Abstract
1. The membrane potentials, currents and contractile responses were measured in the bullfrog ventricular trabeculae under voltage clamp conditions by means of double glycerol-gap technique.
2. The threshold potential for sodium inward current was -62mV and that for tetrodotoxin insensitive current, -58mV. At stronger depolarization up to +20mV, the contractile tension increased, showing a sigmoid curve in the presence of tetrodotoxin.
3. The tension development depended strongly on the duration of depolarization. Below a critical duration, neither active current nor tension was elicited (Phase I). Slightly longer depolarization, up to about 100msec, produced an all-or-none inward current and small tension in normal conditions (Phase II). This changed into graded responses after tetrodotoxin.
4. Further increase in duration of depolarization augmented the peak tension, first by increasing the rate of rise of tension (Phase III) and later by prolonging the time to peak tension (Phase IV). After reaching the peak the tensionde creased gradually when the depolarization was continued further (Phase V). No second rise of tension was observed for long and strong depolarizations.
5. Differences in characteristics of the bullfrog ventricle from those of mammalian myocardium were discussed, and a role of sarcotubular systems in the relation between the action potential and tension development was suggested.
