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Segments of ileum from newborn rabbits contracted in response to histamine but sensitivity declined from age 11 days. Intestine from adult animals barely contracted to histamine, even in a concentration of 3 times 10- minus 3 M. Such variation with age was not observed with acetylcholine. The response to histamine was effected by mepyramine-sensitive receptors which appeared to decrease in number as the animals aged.

1 The inhibitory effect of a commercial preparation of oxytocin (Syntocinon) was studied on the isolated vas deferens of the rat. 2 The inhibition of contractions to agonists and to field stimulation obtained were mimicked by appropriate dilutions of chlorbutol, a preservative present in Syntocinon. 3 Preservative-free synthetic oxytocin had no inhibitory effect on the tissue and slightly potentiated contractions evoked by field stimulation. 4 It is concluded that inhibitory effects of Syntocinon on the vas deferens are due to chlorbutol, not oxytocin.

1 Guinea-pig ileum suspended in Krebs solution showed a continuous increase of tone which was lost by changing the bath fluid. Prostaglandin E(2) was released from the ileum during incubation, and its concentration in the bath fluid appeared to correlate with the increase in tone.2 Supramaximal field stimulation (10 Hz) resulted in increased release of prostaglandin E(2) from the ileum. At lower rates of stimulation, the increase in the release of E(2) compared with the resting output was not significant.3 Indomethacin (1 and 10 mug/ml) produced a significant, dose-related reduction of the amount of prostaglandin E(2) measured in the bath fluid at rest and with field stimulation. Indomethacin inhibited the contraction of the ileum during incubation in Krebs solution.4 Indomethacin (10 and 20 mug/ml) had no significant effect on the release of acetylcholine during field stimulation, but reduced the resting output of acetylcholine from guinea-pig ileum in some experiments.5 The results are discussed in the context of the role ascribed to prostaglandins as physiological modulators in transmitter release. No evidence for a prostaglandin-mediated negative feedback mechanism on acetylcholine release was obtained.

1 Ligation of a coronary artery was performed in conscious rats whose sympathetic system activity had been altered by various treatments. 2 beta-Adrenoceptor blockade with acute (0.2 mg kg-1 plus 0.1 micrograms kg-1 min-1) or chronic (50-60 mg kg-1 daily for 12 days) propranolol treatment had little effect on arrhythmias, or other responses to ligation. 3 Abrupt withdrawal of chronic propranolol two days before ligation was also without effect. 4 Reduction of sympathetic activity acutely with labetalol (5 mg kg-1), or chronically with adrenomedullectomy and 6-hydroxydopamine treatment, accentuated the adverse effects of ligation. 5 The results of this study suggest that, while activity of the sympathetic system is not detrimental during ligation in the conscious rat, it may be important for survival.