The efferent mechanisms of reflex hemodynamic responses to distension of the descending colon in anesthetized dogs.

We have recently shown that distension of the descending colon in anesthetized dogs causes reflex increases in heart rate, aortic blood pressure and the maximal rate of rise of left ventricular pressure, involving afferent pathways in the hypogastric nerves. In this study we have examined the efferent mechanisms involved in those responses. The descending colon was distended using Ringer solution at constant pressure in 13 anesthetized dogs. As previously shown, distension of the colon caused an increase in aortic blood pressure, heart rate and the maximal rate of rise of left ventricular pressure. The increase in the aortic blood pressure was abolished by either bretylium tosylate or phentolamine. Propranolol or bretylium tosylate abolished the increases in heart rate when changes in the aortic blood pressure were prevented, and abolished the increases in the maximal rate of rise of left ventricular pressure when the increases in heart rate were also prevented. These results indicate that the reflex increases in heart rate, arterial blood pressure and the maximal rate of rise of left ventricular pressure involved efferent sympathetic pathways.