The Origin of Increased Cytoplasmic Calcium upon Reversal of the Na +/Ca 2+-Exchanger in Isolated Rat Ventricular Myocytes

Abstract

Reversal of the driving force of the Na +/Ca 2+-exchanger (ΔG exch) by a sufficiently large change of the transsarcolemmal electrochemical potential of sodium and calcium causes a transient increase of cytoplasmic calcium ([Ca 2+] i). The objective of this study was to investigate the origin of this transient increase of calcium. In isolated quiescent rat ventricular myocytes ΔG exchwas abruptly changed by reduction of extracellular sodium ([Na +] o), with or without a simultaneous increase of potassium ([K +] o) or calcium ([Ca 2+] o). [Ca 2+] iwas measured with indo-1. A particular change of ΔG exchinduced either by reduction of [Na +] oalone or in combination with increase of [Ca 2+] o, produced a transient increase of [Ca 2+] iof the same magnitude with a maximum after around 30 s. The response of [Ca 2+] iwas 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 [Ca 2+] iand also the contribution from SR increased with increasing change of ΔG exch. A particular change of ΔG exchinduced by a reduction of [Na +] oin combination with membrane depolarization (increase of [K +] o) increased the response of [Ca 2+] i, compared that induced by reduction of [Na +] oalone at the same change of ΔG exch. This effect increased with the degree of depolarization and was completely abolished by verapamil. Also in depolarized cells the response of [Ca 2+] iwas 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 ΔG exch. Inhibition of the Na +/Ca 2+-exchanger by Ni 2+almost completely abolished the response of [Ca 2+] ito reduction of [Na +] o. Restitution of [Na +] oduring the course of the calcium response greatly accelerated the rate of decay of [Ca 2+] 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 [Ca 2+] i, but the extra influx of calcium associated with depolarization did not affect the contribution from SR.