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.
D000331:Aequorin; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002794:Choline / Q000494:pharmacology; D006329:Heart Conduction System / Q000502:physiology*; D066298:In Vitro Techniques; D008094:Lithium / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D011690:Purkinje Fibers / Q000502:physiology*; D012433:Ryanodine / Q000494:pharmacology; D012756:Sheep; D012964:Sodium / Q000494:pharmacology; D013997:Time Factors
D012433:Ryanodine; D000331:Aequorin; D008094:Lithium; D012964:Sodium; D002794:Choline; D002118:Calcium