Ca2+- and caffeine-induced Ca2+ release from the sarcoplasmic reticulum in rat skinned trabeculae: Effects of pH and P-i

Objective: Our aims were: (1) to examine the effect of pH (7.4-6.5) on Ca2+ release from the sarcoplasmic reticulum (SR) of cardiac muscle, and (2) to see if these effects were altered by phosphate (P-i). Methods: Rat ventricular trabeculae were permeabilised with saponin. Ca2+-induced Ca2+ release (CICR) from the SR was triggered by flash photolysis of nitr-5. Under similar loading conditions, SR Ca2+ loading was assessed using caffeine to release the Ca2+ in the SR. Force and fluo-3 fluorescence (a measure of the cytosolic [Ca2+]) were monitored. Results: SR Ca2+ loading was optimal at pH 7.1 and was significantly reduced at pH 7.4, 6.8 and 6.5. CICR was the same at pH 7.4 as at pH 7.1, but was reduced, by more than Ca2+ loading, in acidic solutions. These differential effects on loading and CICR suggested that Ca2+ activation of the Ca2+ release channel was decreased (by > 50%) as pH was lowered from 7.4 to 6.5. A direct effect on the Ca2+ release channel was confirmed by the finding that Ca2+ release was slower in acidic solutions. Acidosis also slowed the re-uptake of Ca2+ into the SR after CICR, which may account for the reduced Ca2+ loading at low pH. As observed previously, P-i (20 mM) by itself decreased SR Ca2+ loading. However, the inhibitory effects of acidosis and P, on SR Ca2+ loading were independent. Conclusions: A fall of pH over the range 7.4-6.5 directly inhibits the SR Ca2+ release channel. In addition, acidosis inhibits SR Ca2+ accumulation by a mechanism independent of that of Pi. Both effects of acidosis would act to decrease SR Ca2+ release and so would contribute to the negative inotropic actions of intracellular acidosis in intact cardiac muscle.