Arterial baroreflex inhibition by uterine distension in rats

Arterial baroreflexes are inhibited during activation of some viscero-sensory receptors. The present study was designed to determine whether stimulation of mechanoreceptors of the uterus also inhibits the baroreflex bradycardia (BB). The aortic depressor nerve (ADN) was electrically stimulated to elicit BB in chloralose/urethane-anesthetized, succinylcholine-immobilized, and artificially ventilated rats. Hydraulic distension of the uterus with warm 0.9% NaCl solution was found to suppress BB with a threshold intra-uterine pressure of 25-100 mmHg. The inhibition was variably affected by successive transections of the hypogastric nerve (HgN) and the pelvic nerve (PN). In 6 rats out of the 16 tested, preceding transection of either HgN or PN (HgN, 4; PN, 2) abolished the inhibition, whereas in 8 rats the inhibition was not affected by preceding transection of either nerve but was abolished by subsequent transection of the other nerve (PN, 3; HgN, 5). In 2 rats, preceding HgN or PN section reduced the inhibition and subsequent PN or HgN section completed the withdrawal of the inhibition. Recording study confirmed that both the HgN and PN contain afferent fibers, signalling mechanoceptive information arising in the uterine wall. Electrical stimulation of the HgN as well as the PN remarkably suppressed the BB. In conclusion, uterine distension suppresses the BB and this inhibition is mediated by afferent fibers running in the HgN and PN with variable contributions among animals. The reflex suppression of BB due to uterine sensory activation may contribute to cardiovascular regulation during parturition.