CA-DEPENDENT CHANGES OF MEMBRANE POTENTIAL AND THEIR SIGNIFICANCE IN EXCITATION-CONTRACTION COUPLING IN HEART . EXPERIMENTS WITH TETRODOTOXIN IN NA-CONTAINING SOLUTIONS

1. Tetrodotoxin (TTX), at a concentration of less than 10-6 g/ml, had no effect on membrane potential and contraction of isolated, thin ventricular trabeculae of sheep and calf hearts. 10-6 to 2 × 10-5 g/ml TTX decreased the rate of rise, over-shoot, and duration (phase of 90% repolarisation) of the action potential and the amplitude of contraction, without change in the resting potential and the plateau (20% repolarisation phase) of the action potential. Excitation block regularly occurred only with 10-5 to 2×10-5 g/ml TTX. 2. In a solution containing Na and TTX (5×10-6-2×10-5 g/ml) graded depolarisation was possible if the preparations were stimulated by square wave pulses of 500 msec duration across a sucrose bridge. In Ca-containing solutions the time-course of the electrotonic potentials showed two steps. The second step of depolarisation (SSD) began when the membrane potential reached-60 to-50 mV (threshold), and tension was initiated at the same level. Contractions reached their steady-state values only after about 5 depolarisations of the same size. There was no difference in the amplitude of contractions (steady-state level) elicited in Tyrode and in Tyrode + TTX. 3. SSD and contraction were dependent on the [Ca]e. Rate of rise (V/sec) and amplitude (mV) of SSD and tension increased with increasing [Ca]e. In Ca-free solutions the electrotonic potentials reached their steady-state levels in one step. No SSD and tension were observed in the absence of Ca, even with reversal of membrane potential. 4. In Tyrode + TTX the SSD was identical with the changes in membrane potential which could be observed in Na-free solution without TTX, but in Na-free solutions, contractions already reached their maximum during the first SSD. 5. SSD in Na-free solution was not blocked by TTX. The results indicate that the second step of depolarization in Na-free as well as in Na-containing solution + TTX is due to a Ca inward current. It is tentatively concluded that not only in Na-free but also in Na-containing solution Ca ions carry charge across the membrane of cardiac muscle during depolarisation and that this Ca inward current is an important factor in excitation-contraction coupling. © 1969 Springer-Verlag.