Changes in posterior pituitary oxytocin release in vitro during the estrous cycle of female rats

The in vitro release of oxytocin (OXT) from posterior pituitary lobes (PPL) of adult female rats is linked to the stage of the animals' sexual cycles. After incubation in normal Locke's solution (K+ 5,6 mM), the basal OXT release from the PPL of rats on diestrus 1 and estrus amounted to 2.25 +/- 0.53 ng/lobe/10 min (mean +/- S.E.M.) and 4.71 +/- 0.61 ng/lobe/10 min, respectively. Excess K+ (56 mM/l) in the presence of Ca++ (2.2 mM/l) increased OXT liberation from the PPl of diestrous and estrous rats to 12.41 +/- 2.65 ng/lobe/10 min und 36.33 +/- 6.18 ng/lobe/10 min, respectively. When Ca++ was omitted from the incubation medium, the K+-excess induced OXT release from the PPL of estrous rats decreased to 21.54 +/- 2.65 ng/lobe/10 min whereas no change occurred in the OXT-release from the rats' PPL on diestrus 1 in the presence of Ca++. The results indicate a cycle-dependent release and suggest that OXT plays a role in the reproductive processes.