Effects of hypertonic urea solution on the contractility of the bullfrog ventricle

The effects of hypertonic urea solution on the excitation-contraction coupling of the bullfrog's heart muscle were investigated.

Small strips obtained from the ventricular wall of Rana catesbiana were used. Contractile tension as well as transmembrane action potential were recorded in the single sucrose-gapped arrangement. The muscle weight was measured in order to check the changes in the dimension of muscle fiber both during isotonic and hypertonic perfusions. Light microscopic study was also performed to compare the effect of hyperosmolar urea with that of sucrose.

3.0 T-urea solution exerted a triphasic change in twitch tension during hypertonic perfusion and a biphasic change during the recovery perfusion. An initial rapid decline was restored to the original level within 10 min and after an overshoot a secondary gradual decrease was observed during hypertonic perfusion. In spite of parallel change in the action potential duration with the time to peak tension there was an increase in dp/dt during the restoration phenomenon. The calcium sensitivity was altered in the 3.0 T-urea solution and thus in Ca-free solution the rate constant of the slow component of twitch decline was decreased. The muscle weight remained decreased during the whole period of hypertonic perfusion. Histological study revealed that the fiber bundles soaked in hypertonic urea solution show normal appearance whereas those in sucrose exhibit a marked shrinkage. These results show that urea can penetrate the heart muscle membrane easily and that urea has some augmenting action on the E-C coupling process.