Acetyicholine-induced contraction in the rat vas deferens: diurnal variations Carneiro, R.C.G., Cipolla-Neto *, J. and Markus, R.P. Institute of Biomedical Science University of Sao Paulo, Dept. of Pharmacology and * Dept. of Physiology. Av. Lineu Prestes 1524, 05508, Sao Paulo, Brasil
In rat isolated vas deferens the effects of cholinomimetic drugs are controversial. While some authors observed modulation effects of cholinomimetic drugs on noradrenaline release (Lee et al., 1986), others showed that contraction induced by these agonists was higher than contraction induced by sympathomimetic agonists (Brito et al., 1989). This study was undertaken to characterize cholinomimetic-induced contractions and to explore a circadian variation of this response in the prostatic portion of rat isolated vas deferens (PRVD). Male Wistar rats (4 months) were placed on a light/dark cycle (12:20 h, lights on 6 : 0 0 h), two weeks before starting the experiments. The mechanism of acetylcholine (Ach)-induced contraction and the diurnal variation of this response was determinated in animals killed at 9 : 0 0 h and every three hours from 6 : 0 0 to 24:00 h, respectively. Isometric contraction of PRVD bathed in nutritive solution (Brito et al., 1989) was recorded at 30°C under a tension of 0.5 g. Single concentration-effect curves to Ach (10/tM up to 1.0 mM) were recorded each 10 rain. The antagonists were incubated for 60 rain. Hexamethonium (0.1 raM), but not atropine (up to 0.3/tM), reduced the Ach-induced contraction. Reserpine-treatment (10 mg/kg, i.p. 24 h), prazosin- (1.0 nM up to 1.0/tM) and a,fl-methylene ATP-incubation (3.0/tM) attenuated the response up to 50~;. Simultaneous blockade of a-adrenergic and purinergic receptors completely blocked the response, suggesting that Ach releases noradrenaline and ATP in PRVD. The sensitivity (PD2) to Ach-induced response and the maximal contraction to noradrenaline and K + did not change throughout the day. Otherwise, the maximal response ~howed a hour of the day effect (table 1) (ANOVA P < 0.001). The best-fitting cosine curve (tested for amplitude different from zero) shows a statistically significant rhythm of 12:20 h (P = 0.01) (ultradian rhythm) with peak~ at maximal response during the mght (CI 9570, 22 : 30-6 : 09 h). Table 1 Ach-induced contraction (g) in PRVD. hours
contraction (g) (s.e. mean)
These results show that in PRVD probably Ach stimulates nicotinic receptors inducing relea~ .)f noradrenaline and ATP from sympathetic neurones. The contractile response induced by Ach shows a diurnal variation modulated by a circadian (24:00 h) and an ultradian (12:20 h) rhythm. The increase in cholinergic response by night is coincident with the increase in sexual activity. Supported by FAPESP, CNPq. References Bfito, A.R.M.S., Medeiros, I.A., Markus, R.P., 1989, Gen. Pharmacol. 20, 65. Lee, C.N., 1985, Br. Pharmacol. 86, 671.