Mechanisms of the modulation of Ca movement and contracti in acidosis

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Effects of intracellular acidification on Ca^<2+> handling mechanisms and tension development were studied using intact and skinned preparations. CO_2 acidosis (ACD) increased Ca^<2+> transient (CaT) and prolonged its decay time, although tension was inhibited without a significant change of its time course. ACD decreased the Ca sensitivity of the contactile elememts and suppressed the maximal tension in tetanized-preparations. A transient increase in CaT (extra-Ca), which was induced by a quick release of muscle from Lmax to 92% Lmax during a twich contraction, was decreased by ACD.This further suggests a decrease in the Ca sensitivity of the contractile elements. The effects of H^+ on the Ca^<2+> handling of the sarcoplasmic reticulum (SR) weere examined using skinned trabeculae in which Ca^<2+> measured using the fluorescent Ca indicator, fluo-3. Ca-induced Ca release (CICR) and Ca^<2+> uptake by tge SR were all inhibited by H^+. CICR induced by pCa 6 was particularly inhibited by H^+. The increase in CaT in ACD is due to a decrease in the Ca sensitivity and the slow decay of the CaT in acidosis is due to the slow uptake of Ca^<2+> by the SR.