Oxytocin release from isolated posterior pituitary lobes of adult male rats as determined by radioimmunoassay.

In vitro oxytocin (OXT) release from isolated posterior pituitary lobes (PPL) of adult male Wistar rats was measured under basal and K+- stimulated conditions using a specific, sensitive radioimmunoassay. A basal release of 0.95 +/- 0.20 ng OXT/lobe/10 min was estimated in standard Locke's bathing solution. An excess of K+ (56 mmol/l) augmented the OXT secretion to 18.1 +/- 2.24 ng OXT/lobe/10 min in the presence of 2.2 mmol/Ca++. A stimulatory effect of K+ excess was also determined in Ca++-free medium and in Ca++ free medium enriched with 0.7 mmol/l EGTA. An inhibitory effect on K+-stimulated OXT release was achieved by raising the Mg++ concentration from 1.0-8.0 mmol/l of bathing fluid. During prolonged K+ stimulation the rate of OXT release declined exponentially. Estimation of the OXT content of PPLs after prolonged stimulation with K+ excess revealed that the lobes still contained 80% of their original OXT content.