Contractility of the frog ventricular myocardium in sodium-free lithium solution

The effects of Na-free Li solution on the electrical and mechanical activities were examined in a bullfrog's ventricular strip. The action potential showed a triphasic change during Na-free perfusion: the initial decrease in the overshoot and duration, the secondary restoration and the gradual decrease in the membrane potential. The twitch contraction also changed in a triphasic manner: the initial rapid decrease with a transient development of contracture and the secondary slow decrease followed by a partial recovery. A negative staircase, which was accompanied by full-sized twitch contractions, was induced by lowering the stimulation rate in this medium, indicating an involvement of some intracellular Ca releasing sites for activating the well-maintained contractile proteins. When CaCl2 was omitted during Na-free Li perfusion the twitch decreased only slightly down to 71.8% whereas in Na-containing medium it decreased to 11.9%. The sensitivity of twitch to the external Ca was thus greatly diminished in the Na-free condition. A fairly large contracture could be elicited by high K solution in Ca-free Li solution even after a prolonged Ca-free perfusion. Modification of the action potential by passing a hyperpolarizing current did not affect the twitch occurring in the Li solution, in contrast to its marked effect on the muscle perfused with normal Ringer. From these results it is concluded that the twitch is activated mainly by Ca released from some intracellular site in the Na-free Li solution. The inhibitory effect of Mn on this contraction was discussed in relation to its intracellular action.