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

Visceromotor responses and vasopressin release before and aftercolonic visceral distension were compared between male (n=5(n=4 for vasopressin)) and female rats and between females duringthe oestrous cycle (proestrus n=6, oestrus n=5, metestrus n=5,diestrus n=6) at a controlled depth of anaesthesia. Pre-stimulationvasopressin and blood pressures demonstrated oestrous cyclevariability. The mean (SEM) colonic balloon pressure triggeringvisceromotor 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 significantlyin 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 measuredin 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 vasopressinconcentrations coupled with low-pressure stimulation duringproestrus shows sex-hormone dependent integration of the neuroendocrineresponse to noxious visceral stimulation. Br J Anaesth 2000; 85: 907–10