Cardiovascular changes in response to uterine stimulation

Abstract

These experiments examined cardiovascular responses to uterine horn distension in anesthetized rats. Changes in blood pressure and heart rate following distension of the uterine horn were compared in estrous and diestrous rats. In the estrous rats, there was a significant decrease in both heart rate and blood pressure following uterine horn pressure of 150 mmHg or greater. This depressor and bradycardia response did not reliably occur in the diestrous rats. To determine the afferent limb of this uterine-cardiovascular reflex, the pelvic and hypogastric nerves were cut consecutively and the effect of uterine horn distension was tested after each nerve cut. Following hypogastric, but not pelvic nerve transection, the cardiovascular responses to uterine horn distension were eliminated or severely attenuated. The importance of hypogastric afferent nerve fibers was further demonstrated by recording single unit activity in these fibers to uterine horn distension. The units responded with a pressure-dependent increase in firing rate, at uterine pressure levels of 50 mmHg and higher. To determine the efferent limb of the reflex, the cardiovascular responses were measured before and after vagus nerve transection. Elimination of this parasympathetic neural activity did not affect the inhibitory cardiovascular responses to uterine horn distension, suggesting that these fibers may not be responsible for the decrease in heart rate and blood pressure. Recordings of multi-unit activity from sympathetic efferent fibers in the renal nerve, a vasoconstrictor nerve, showed a decrease to uterine horn distension. A decrease in renal nerve efferent activity was observed at uterine horn pressures of 150 mmHg or greater and the time course of the renal nerve response paralleled the cardiovascular response. Thus, the results of these experiments indicate that uterine horn distension produces decreases in heart rate and blood pressure. These cardiovascular responses are more strongly and reliably evoked in the estrous rat compared to the diestrous rat. The afferent projection of this uterine-cardiovascular reflex is through the hypogastric nerve and the efferent pathway appeared to be through sympathetic, rather than parasympathetic, nerves.