Increase of cardiodepressant activity in medium incubating the posterior pituitary lobe in situ during vagal nerve stimulation in rat.

Previous studies have indicated that there is a cardiodepressant factor in the medium incubating the posterior pituitary lobe in situ. The cardiodepressant activity of the medium incubating the posterior pituitary lobe before and during stimulation of the vagus nerves was tested on isolated auricles of the right heart atrium of a two-day-old rat. It was found that the medium incubating the posterior pituitary lobe collected before stimulation decreased the contraction rate of the auricle by 34%, while that collected during the intermittent stimulation of the central ends of the cut vagus nerves caused a decrease of the auricle contractions frequency by 52%. The addition of cholinergic, serotoninergic, histaminergic receptor blockers or prostaglandin synthetase into Ringer-Lock's solution bathing the auricle has no effect on the changes of the contraction rate caused by the incubation medium.