The origin of increased cytoplasmic calcium upon reversal of the Na+/Ca(2+)-exchanger in isolated rat ventricular myocytes.

Reversal of the driving force of the Na+/Ca(2+)-exchanger (delta Gexch) by a sufficiently large change of the transsarcolemmal electrochemical potential of sodium and calcium causes a transient increase of cytoplasmic calcium ([Ca2+]i). The objective of this study was to investigate the origin of this transient increase of calcium. In isolated quiescent rat ventricular myocytes delta Gexch was abruptly changed by reduction of extracellular sodium ([Na+]o), with or without a simultaneous increase of potassium ([K+]o) or calcium ([Ca2+]i). [Ca2+]i was measured with indo-1. A particular change of delta Gexch induced either by reduction of [Na+]o alone or in combination with increase of [Ca2+]o, produced a transient increase of [Ca2+]i of the same magnitude with a maximum after around 30s. The response of [Ca2+]i was insensitive to verapamil, but was greatly reduced by ryanodine, thapsigargin and caffeine, indicating a large contribution originating from the sarcoplasmic reticulum (SR). The magnitude of the response of [Ca2+]i and also the contribution from SR increased with increasing change of delta Gexch. A particular change of delta Gexch. Induced by a reduction of [Na+]o in combination with membrane depolarization (increase of [K+]o) increased the response of [Ca2+]i, compared that induced by reduction of [Na+]o alone at the same change of delta Gexch. This effect increased with the degree of depolarization, and was completely abolished by verapamil. Also in depolarized cells the response of [Ca2+]i was reduced by ryanodine. However, the contribution from SR to the response did not depend on the degree of depolarization, but only on the magnitude of the change of delta Gexch. Inhibition of the Na+/Ca(2+)-exchanger by Ni2+ almost completely abolished the response of [Ca2+]i to reduction of [Na+]o. Restitution of [Na+]o during the course of the calcium response greatly accelerated the rate of decay of [Ca2+]i. It is concluded that in quiescent rat ventricular myocytes, a large part of the transient increase of cytoplasmic calcium associated with reversal of the driving force of the Na+/Ca(2+)-exchanger originates from SR. Reversal of the exchanger combined with sustained depolarization increased the transient of [Ca2+]i, but the extra influx of calcium associated with depolarization did not affect the contribution from SR.