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 the 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.