A value of Ca2+ binding in 1 second by sarcoplasmic reticulum fragments from guinea pig hearts increased proportionally to elevation of Ca2+ concentration from 0.1 to 3.0(-6) M at pH 7.2. Decline of pH to 6.8 and 6.2 decreased the Ca2+ bound in 1 second, but increased the Ca2+ bound in 30 seconds when membrane permeability has become a crucial factor in determining Ca2+ bound. In experiments on guinea pig papillary muscles, a decline of pH from 7.35 to 6.83 resulted in a nearly equal fall in velocities of isometric contraction and relaxation (27 and 21%, respectively), but during isotonic shortening when the relaxation proceeds much faster, a fall in relaxation velocity (44 +/- 3%) was significantly more profound as compared to the fall in contraction velocity (29 +/- 4%). This effect as well as a decrease in initial rate of Ca2+ binding by sarcoplasmic reticulum suggest a direct inhibitory effect of acidosis on Ca2+ removal from myofibrils in myocardial cells.
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