Voltage dependence and time dependence of contraction in sheep cardiac Purkinje fibers.

Contractile behavior of the sheep cardiac Purkinje fiber was examined under voltage-clamp conditions in Tyrode's solution. For square depolarizing clamps to a given voltage, peak twitch tension depended on clamp duration if the duration was less than 80 to 90 msec, but measurable contractions could be obtained following 2-msec square clamps. Time to peak tension at 37°C remained constant as clamp duration was shortened. Twitch tension also depended on clamp voltage. The voltage at which tension was first seen, the position of the voltage-tension curve on the voltage axis, and the shape of the curve all were altered by a shift in the steady membrane voltage. After a contraction, recovery of trie contractile system was voltage-dependent; recovery was complete in approximately 2 seconds if the muscle was repolarized to potentials more negative than —75 mv, and no recovery was found at potentials more positive than —20 mv. Step changes of voltage to potentials just below tension threshold rapidly inactivated the contractile mechanism without development of tension. These results show a voltage dependence and time dependence of the contractile mechanism which may be of considerable importance to understanding excitation-contraction coupling.