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1. 45Ca efflux and uptake, net Ca movements, and catecholamine secretion were studied in thin slices of bovine adrenal medulla. 2. There was a slow component of 45Ca wash-out which is attributed to efflux to intracellular Ca. This efflux was strongly influenced by cations in the external solution, being reduced by 50% when Mg replaced Ca, and the residual efflux being reduced by 80% when choline replaced Na. 3. None of the substitutes tested, choline, K, Li, Tris or sucrose, could replace Na in maintaining 45Ca efflux into Ca-free solution. The Na-dependent Ca efflux showed sigmoidal activation by Na, indicating a requirement for the co-operative action of two or more Na ions in the extrusion of each Ca. 4. In the presence of 3-6 mM-Ca, Na deprivation failed to reduce 45Ca efflux. When K, Li and choline replaced Na the rate constant of 45Ca efflux increased. 5. Ca-dependent 45Ca efflux was studied by adding back Ca to Ca-free solutions. Its activation by Ca was hyperbolic indicating that one external Ca is involved in the extrusion of each Ca, consistent with a Ca:Ca exchange process. The apparent affinity for Ca and the maximal efflux were different in the presence of different monovalent cations. 6. 45Ca uptake was increased when Li or choline replaced Na in a solution containing 3-6 mM-Ca. Net Ca uptake also increased, but to a much smaller extent, supporting the idea of a Ca:Ca exchange process. 7. Mg had little effect in activating or inhibiting 45Ca efflux. Co appeared to act as a weak agonist and weak inhibitor of Ca-dependent 45Ca efflux. Ba strongly activated 45Ca efflux.8. Elevation of [Na]t, with ouabain treatment, did not appreciably affect Na-dependent 45Ca efflux. This may indicate that while this Ca efflux is dependent on external Na, it is not dependent on the Na gradient. Elevation of [Na]i had rather little effect on 45Ca uptake. 9 Exposure of slices to Li or choline solutions evoked a Ca-dependent increase in catecholamine output. This could be attributed to the observed increase in net Ca uptake in these conditions. Sucrose and Tris solutions produced Ca-independent secretory responses with quite different time courses. These results emphasize the importance of the Na substitute used in determining the secretory effect of Na deprivation. 10. Elevation of [Na]i did not change basal catecholamine release, nor did it greatly affect the secretory response to Na deprivation. 11. Only some of the secretory effects of NA deprivation can be attributed to the influence of Na on Ca movements.

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