PH-DEPENDENT INHIBITORY EFFECTS OF CA2-RETICULUM ATPASE(, MG2+, AND K+ ON CA2+ EFFLUX MEDIATED BY SARCOPLASMIC)

The effect of pH on the rate of Ca2+ efflux mediated by the Ca2+ adeno sinetriphosphatase (ATPase) was measured. The cations Ca2+, Mg2+, and K+ decrease the rate of Ca2+ efflux at pH 7.5 but not at pH 6.0. The e ffect of pH on the affinity to Ca2+ during Ca2+ efflux was found to be similar to the pH dependence of the high-affinity Ca2+ binding sites of the ATPase. The inhibitory activity of cations was significantly in creased by a rise in pH, whereas acidification amplified the magnitude of an efflux component insensitive to cations. Acidification of the a ssay medium allows efflux of Ca2+ through the Ca2+ pump, even in the p resence of high concentrations of monovalent and divalent cations. The efflux rate was severalfold increased by addition of the hydrophobic drugs trifluoperazine, dibucaine, and imipramine. At neutral pH, the C a2+ efflux induced by trifluoperazine was antagonized by the cations C a2+, Mg2+, and K+ and by thapsigargin, a highly specific inhibitor of the Ca2+ pump. In contrast to that observed at neutral pH, the cations did not antagonize the effect of trifluoperazine on Ca2+ efffux at ac id pH. It is concluded that H+ produces functional alterations in ATPa se domains involved in Ca2+ efflux.