Abstract
The effects of colorectal distension on heart rate (HR) and mean arterial blood pressure (MAP) were examined in anesthetized rats. A 2 cm balloon was inserted into the colorectum through the anus; the tip of the balloon was placed 6 cm proximally to the anal verge. The balloon was distended by inflating it with air using a syringe, and intra-balloon pressures of 40, 60 or 80 mmHg were applied for 20 s. HR and MAP were decreased by colorectal distension when intra-balloon pressures went up to 60 and 80 mmHg, but not 40 mmHg. The decreases in HR and MAP to colorectal distension were accompanied by decreases in the activities of the cardiac sympathetic efferent nerve and the renal sympathetic nerve which are indexes of vasoconstrictor. In acutely spinalized rats at the C2 level, the depressor response elicited by noxious colorectal distension was reversed to pressor response which was accompanied by an increase in the renal sympathetic nerve activity, while the response of HR was abolished. We concluded that noxious colorectal distension elicits 1) reflex decreases in HR and MAP via depression of sympathetic efferent nerve activity, and its reflex center requires the presence of brain; 2) reflex increase in MAP via excitation of sympathetic efferent nerve activity in spinalized rats. When central nervous system is intact, this spinally induced pressor response to noxious colorectal distension appears to be suppressed by inhibitory pathways from the brain. [J Physiol Sci. 2007;57 Suppl:S81]
