Flow-dependence of vasodilatory response in renal vascular bed to hemorrhage in dogs.

The vasodilatory response in the renal vascular bed was studied after quick mild hemorrhage under different levels of renal blood flow. In neuraxis intact dogs, the renal vascular resistance increased or decreased within 5 s after hemorrhage and reached a stable level close to the control within 10 s. The responses under neuraxis intact conditions may involve both responses of the renal vascular resistance to changes in renal sympathetic nerve activity caused by the baroreceptor reflex and responses to decrease in renal perfusion pressure after hemorrhage. In order to exclude baroreflex neural regulation of renal blood flow, we sectioned the carotid sinus and vagus nerves. The renal vascular resistance decreased within 5 s after hemorrhage, and was maintained at around this level for about 2 min. The decrease in renal vascular resistance and control (prehemorrhage) renal blood flow revealed a significant hyperbolic relationship. Since the systemic arterial pressure was not restored after hemorrhage and reflexogenic changes in sympathetic nerve activity were excluded after barodenervation, the vasodilatory response in the renal vascular bed after hemorrhage should reflect only hemodynamic changes. We conclude that the vasodilatory response to pressure fall in the renal vascular bed increases with a decrease in the control renal blood flow.