Dependence of ionophore- and caffeine-induced calcium release from sarcoplasmic reticulum vesicles on external and internal calcium ion concentrations

The effects of the ionophore, X537A, and caffeine on ATP-dependent calcium transport by fragmented sarcoplasmic reticulum were studied in the absence (calcium storage) or presence (calcium uptake) of calcium-precipitating anions. The ionophore caused rapid calcium release after calcium storage, the final level of calcium storage being the same whether a given concentration of X537A was added prior to initiation of the reaction or after calcium storage had reached a steady state. Although 10 to 12 muM X537A caused approximately 90% inhibition of oxalate-supported calcium uptake when added prior to the start of the reaction, this ionophore concentration caused only a small calcium release when added after a calcium oxalate precipitate had formed within the vesicles, and only slight inhibition of calcium uptake velocity when added during the calcium uptake reaction. When low initial calcium loads limited calcium uptake to 0.4 mumol of calcium/mg of protein, subsequent calcium additions in the absence of the ionophore led to renewed calcium uptake. Uptake of the subsequent calcium additions was not significantly inhibited by 10 to 12 muM X537A. These phenomena are most readily understood in terms of constraints imposed by fixed Cai (calcium ion concentration inside the vesicles) on the pump-leak situation in sarcoplasmic reticulum vesicles containing a large amount of an insoluble calcium precipitate, where most of the calcium is within the vesicles and Cai is maintained at a relatively low level. These constraints restrict calcium loss after calcium permeability is increased because calcium release can end when the calcium pump is stimulated by the increased Cao (calcium concentration outside the vesicles) so as to compensate for the increased efflux rate. In contrast, an increased permeability in vesicles that have stored calcium in the absence of a calcium-precipitating ion causes a much larger portion of the internal calcium store to be released. Under these conditions calcium storage capacity is low so that release of stored calcium is less able to raise Cao to levels where the calcium pump can compensate for the increased efflux rate. The constraints imposed by anion-supported calcium uptake explain the finding that more calcium is released by X537A or caffeine when these agents are added at higher levels of Cao, and that more calcium leaves the vesicles in response to a given increase in calcium permeability at higher Cai. Although such calcium release is amplified by increased Cao, the amplification is attributable to the constraints described above and does not represent a "calcium-triggered calcium release."