Anion effects on in vitro sarcoplasmic reticulum function. The relationship between anions and calcium flux

Abstract

Isolated sarcoplasmic reticulum vesicles exhibited different functional characteristics in the presence of zwitterionic as compared to anionic buffers. In the absence of oxalate, dicarboxylic anions (e.g. maleate, succinate) in a dose-dependent manner enhanced ATP-supported Ca2+ accumulation, the ensuing spontaneous Ca2+ release, and Ca2+-dependent ATPase activity compared to zwitterionic buffers (e.g. piperazine-N,N'-bis(2-ethanesulfonic acid) (Pipes) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) (Hepes). This was not attributed to ionic strength and osmotic effects. The additional anion-dependent Ca2+ accumulation was linked to augmented Ca2+-dependent ATPase activity, and both could be induced by the addition of anion at any time during Ca2+ accumulation as long as ATP was present. Since the initial Ca2+ accumulation rates and acyl phosphoenzyme formation were the same between the two buffer classes, and the presence of either oxalate (a Ca2+-precipitating anion) or A23187 (a Ca2+ ionophore) abolished differences in Ca2+-dependent ATPase activity between the two buffer classes, it is likely that conditions favoring high intravesicular Ca2+ concentration allow the expression of the observed effect of the anions. Initial spontaneous Ca2+ release in the presence of maleate was not caused by ATP depletion, and it was virtually absent in Pipes buffer. The rate of spontaneous release was also stimulated in a dose-dependent manner by the dicarboxylic anions, with the time of release being related to the time of anion addition and not ATP addition. A later, more rapid release phase in either maleate or Pipes buffer corresponded to ATP depletion, and could be duplicated at any time in the Ca2+ accumulation/release cycle by the addition of an ATP trap. With an ATP-regenerating system present or with very high ATP concentrations, the maximal peak Ca2+ accumulation in Pipes buffer could approach that in maleate buffer. The data suggest that dicarboxylic anions stimulate the filling of a Ca2+ compartment from which spontaneous Ca2+ release occurs.