A study on the mechanism of twitch potentiation by hypertonic solution in the frog atrial muscle

Hypertonic solutions were found to exhibit both positive and negative inotropic effects on the contraction of the isolated atrial myocardium of bullfrog. The optimum tonicity for twitch potentiation was about 1.5 T. The mechanism for the positive inotropic action was investigated. The possibility of involvement of an increase in calcium influx during each action potential was excluded, since both the overshoot and the plateau of action potential were strongly depressed by perfusion of hypertonic solution. The effect on the time course of twitch potentiation was similar to that of muscle shrinkage, regardless of the type solute (sucrose, NaCl or LiCl) used for elevating the tonicity, except that excess sodium showed an initial rapid inhibitory phase of contraction. A marked post-rest potentiation was observed even after "zero" calcium perfusion, provided that the tonicity of the bathing medium was elevated previously. Potassium contracture occurred during the prolonged hypertonic perfusion in "zero" calcium condition. In addition, caffeine contracture was strongly augmented in hypertonic solution. The results suggest that an elevation in both the [Ca2+]i and amount of calcium bound intracellularly may play an important role in the positive inotropic action.