Characterization of the effect of pH on the excitation-contraction coupling system of canine masseter muscle.

The effect of pH on the excitation-contraction coupling system of canine masseter muscle was studied by evaluating the functional integrity of the sarcoplasmic reticulum (SR) and myofibrils. Increasing proton concentration (pH 7.0-5.8) significantly reduced oxalate supported SR calcium uptake velocity, while Ca2+-stimulated, Mg2+-dependent ATPase activity was unaffected by pH. The efficiency ratio of calcium transport, or the coupling ratio (mumoles Ca2+ transported/mumoles ATP hydrolyzed), decreased from 1.094 +/- 0.042 at pH 7.0 to 0.946 +/- 0.036 at pH 6.0 (P less than 0.05) and to 0.780 +/- 0.024 at pH 5.8 (P less than 0.01). Myofibrillar pCa (-log [free Ca2+] )-ATPase activity was unaffected between pH 7.0 and pH 6.5. At pH 6.0, increasing Ca2+ concentration inhibited myofibrillar ATPase activity, and this inhibitory phenomenon was accentuated at pH 5.8. Kinetic analysis of the myofibrillar pCa-ATPase data, utilizing double-reciprocal plots, demonstrated an increase in Km at low pH. It is concluded that acidosis significantly uncouples calcium transport from ATP hydrolysis in the SR of masseter muscle and significantly alters myofibrillar ATPase activity. It is hypothesized that these defects may explain an observed depression in skeletal muscle cell function during ischemia.