CA2-INDUCED 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). Metho ds: Rat ventricular trabeculae were permeabilised with saponin. Ca2+-i nduced Ca2+ release (CICR) from the SR was triggered by flash photolys is of nitr-5. Under similar loading conditions, SR Ca2+ loading was as sessed using caffeine to release the Ca2+ in the SR. Force and fluo-3 fluorescence (a measure of the cytosolic [Ca2+]) were monitored. Resul ts: SR Ca2+ loading was optimal at pH 7.1 and was significantly reduce d at pH 7.4, 6.8 and 6.5. CICR was the same at pH 7.4 as at pH 7.1, bu t was reduced, by more than Ca2+ loading, in acidic solutions. These d ifferential effects on loading and CICR suggested that Ca2+ activation of the Ca2+ release channel was decreased (by > 50%) as pH was lowere d from 7.4 to 6.5. A direct effect on the Ca2+ release channel was con firmed by the finding that Ca2+ release was slower in acidic solutions . Acidosis also slowed the re-uptake of Ca2+ into the SR after CICR, w hich may account for the reduced Ca2+ loading at low pH. As observed p reviously, P-i (20 mM) by itself decreased SR Ca2+ loading. However, t he inhibitory effects of acidosis and P, on SR Ca2+ loading were indep endent. Conclusions: A fall of pH over the range 7.4-6.5 directly inhi bits the SR Ca2+ release channel. In addition, acidosis inhibits SR Ca 2+ accumulation by a mechanism independent of that of Pi. Both effects of acidosis would act to decrease SR Ca2+ release and so would contri bute to the negative inotropic actions of intracellular acidosis in in tact cardiac muscle.