CALCIUM INFLUX IN SKELETAL MUSCLE AT REST, DURING ACTIVITY, AND DURING POTASSIUM CONTRACTURE

Calcium influx in the sartorius muscle of the frog (Rana pipiens) has been estimated from the rate of entry of Ca⁴⁵. In the unstimulated preparation it is about equal to what has been reported for squid giant axons, but that per impulse is at least 30 times greater than in nerve fibers. The enhanced twitch when NO^-₂ replaces Cl^- in Ringer's is associated with at least a 60 per cent increase in influx during activity, whereas this anion substitution does not affect the passive influx significantly. Calcium entry during potassium contracture is even more markedly augmented than during electrical stimulation, but only at the beginning of the contracture; thus, when a brief Ca⁴⁵ exposure precedes excess K⁺ application, C⁴⁵ uptake is increased three- to fivefold over the controls not subjected to K⁺, whereas when C⁴⁵ and K⁺ are added together, no measurable increase in Ca⁴⁵ uptake occurs. These findings are in keeping with the brevity of potassium contracture in "fast (twitch)" fibers such as in sartorius muscle.