Potentiation by urethane and inhibition by pentobarbitone of oxytocin release in vitro.

Isolated rat neural lobes were incubated in vitro in Locke's solution containing anaesthetic quantities of urethane, pentobarbitone or tribromoethanol. The oxytocin content of the incubation medium was estimated before, during and after stimulation of the tissue by raising the potassium chloride concentration from 5-6 to 56 mmol/l. Urethane (25 mmol/l) significantly potentiated oxytocin release (P less than 0-01) whereas tribromoethanol (0-5 mmol/l) had no obvious effect and pentobarbitone (0-4 mmol/l) significantly (P less than 0-01) inhibited its release. Reduction of the sodium chloride concentration in the medium potentiated the release of oxytocin in each case but did not alter its pattern. Urethane which increased secretion of oxytocin also increased calcium ion uptake by the neural lobes and pentobarbitone which decreased oxytocin release decreased calcium ion uptake. The results may explain why the blood concentration of the neurohypophysial hormones tends to be higher in rats anaesthetized with urethane than with tribromoethanol. Inhibition of hormone release by pentobarbitone suggests that this anaesthetic is unsuitable for use in studies of neurohypophysial hormone release. A partial explanation of the anaesthetic properties of urethane and pentobarbitone may also have been found if the release of neurotransmitter substances is influenced in a similar manner.