Alterations in cellular calcium handling as a result of systemic calcium deficiency in the developing chick embryo: I. Erythrocytes.

Abstract

Chick embryos rendered calcium (Ca) deficient by shell-less (SL) culture develop hypertension and tachycardia. Since hypocalcemia is accompanied by hypernatremia systemically but not by lower cellular Ca (Koide and Tuan, 1989), we speculate that cellular Ca handling may be altered in the SL embryo, perhaps involving Na transport. Using erythrocytes (RBC) from day-14 SL and normal (NL) embryos as the experimental cell, cellular Ca handling was studied under varying extracellular osmotic and ionic conditions by analyzing 45Ca uptake and cell volume regulation. Two agents, p-chloromercuriphenylsulfonate (PCM), and inosine/iodoacetamide (INI) were used to treat the RBCs to modify plasma membrane ion permeability and to deplete cellular ATP, respectively. Other cellular functions and activities related to Ca homeostasis, including ATP content and Ca(2+)-ATPase activity, were also analyzed. These analyses showed: (1) in NaCl, Ca uptake was similar in NL and SL cells, except after INI treatment, which resulted in slower Ca uptake by the SL cells, (2) in choline and sucrose, Ca uptake by SL RBCs was higher, (3) Ca uptake by RBCs of both embryos changed depending on the osmotic agent (Na < K < or = choline < sucrose), (4) Ca(2+)-ATPase activity was higher in SL RBC, although there was no change in the size or charge of the enzyme, and (5) in any osmotic agent, cellular Na was significantly lower, whereas cellular K was higher, in SL RBC. Based on these results, three features of RBC Ca handling were apparent: (1) Na-Ca exchange was functional and was more active in SL RBCs, (2) Ca uptake was dependent on the total ionic electrochemical gradient but not on bulk H2O movement, and (3) Ca pumping out capacity was directly correlated with Ca(2+)-ATPase activity. Elevated Ca uptake in sucrose-treated SL RBC is therefore indicative of its greater ion permeability. Taken together, these findings indicate that cellular Ca handling of the RBCs of SL chick embryos is characterized by a more active Na-Ca exchange system, greater ion permeability, and higher Ca pumping out capacity, thereby suggesting an up-regulated Ca handling function in the SL RBCs. The abnormal cellular Ca handling may be a direct result of the systemic Ca deficiency of the SL chick embryo and may be functionally related to its hypertension and tachycardia.