Dynamic and static baroreflex control of muscle sympathetic nerve activity (SNA) parallels that of renal and cardiac SNA during physiological change in pressure

Abstract

Despite accumulated knowledge on human baroreflex control of muscle sympathetic nerve activity (SNA), whether baroreflex control of muscle SNA parallels that of other SNAs, in particular renal and cardiac SNAs, remains unclear. Using urethane and alpha-chloralose-anesthetized, vagotomized and aortic-denervated rabbits (n = 10), we recorded muscle SNA from tibial nerve by microneurography, simultaneously with renal and cardiac SNAs by wire electrode. To produce a baroreflex open-loop condition, we isolated the carotid sinuses from systemic circulation and altered the intracarotid sinus pressure (CSP) according to a binary white noise sequence of operating pressure +/- 20 mmHg (for investigating dynamic characteristics of baroreflex) or in stepwise 20-mmHg increments from 40 to 160 mmHg (for investigating static characteristics of baroreflex). Dynamic high-pass characteristics of baroreflex control of muscle SNA, assessed by the increasing slope of transfer gain, showed that more rapid change of arterial pressure resulted in greater response of muscle SNA to pressure change and that these characteristics were similar to cardiac SNA but greater than renal SNA. However, numerical simulation based on the transfer function shows that the differences in dynamic baroreflex control at various organs result in detectable differences among SNAs only when CSP changes at unphysiologically high rates (i.e., 5 mmHg/s). On the other hand, static reverse-sigmoid characteristics of baroreflex control of muscle SNA agreed well with those of renal or cardiac SNAs. In conclusion, dynamic-linear and static-nonlinear baroreflex control of muscle SNA is similar to that of renal and cardiac SNAs under physiological pressure change.