EFFECTS OF INTRACELLULAR ACIDOSIS ON [CA-2+]I TRANSIENTS, TRANSSARCOLEMMAL CA-2+ FLUXES, AND CONTRACTION IN VENTRICULAR MYOCYTES

We examined the effects of intracellular acidosis produced by washout of NH4Cl on [Ca2+](i) transients (indo-1 fluorescence), cell contraction (video motion detector), and 45Ca and 24Na fluxes in cultured chick embryo ventricular myocytes. Exposure of cells to 10 mM NH4Cl produced intracellular alkalosis (pH 7.6), and subsequent washout resulted in a transient acidosis (pH 6.5). Exposure to 10 mM NH4Cl slightly decreased [Ca2+](i) transients but increased the amplitude of cell contraction. Subsequent washout of NH4Cl initially increased diastolic [Ca2+](i) and decreased the peak positive and negative d[Ca2+](i)/dt, while the amplitude of cell contraction was markedly decreased. Subsequently, peak [Ca2+](i) increased with partial recovery of contraction. A similar increase in [Ca2+](i) and decrease in contraction after washout of NH4Cl was observed in single paced adult guinea pig ventricular cells. Acidosis decreased 45Ca uptake by sarcoplasmic reticulum vesicles isolated from chick embryo ventricle. However, the [Ca2+](i) increase caused by intracellular acidosis was also observed in the presence of 10 mM caffeine, suggesting that altered sarcoplasmic reticulum handling of calcium is not the only mechanism involved. Intracellular acidosis only slightly increased total 24Na uptake under these conditions, an effect resulting from the combination of a stimulation of amiloride-sensitive sodium influx (Na+-H+ exchange) and inhibition of sodium influx via Na+-Ca2+ exchange, manifested by a significant decrease in 45Ca efflux. Further support for a lack of involvement of an increased [Na+](i) in the observed increase in [Ca2+](i) during acidosis was provided by the observation that intracellular acidosis could produce an increase in [Ca2+](i) in low-sodium, nominal 0-calcium extracellular solution, an experimental condition that minimizes the possible effects of Na+-H+ exchange and Na+-Ca2+ exchange. We conclude that the [Ca2+](i) increase caused by intracellular acidosis in cultured ventricular cells is primarily due to changes in [Ca2+](i) buffering and [Ca2+](i) extrusion, rather than to an increase in transsarcolemmal calcium influx. Intracellular acidosis also markedly decreases the sensitivity of the contractile elements to [Ca2+](i) in cultured chick embryonic and adult guinea pig ventricular myocytes.