Effect of intracellular pH on spontaneous Ca2+ sparks in rat ventricular myocytes

A fall of intracellular pH (pH i ) typically depresses cardiac contractility. Among the many mechanisms underlying this depression, an inhibitory effect of acidosis upon the sarcoplasmic reticulum (SR) Ca 2+ release channel has been predicted, but not so far demonstrated in the intact cardiac myocyte. In the present work, pH i was manipulated experimentally while confocal imaging was used to record spontaneous ‘Ca 2+ sparks’ (local SR Ca 2+ release events) in rat isolated myocytes loaded with the fluorescent Ca 2+ indicator fluo-3. In other experiments, whole cell (global) pH i or [Ca 2+ ] i was measured by microfluorimetry (using, respectively, intracellular carboxy SNARF-1 and indo-1). Reducing pH i (i) increased whole cell intracellular [Ca 2+ ] transients induced either electrically or by addition of caffeine, whereas (ii) it decreased spontaneous Ca 2+ spark frequency. Conversely, raising pH i increased spontaneous Ca 2+ spark frequency. Blocking sarcolemmal Ca 2+ influx with 10 m m Ni 2+ , or reducing external pH by 1.0 unit, had no effect on the pH i -dependent changes in spontaneous Ca 2+ spark frequency. Decreasing pH i over the range 7.78–7.20, decreased Ca 2+ spark frequency exponentially as a function of pH i , with frequency declining by ∼33 % for a 0.2 unit fall in pH i . In contrast, over the same pH i range, Ca 2+ spark amplitude was unaffected. Intracellular acidosis produced a slight slowing of Ca 2+ spark relaxation. The results indicate that, in the intact myocyte, a reduced pH i decreases the probability of opening of the SR Ca 2+ release channel. This phenomenon may contribute to the negative inotropic effects of acidosis.