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Abstract:
We investigated the effect of temperature on (a) calcium ion binding by ethylene glycol bis(β-aminoethyl ether)-N,N'-tetraacetic acid (EGTA), (b) the calcium ion dependence of rabbit skeletal muscle myosin B and myofibrillar adenosine triphosphatase activity, and (c) myofibrillar syneresis. Temperature changes between 0° and 40° had no significant effects on calcium ion concentrations established by an EGTA-CaEGTA buffer system. Both initial and basic rates of myosin B ATPase activity showed a constant calcium ion requirement from 20° to 40° with [Ca++] for half-maximal activity of 7.1 ± 1.0 x 10-7 m. During a brief initial period the rate of myofibrillar ATPase activity showed the same calcium ion dependence. The initial high rate of calcium ion-dependent myofibrillar ATPase activity was associated with a rapid increase in absorbance, which lasted for about 1 min at 20° and only 20 set at 37°. After the initial rapid increase in absorbance was complete, the rate of calcium ion-dependent myofibrillar ATPase activity declined (although considerable calcium ion-insensitive ATPase activity remained). The extent to which calcium ion-dependent myofibrillar ATPase activity was lost increased with the extent of syneresis, both factors being promoted by an increase in temperature. At all temperatures there was a correlation between the initial calcium ion-dependent rate of myofibrillar ATPase activity and the over-all rate of the initial rapid increase in absorbance. The decrease in calcium ion-dependent ATPase activity in myofibrils, but not in myosin B, following the rapid phase of syneresis appears to be a consequence of the contractile event.
