Effects of ouabain on cytosolic calcium in lymphocytes, platelets and adrenomedullary cells

It has been suggested that a circulating inhibitor of Na/K ATPase can stimulate natriuresis and cause vasoconstriction in essential hypertension by stimulating transmembrane Na/Ca exchange to produce increased cytosolic concentrations of ionized calcium ([Ca⁺⁺]_i) in renal tubular and arteriolar smooth muscle cells. If this inhibitor affected ([Ca⁺⁺]_i) in all cell types, then clinical assays for its presence could be applied to easily accessible cells such as blood cells or platelets, and the inhibitor could exert a hormonal action on Ca⁺⁺-dependent adrenomedullary secretion of catecholamines. We used the Quin 2 technique for measuring [Ca⁺⁺]_i in lymphocytes, platelets and adrenomedullary cells in response to ouabain. Inhibition of Na/K ATPase by ouabain (10^-7 to 10^-3 mol/l enhanced transient [Ca⁺⁺]_i responses during Ca repletion but had no effect on steady-state [Ca⁺⁺]_i in any of the cell lines. Although it is possible that a Na/Ca exchange mechanism may exaggerate transient increases in [Ca⁺⁺]i during Na/K ATPase inhibition, other mechanisms appear to buffer these acute perturbations of [Ca⁺⁺]_i in lymphocytes, platelets and adrenomedullary cells.