INFLUENCE OF PH ON THE CA2+-REGULATED ATPASE OF CARDIAC AND WHITE SKELETAL MYOFIBRILS

An intracellular acidosis depresses the twitch tension of cardiac muscle but in general increases that of skeletal muscle. To determine whether these contrasting effects might be due to a difference in the pH-sensitivities of the Ca2+-activation of the myofibrils, we assayed the ATPase activity of rabbit and guinea-pig cardiac, and rabbit white skeletal, myofibrils at 37°C in solutions of different pCa (8.0 to 4.3) and pH (7.20, 7.00, 6.80, 6.40). Care was taken to free the cardiac myofibrils of contaminant membrane ATPases. Reducing the pH from 7.20 to 6.40 decreased the maximum Ca2+-regulated portion of the cardiac myofibrillar ATPase by 20% but had no significant effect on that of the skeletal myofibrillar ATPase. This reduction of pH also produced a five-fold increase in the [Ca2+] required for half-maximal cardiac ATPase activity at each pH; the corresponding [Ca2+] for the skeletal myofibrillar ATPase was increased four-fold. These slight differences in the pH-sensitivities of cardiac and skeletal myofibrillar ATPases probably do not account for more than a small part of the contrasting effects of acidosis on the two types of muscle. © 1979.