The regulation of endoplasmic reticulum calcium uptake of adipocytes by cytoplasmic calcium.

The endoplasmic reticulum of adipocytes can accumulate calcium by an active transport system linked to (Ca2+ + Mg2+)-ATPase activity (Black, B. L., Jarett, L., and McDonald, J. M. (1980) Biochim. Biophys. Acta 596, 359-371). The ability of this transport system to regulate intracellular calcium levels was tested by incubating adipocytes with various concentrations of calcium and measuring the calcium transport activity in subsequently isolated endoplasmic reticulum vesicles (microsomes). The calcium transport system responded to changes in extracellular calcium concentration; 2.5 mM calcium elicited a transport rate 21 to 34% greater than that of microsomes from cells incubated in the absence of calcium. The stimulatory effect of extracellular calcium was maximum at 2.5 mM, was reduced in magnitude at higher calcium concentrations, was demonstrable after a cell incubation period as short as 5 min, and was maintained for an additional 60 min. The transport system also could be modulated by preincubating isolated microsomes in calcium-containing medium prior to assay of calcium transport. Preincubation with low calcium concentrations elicited a 22 to 33% stimulation of subsequent calcium uptake with half-maximal stimulation at 0.19 +/- 0.02 microM free calcium. Preincubation with high calcium (200 microM) resulted in a 28% inhibition of calcium uptake. The stimulatory effect required ATP during the preincubation period and was modulated by ATP in the 0.05 to 0.5 mM range. The stimulation was not due to changes in efflux of calcium or in affinity of the transport system for calcium. The (Ca2+ + Mg2+)-ATPase activity of a deoxycholate-treated microsomal preparation was increased under preincubation conditions identical with those eliciting stimulation of calcium uptake, suggesting that modulation of enzyme activity produced the stimulatory effect. The results indicate that the endoplasmic reticulum plays a homeostatic role in control of intracellular calcium levels and suggest that the homeostatic response is mediated through interaction of the organelle with calcium and ATP.