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 vis ceral 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 anaesthe sia. Pre-stimulation vasopressin and blood pressures demonstrated oestrous cycle variability. The mean (SEM) colonic balloon pressure triggering visce romotor responses was significantly higher in males (64 (4) mm Hg) than fem ales (41 (1) mm Hg), P=0.002 and within females, proestrus rats had the low est thresholds, (29 (1) mm Hg, P<0.01). Post-stimulation, vasopressin conce ntrations 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 l ow-pressure stimulation during proestrus shows sex-hormone dependent integr ation of the neuroendocrine response to noxious visceral stimulation.