CALCIUM EXPORT BY SODIUM-CALCIUM EXCHANGE IN BOVINE CHROMAFFIN CELLS

Calcium efflux from bovine chromaffin cells in tissue culture has been examined after loading them with small amounts of Ca2+ by brief depolarization in media containing 20-mu-mol/l to 1 mmol/l Ca2+ and Ca-45(2+) in trace amounts. In the presence of normal external Na+ and Ca2+ concentrations cells depolarized in media containing up to 200-mu-mol/l Ca2+ exported nearly 100% of their accumulated Ca2+ loads within 10 min and 20% within the first 5 s. In the absence of external Na+ and Ca2+ the proportion of a small (i.e., depolarization in 20-mu-mol/l calcium) Ca2+ load exported at any time point in the range to 10 min was approximately two thirds of the total efflux measured in their presence indicating that under these conditions the external Na+/Ca2+-dependent and Na+/Ca2+-independent mechanisms both contribute significantly to the export of calcium. At higher cellular loads of calcium (i.e., depolarization in 200-mu-mol/l to 1 mmol/l calcium) the Na+/Ca2+-dependent mechanism exported a progressively greater proportion of the accumulated Ca2+. Both sodium and calcium alone promoted a component of Ca2+ efflux; Ca2+ (i.e. calcium-calcium exchange) was as effective as Na+ (i.e. sodium-calcium exchange). The K(m) for Na+ stimulation of Ca2+-efflux (K(Na)) was approximately 65 mM. Increased external Mg2+ (from 1.2 to 10 mmol/l) increased the apparent K(Na) to 90 mM. The dependence of Ca2+-efflux on external Na+ can be described by a relationship of co-operativity (Hill equation) with a Hill coefficient of 2-3 which suggests a stoichiometry of 2-3 Na+ per Ca2+ transported. In cells loaded with Na+ labelled with Na-22+ a component of Na+ efflux was dependent upon external Ca2+. It is concluded that Na+:Ca2+ exchange does operate in bovine chromaffin cells and may play a role in normal Ca2+ homeostasis. At loads of internal Ca2+ that correspond approximately with those calculated to accompany maximal depolarization of chromaffin cells, the external Na+/Ca2+-dependent export mechanism is more prominent than the Na+/Ca2+-independent mechanism.