Role of calcium in the regulation of sugar transport in the avian erythrocyte: effects of the calcium ionophore, A23187.

The tissue/medium distribution of the nonmetabolized glucose analog [14C]-3-O-methyl-D-glucose was measured in pigeon erythrocytes and related to changes in 45Ca uptake and efflux, total calcium content and ATP levels. Sugar transport was not affected by changes in external Ca2+. However, both sugar and 45Ca influx were increased by the Ca-ionophore A23187. In the absence of external Ca2+, the ionophore caused a delayed increase in sugar transport and net loss of calcium, probably through releasing Ca2+ from internal storage sites into the cytoplasm. Increasing internal Na+ through Na+ pump inhibition or using the sodium ionophore monensin did not alter influx of sugar or 45Ca, indicating Na+-Ca2+ exchange was absent in these cells. The results are consistent with A23187 causing increased Ca2+ influx or release from mitochondrial storage and the resulting rise in cytoplasmic Ca2+ stimulating hexose transport. Experiments with low Mg++ and high K+ media and measurements of ATP levels exclude alternative explanations for the action of A23187. We conclude that sugar transport regulation in avian erythrocytes is Ca2+-dependent and resembles that in muscle in its basic mechanism. It differs in the response to some modulating agents, largely because of a different pattern of Ca2+ fluxes in these cells.