Sex and oestrous cycle differences in visceromotor responses and vasopressin release in response to colonic distension in male and female rats anaesthetized with halothane.

Visceromotor responses and vasopressin release before and after colonic visceral distension were compared between male (n=5 (n=4 for vasopressin)) and female rats and between females during the oestrous cycle (proestrus n=6, oestrus n=5, metestrus n=5, diestrus n=6) at a controlled depth of anaesthesia. Pre-stimulation vasopressin and blood pressures demonstrated oestrous cycle variability. The mean (SEM) colonic balloon pressure triggering visceromotor responses was significantly higher in males (64 (4) mm Hg) than females (41 (1) mm Hg), P=0.002 and within females, proestrus rats had the lowest thresholds, (29 (1) mm Hg, P<0.01). Post-stimulation, vasopressin concentrations increased significantly in all groups (males 1.34 (0.39) to 2.24 (0.74) pmol litre(-1); females 1.54 (0.24) to 2.88 (0.58) pmol litre(-1); P=0.002). Within groups statistically significant differences were measured in proestrus 2.06 (0.56) to 3.42 (1.12) and oestrus 1.16 (0.38) to 2.76 (0.60) pmol litre(-1) (P<0.05). High vasopressin concentrations coupled with low-pressure stimulation during proestrus shows sex-hormone dependent integration of the neuroendocrine response to noxious visceral stimulation.