CATECHOLAMINE RELEASE FROM BOVINE CHROMAFFIN CELLS - THE ROLE OF SODIUM CALCIUM EXCHANGE IN OUABAIN-EVOKED RELEASE

Spontaneous catecholamine (CA) release from bovine chromaffin cells maintained in primary tissue culture has been measured after pre‐loading the cells with [3H]noradrenaline. Ouabain inhibited 86Rb+ uptake and increased 3H release in a concentration‐dependent manner during a 60 min incubation period. Low external Na+ (5 mM: Li+ substitution) also increased 3H release. Whereas the 3H‐releasing action of ouabain was maintained, the Li(+)‐evoked release decreased with time. The effects of both ouabain and low Na+ solution on 3H release were completely inhibited by removal of Ca2+ from the external medium even though in Ca2(+)‐free solution ouabain further inhibited 86Rb+ uptake into the cells. Readmission of Ca2+ to Na(+)‐loaded cells (10‐4 M‐ouabain in Ca2(+)‐free‐1 mM‐EGTA solution for 60 min) markedly increased the release of 3H. In the additional presence of diphenylhydantoin (DPH, 10‐4 M) 3H release was significantly less on Ca2+ readmission. The 3H release from Na(+)‐loaded cells was proportional to the concentration of Ca2+ readmitted. The 3H release was further increased from Na(+)‐loaded cells in response to Ca2+ readmission when [Na+]o was lowered from 149 to 5 mM (Li+, choline+, Tris+ or sucrose substitution) though Li+ was less effective than the other Na+ substitutes. Potassium removal from the external medium significantly inhibited the 3H release evoked by Ca2+ readmission to Na(+)‐loaded cells, even when [Ca2+]o was greater than normal (7.5 mM) or if Ca2+ was readmitted in low [Na+]o solution. Rb+, Cs+ or Li+ could substitute for K+ with the order of potency: Rb+ greater than or equal to K+ greater than Cs+ greater than Li+. A slight increase of external K+ (10.8 mM) potentiated the 3H release from Na(+)‐loaded cells on Ca2+ readmission, but a higher concentration of K+ (149.4 mM) had the opposite action. The data is consistent with the hypothesis that ouabain‐evoked CA release from bovine chromaffin cells is, in part, a consequence of an internal Na(+)‐dependent Ca2+ influx. The evidence also suggests that there is Na(+)‐Ca2+ competition at the external arm of the exchanger together with a monovalent cation activation site. © 1990 The Physiological Society