Effects of calcium and sodium on vasopressin release in vitro induced by a prolonged potassium stimulation

Groups of isolated hemilobes of rat neurohypophyses were stimulated for 90 min with a 56 mM K+ concentration in the surrounding medium. The rate of release of vasopressin into the medium showed an early maximum followed by a long phase of decline which lasted for most of the stimulation period. When 3 min collection periods were employed, higher secretion rates were obtained than when using 10 min periods. This may be due to a higher rate of removal of vasopressin from the tissue and a lower vasopressin inactivation when 3 min periods are used. When Na+ was replaced by sucrose in the incubation medium for the whole stimulation period, much higher secretion rates than previously reported were obtained (13% of the total hormone content in the first 30 min period and 21% for the total 90 min period). When Na+ was removed from the incubation medium late in the stimulation period, a new high secretion rate was induced, provided that extracellular calcium was present. When Ca-2+ was removed from the medium 3 min after start of the stimulation with 56 mM K+, the secretion rate fell very rapidly. Reintroduction of a normal Ca-2+ concentration caused a new marked increase in secretion rate. These results further stress the importance of extra-cellular calcium for release of vasopressin. In addition they show that a considerable fraction of the vasopressin present in the neurohypophysis can be mobilized during a continued strong stimulation if sodium is omitted from the medium.