Differential effects of metaboreceptor and chemoreceptor activation on sympathetic and cardiac baroreflex control following exercise in hypoxia in human

Muscle metaboreceptors and peripheral chemoreceptors exert differential effects on the cardiorespiratory and autonomic responses following hypoxic exercise. Whether these effects are accompanied by specific changes in sympathetic and cardiac baroreflex control is not known. Sympathetic and cardiac baroreflex functions were assessed by intravenous nitroprusside and phenylephrine boluses in 15 young male subjects. Recordings were performed in random order, under locally circulatory arrested conditions, during: (1) rest and normoxia (no metaboreflex and no chemoreflex activation); (2) normoxic post-handgrip exercise at 30% of maximum voluntary contraction (metaboreflex activation without chemoreflex activation); (3) hypoxia without handgrip (10% O2 in N2, chemoreflex activation without metaboreflex activation); and (4) post-handgrip exercise in hypoxia (chemoreflex and metaboreflex activation). When compared with normoxic rest (-42 +/- 7% muscle sympathetic nerve activity (MSNA) mmHg(-1)), sympathetic baroreflex sensitivity did not change during normoxic post-exercise ischaemia (PEI; -53 +/- 9% MSNA mmHg(-1), P = 0.5) and increased during resting hypoxia (-68 +/- 5% MSNA mmHg(-1), P < 0.01). Sympathetic baroreflex sensitivity decreased during PEI in hypoxia (-35 +/- 6% MSNA mmHg(-1), P < 0.001 versus hypoxia without exercise; P = 0.16 versus normoxic PEI). Conversely, when compared with normoxic rest (11.1 +/- 1.7 ms mmHg(-1)), cardiac baroreflex sensitivity did not change during normoxic PEI (8.3 +/- 1.3 ms mmHg(-1), P = 0.09), but decreased during resting hypoxia (7.3 +/- 0.8 ms mmHg(-1), P < 0.05). Cardiac baroreflex sensitivity was lowest during PEI in hypoxia (4.3 +/- 1 ms mmHg(-1), P < 0.01 versus hypoxia without exercise; P < 0.001 versus normoxic exercise). The metaboreceptors and chemoreceptors exert differential effects on sympathetic and cardiac baroreflex function. Metaboreceptor activation is the major determinant of sympathetic baroreflex sensitivity, when these receptors are stimulated in the presence of hypoxia.