Abstract
The presence of an Na/Ca exchange system in fasciculata cells of the bovine adrenal gland was tested using isolated plasmalemmal vesicles. In the presence of an outwardly Na + gradient, Ca 2+ uptake was about 2-fold higher than in K + condition. Li + did not substitute for Na + and 5 mM Ni 2+ inhibited Ca 2+ uptake. Ca 2+ efflux from Ca 2+-loaded vesicles was Na +-stimulated and Ni 2+-inhibited. The saturable part of Na +-dependent Ca 2+ uptake displayed Michaelis–Menten kinetics. The relationship of Na +-dependent Ca 2+ uptake versus intravesicular Na + concentration was sigmoid (apparent K 0.5≈24 mM; Hill number ≈3) and Na + acted on V max without significant effect on K m. Na +-stimulated Ca 2+ uptake was temperature-dependent (apparent Q 10≈2.2). The inhibition properties of several divalent cations (Cd 2+, Sr 2+, Ni 2+, Ba 2+, Mn 2+, Mg 2+) were tested and were similar to those observed in kidney basolateral membrane. The above results indicate the presence of an Na/Ca exchanger located on plasma membrane of zona fasciculata cells of bovine adrenal gland. This exchanger displays similarities with that of renal basolateral cell membrane.
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