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INSERM U. 176, Rue Camille Saint-Saëns, 33077 Bordeaux Cedex, France

1. Calcium efflux from isolated rat neurophypophyses has been studied. Curve fitting of the wash-out curves suggests three phases with t of ca. 3, 15 and 130 min.

2. The slow component of the ⁴⁵Ca efflux is attributed to efflux of intracellular Ca. On the basis of the temperature sensitivity of the Ca efflux, the activation energy has been calculated to be approximately 12,000 cal/mole, corresponding to a Q₁₀ of ca. 2·0.

3. Ca efflux decreased by approximately 32% when external Na was replaced by choline. Li_o, in the presence or absence of Ca_o, was as effective as Na_o in stimulating the Ca efflux.

4. The curve relating Ca efflux to [Na]_o or [Li]_o is sigmoid and suggests that at least two Na (or Li) ions are necessary to activate the efflux of each Ca ion. Ca_o does not modify the absolute Na-dependent Ca efflux but decreases the affinity for Na of the site involved in Ca extrusion.

5. Removal of Ca_o decreased the Ca efflux by ca. 44% in Na-free media. The apparent affinity for Ca_o of the Ca_o-activated Ca efflux (K_m^Ca_o = 20 µM) is greatly decreased by the presence of 150 mM-Na (K_m^Ca_o = 0·8 mM).

6. Lanthanum decreased the total Ca efflux by ca. 60% and totally abolished the Na_o-activated and Ca_o-activated Ca efflux.

7. Vanadate reduced the Ca efflux remaining in Na-, Ca-free saline by 73%.

8. Elevation of Na_i with ouabain did not modify the rate of loss of ⁴⁵Ca.

9. Increased concentration of K_o stimulated transiently the ⁴⁵Ca loss. The time course of this increase depends on the Ca_o concentration ([Ca]_o).

10. Cyanide or CCCP (carbonyl cyanide m-chlorophenylhydrazone) increased transiently the Ca efflux. The increase induced by cyanide could only be observed when the neural lobes had been over-loaded with ⁴⁵Ca.

11. Membrane destruction induced by high temperature eliminated the effect of [Na]_o and [Ca]_o on ⁴⁵Ca efflux.

12. In 150 mM-Na-containing saline, half-maximum activation of ⁴⁵Ca uptake occurs in the 0·2-0·4 mM [Ca]_o range.

13. The Ca efflux from isolated pituicytes was not affected by removal of Na_o.

14. In conclusion we show that Ca efflux from neurosecretory nerve terminals can be subdivided into three components of approximately the same magnitude, one which is activated by Na_o, another by Ca_o and a third component which is independent of Na_o and Ca_o.