RELATIVE CONTRIBUTIONS OF CARDIOPULMONARY AND SINOAORTIC BAROREFLEXESIN CAUSING SYMPATHETIC ACTIVATION IN THE HUMAN SKELETAL-MUSCLE CIRCULATION DURING ORTHOSTATIC STRESS

The aim of this study was to reexamine the hypothesis that cardiopulmo nary baroreflexes are more important than sinoaortic baroreflexes in c ausing vasoconstriction in the skeletal muscle circulation during orth ostatic stress. We recorded muscle sympathetic nerve activity (MSNA) w ith microelectrodes in the peroneal nerve (and forearm blood How with venous occlusion plethysmography) in normal subjects (innervated ventr icles) and in heart transplant recipients (denervated ventricles) duri ng graded lower body negative pressure (LBNP) performed alone and in c ombination with intravenous infusion of phenylephrine, which was titra ted to eliminate the orthostatically induced fall in blood pressure an d thus the unloading of both carotid and aortic baroreceptors. The pri ncipal new findings are as follows: (1) The increases in both MSNA and forearm vascular resistance during multiple levels of LBNP were not a ttenuated by heart transplantation, which causes ventricular but not s inoaortic deafferentation. (2) In heart transplant recipients, a small increase in MSNA during mild LBNP was dependent on a decrease in arte rial pressure, but in normal subjects, a similar increase in MSNA occu rred in the absence of any detectable decrease in the aortic pressure stimulus to the sinoaortic baroreceptors. (3) In normal subjects, the large increase in MSNA during a high level of LBNP was dependent on a decrease in arterial pressure and could be dissociated from the decrea se in central venous pressure. Taken together, the findings strongly s uggest that sinoaortic baroreflexes are much more important and ventri cular baroreflexes are much less important than previously thought in causing reflex sympathetic activation and vasoconstriction in the huma n skeletal muscle circulation during orthostatic stress.