Epidermal growth factor stimulates Ca2+ uptake of human erythrocytes

To examine the functional significance of epidermal growth factor (EGF) binding sites present on the human erythrocyte membrane [Engelmann et al. (1992) Am J Hematol 39:239–241], the effect of EGF on 45Ca2+ uptake and on 22Na+ efflux from these cells has been studied. In all cases media contained 1.25 mM Ca2+, whereas Na+ and K+ were varied. In 140 mM Na+/5 mM K+ medium EGF (250 ng/ml) stimulated 45Ca2+ uptake by 50%–90% in quin-2-loaded cells, and by up to threefold in untreated cells. Increasing extracellular K+ up to 75 mM at the expense of extracellular Na2+ stimulated the EGF-induced 45Ca2+ uptake by about twofold compared to 145 mM Na+ medium both in quin-2-loaded and in untreated cells. In 145 mM K+ medium, however, no EGF-induced 45Ca2+ uptake was detectable in quin-2-loaded cells, while in untreated cells Ca2+ entry was stimulated twofold by EGF. After increasing intracellular Na+ from 6 mmol/l cells to 18 mmol/l cells in untreated cells suspended in 145 mM K+ medium, 45Ca2+ uptake induced by EGF gradually increased. In contrast, in 140 mM Na+/5 mM K+ as well as in 70 mM Na+/75 mM K+ medium, 45Ca2+ uptake accelerated by EGF was largely unaffected by a modified red cell Na+ content. When 22Na-loaded untreated red cells were suspended in 145 mM K+ medium EGF stimulated red cell 22Na+ efflux by more than threefold. In 140 mM Na+/5 mM K+ as well as in 70 mM Na+/75 mM K+ medium, no 22Na+ efflux induced by the growth factor was evident. The results are consistent with the idea that EGF stimulates (at least) two components of 45Ca2+ uptake in human erythrocytes. One of the two is unmasked in 145 mM K+ medium, inhibited by quin-2 loading, accelerated by intracellular Na+ and appears to involve reversed Na+/Ca2+ exchange.