Effects of membrane potential on intracellular calcium concentration in sheep Purkinje fibres in sodium‐free solutions.

1. The intracellular Ca2+ concentration [( Ca2+]i) was measured in voltage‐clamped sheep cardiac Purkinje fibers while recording tension simultaneously. 2. When [Na+]i was elevated (by Na+‐K+ pump inhibition) depolarization produced an increase of tonic tension. 3. Replacement of external Na+ by Li+ or choline produced a contracture which then relaxed spontaneously. Following this relaxation, depolarization either had no effect on tonic tension or produced a small decrease. 4. When external Na+ was replaced by Ca2+, depolarization (over the range ‐120 to ‐20 mV) produced a decrease of tonic tension and [Ca2+]i. Hyperpolarization increased tonic tension and [Ca2+]i. 5. An after‐contraction and accompanying increase of [Ca2+]i were produced by repolarization in both Na+‐free and Na+‐containing solution. This eliminates the possibility that the stimulus for the after‐contraction is the increase of [Ca2+]i during the depolarization and suggests that the stimulus may be the change of membrane potential. 6. The increase of [Ca2+]i on hyperpolarization seen in Na+‐free solutions persisted in the presence of ryanodine. 7. These results show, in contrast to previous work, that in Na+‐free solutions tonic tension is still sensitive to membrane potential. The results support the hypothesis that, in Na+‐containing solutions, the increase of tonic tension on depolarization results from a voltage‐dependent Na+‐Ca2+ exchange. The reduction of tonic tension on depolarization in Na+‐free solutions may be due to the decrease of the electrochemical gradient for Ca2+ to enter the cell.