SODIUM-EVOKED, CALCIUM-INDEPENDENT VASOPRESSIN RELEASE FROM RAT ISOLATED NEUROCHYPOPHYSIAL NERVE-ENDINGS

1. The effects of Na+ on vasopressin release and on redistribution of Ca2+ Na+ and H+ in isolated rat neurohypophysial nerve endings have be en studied. 2. Substituting Na+ for a non-permeant cation produced a p ronounced and sustained release of vasopressin. This increase occurred in the absence of external Ca2+ and in nerve endings loaded with the Ca2+ chelator dimethyl-BAPTA 2-bis-(0-aminophenoxy)ethane-N,N,N',N'-te traacetic acid). 3. The effect of Na+ was independent of a rise in int racellular Ca2+ as judged by the measurement of [Ca2+]i using the indi cator fura-2 and Ca-45(2+) efflux studies. Although Na+ could release Ca2+ from internal reservoirs the small elevation in [Ca2+]i induced b y Na+ could not explain the large and sustained increase in vasopressi n secretion. 4. The channel blockers TTX (tetrodotoxin), D888 (desmeth oxyverapamil), N144 (5-nitro-2-(phenylpropylamino)-benzoic acid) or SI TS etamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) could not prevent the Na+-dependent increase in vasopressin release. Similarly t his increase was not affected by metabolic inhibitors (Ruthenium Red a nd KCN) nor by CCCP (carbonyl cyanide m-chlorophenylhydrazone), an unc oupler of oxidative phosphorylation. 5. Selectivity among monovalent c ations to promote secretion was found with the largest effect on the s ecretory response being produced by Na+. Similarly Cl-was found to be the most potent anion studied for inducing, in the presence of Na+, an increase in neurohormone release. 6. Measuring [Na+]i by means of the Na+ indicator SBFI showed that the extent of the secretory response w as correlated with the intraterminal Na+ concentration. 7. The Na+-ind uced, Ca2+-independent release of vasopressin occurred by exocytosis a s judged (i) by the linear relationship between the amount of vasopres sin secreted and that of the co-localized neurophysin and (ii) by the demonstration that the extracellular marker horseradish peroxidase was only found in endocytotic vacuoles and not in the cytoplasm of the st imulated nerve endings. 8. The Na+-dependent secretory response found on addition of extracellular Na+ was not the result of the change in i nternal pH as measured with the indicator BCECF and as mimicked by add ition of propionic acid. 9. Addition of Na+ to digitonin- or streptoly sin-O-permeabilized nerve endings in the presence or absence of Ca2+ a lso gave rise to an increase in vasopressin secretion. 10. It is concl uded that an increase in internal Na+ per se can promote, in the absen ce of a rise in intracellular Ca2+, an increase in neuropeptide secret ion.