and increase in recovery from, the motor deficits produced by (+/-)-BAY K 8644. Furthermore, chronic treatment with (+/-)-BAY K 8644 increased the onset time, but did not reduce the number of mice having convulsions to (+/-)-BAY K 8644. Chronic injection of nifedipine did not affect the motor deficit and convulsive activity of (+/-)-BAY K 8644. The behavioral effects of (+/-)-BAY K 8644 were observed at significant brain levels of drug. [3H]Nitrendipine binding to mouse brain dihydropyridine binding sites was unchanged in mice chronically injected with either (+/-)-BAY K 8644 or nifedipine.(ABSTRACT TRUNCATED AT 250 WORDS)