ESTROUS-CYCLE PHASE VARIATIONS IN VISCEROMOTOR AND CARDIOVASCULAR-RESPONSES TO COLONIC DISTENSION IN THE ANESTHETIZED RAT

Visceromotor and cardiovascular responses to colonic distension were m easured in female rats, anesthetized with halothane in oxygen, in the proestrus, estrus, metestrus and diestrus phases of the estrous cycle. Ten rats were studied in each group and responses were measured at 5- min intervals for 60 min. A mixed model analysis of variance showed th at there was no real change in either the visceral or cardiovascular r esponse with time. There was a highly significant difference in viscer omotor responses between the phases of the estrous cycle (P < 0.001). During the phase of proestrus the balloon pressure at which a response was triggered was much lower, with a mean value (95% confidence inter val) of 18.7 (16.1, 21.8) mmHg, than the other phases with mean values (95% confidence interval) of 31.9 (27.4, 37.2) mmHg for estrus, 28.1 (24.2, 32.8) mmHg for metestrus, and 31.1 (26.7, 36.3) mmHg for diestr us. The mean arterial blood pressure increased in all groups (range 3. 2, 5.4%) as a response to the stimulus, but there was no associated he art rate variability and no significant differences in cardiovascular changes between the groups (P = 0.6). The visceromotor responses measu red during the phase of proestrus occurred at a significantly lower th reshold than in the other phases of estrous.