Abstract
Calcium efflux from bovine chromaffin cells in tissue culture has been examined after loading them with small amounts of Ca 2+ by brief depolarization in media containing 20 μmol/l to 1 mmol/l Ca 2+ and 45Ca 2+ in trace amounts. In the presence of normal external Na + and Ca 2+ concentrations cells depolarized in media containing up to 200 μmol/l Ca 2+ exported nearly 100% of their accumulated Ca 2+ loads within 10 min and 20% within the first 5 s. In the absence of external Na + and Ca 2+ the proportion of a small (i.e., depolarization in 20 μmol/l calcium) Ca 2+ 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 + Ca 2+ -dependent and Na + Ca 2+ -independent mechanisms both contribute significantly to the export of calcium. At higher cellular loads of calcium (i.e., depolarization in 200 μmol/l to 1 mmol/l calcium) the Na + Ca 2+ -dependent mechanism exported a progressively greater proportion of the accumulated Ca 2+. Both sodium and calcium alone promoted a component of Ca 2+ efflux; Ca 2+ (i.e. calcium-calcium exchange) was as effective as Na + (i.e. sodium-calcium exchange). The K m for Na + stimulation of Ca 2+-efflux (K Na) was approximately 65 mM. Increased external Mg 2+ (from 1.2 to 10 mmol/l) increased the apparent K Na to 90 mM. The dependence of Ca 2+-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 Ca 2+ transported. In cells loaded with Na + labelled with 22Na + a component of Na + efflux was dependent upon external Ca 2+. It is concluded that Na +:Ca 2+ exchange does operate in bovine chromaffin cells and may play a role in normal Ca 2+ homeostasis. At loads of internal Ca 2+ that correspond approximately with those calculated to accompany maximal depolarization of chromaffin cells, the external Na + Ca 2+ -dependent export mechanism is more prominent than the Na + Ca 2+ -independent mechanism.
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