[Transmembrane potential formation upon ATP hydrolysis in sarcoplasmic reticulum]

Abstract

Fluorescent cyanine (diS-C3-(5), diS-C2-(5), diO-C3-(5)) and oxonol (diBA-C4-(5)) potential-dependent dyes appeared to be extremely effective in detecting and studying the potential formed on the fragmented sarcoplasmic reticulum membrane under Ca2+ transport When [Ca2+] less than 5 X 10(-7) M ATP hydrolysis leads to formation of transmembrane potential (positive inside vesicules) caused by the Ca-independent ATPase activity. The potential is formed by a monovalent ion, presumably by H+, and possibly by Mg2+ ions. Ca-dependent ATPase activation by Ca2+ makes the potential to drop sharply and successive Ca2+ transport proceeds at low potential value. When Ca2+ has been accumulated by vesicules the Ca-independent ATPase restores positive potential. The potentials generated by both Ca-independent (10--30 mv) and Ca-dependent (-20 divided by -40 mv) ATPases have been estimated on the basis of the Nernst's equation with the help of positive and negative diffusion potentials formed by MgCl2 and CaCl2 gradients. The Ca2+ transport is shown not to be due to transmembrane electrophoresis but Ca-dependent ATPase action. The results suggest quite clearly that Ca-dependent ATPase operates as electrogenic Ca2+/H+, Mg2+-exchanger. The functional role of Ca-independent ATPase is, possibly, in compensation of charge effects when Ca2+ ions are passing through the membranes. The model illustrating the electrogenicity of Ca-independent and Ca-dependent ATPases action during Ca2+ transport in SR membranes has been proposed.