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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.
Cannell et al. (1986) studied this question.