Role of calcium in osmotic and nonosmotic release of vasopressin from rat organ culture.

In the present study the role of calcium (Ca) in the stimulation of arginine vasopressin (AVP) release from the cultured rat hypothalamoneurohypophyseal complex (HNC) was examined in response to three different stimuli, 56 mM potassium chloride, an increase in medium osmolality from 290 to 310 mosmol/kg H2O, or 1 X 10(-6) M angiotensin II (ANG II). With all three stimuli AVP release from rat HNC explants was enhanced by increasing Ca concentration in the medium from 0 to 1.8 mM Ca. However, high concentrations of Ca (8 mM) inhibited the response of AVP release to either hyperosmolality or angiotensin II. Chemically dissimilar blockers of cellular Ca uptake, verapamil (5.2 X 10(-6) or 5.2 X 10(-5) M) or nifedipine (5.8 X 10(-6) or 5.8 X 10(-5) M), completely abolished AVP release from rat HNC explants in response to the three different stimuli in 1.8 mM Ca. In a normal concentration of medium Ca (1.8 mM) a Ca ionophore, A23187 (3.8 X 10(-5) M), significantly enhanced the osmotic and nonosmotic (ANG II-stimulated) release of AVP from rat HNC explants compared with controls without Ca ionophore. This effect of Ca ionophore to enhance AVP release was more evident in a lower Ca medium (0.9 mM Ca in the hyperosmolality study and 0.3 mM Ca in the ANG II study). These results therefore indicate that cellular Ca uptake is an important modulator of osmotic and nonosmotic AVP release from the intact rat hypothalamoneurohypophyseal system. The influence of extracellular Ca on the osmotic and nonosmotic release of AVP is also demonstrated.