Potassium and calcium conductance in slow muscle fibres of the toad.

Slow muscle fibres in isotonic potassium sulphate saline could be easily repolarized to -90 mV. From this membrane potential a regenerative response could be elicited with short depolarizing pulses. 2. This response is blocked by TEA, suggesting that potassium is the main ion involved. 3. In the presence of TEA, a transient depolarization is recorded when the steady hyperpolarization is withdrawn. This anode break response is dependent upon the external calcium and is blocked by cobalt, suggesting that it is due to a calcium conductance. 4. The membrane conductance change was continuously recorded with short pulses at the end of the hyperpolarization. The membrane conductance decayed with at least two components with an average t1/2 of 1-2 and 6-8 sec. TEA blocked the slow component, and the fast one was dependent upon calcium and was blocked by cobalt.