Relative contributions of cardiopulmonary and sinoaortic baroreflexes in causing sympathetic activation in the human skeletal muscle circulation during orthostatic stress.

The aim of this study was to reexamine the hypothesis that cardiopulmonary baroreflexes are more important than sinoaortic baroreflexes in causing vasoconstriction in the skeletal muscle circulation during orthostatic stress. We recorded muscle sympathetic nerve activity (MSNA) with microelectrodes in the peroneal nerve (and forearm blood flow with venous occlusion plethysmography) in normal subjects (innervated ventricles) and in heart transplant recipients (denervated ventricles) during graded lower body negative pressure (LBNP) performed alone and in combination with intravenous infusion of phenylephrine, which was titrated to eliminate the orthostatically induced fall in blood pressure and thus the unloading of both carotid and aortic baroreceptors. The principal new findings are as follows: (1) The increases in both MSNA and forearm vascular resistance during multiple levels of LBNP were not attenuated by heart transplantation, which causes ventricular but not sinoaortic deafferentation. (2) In heart...