Absence of arterial baroreflex modulation of skin sympathetic activity andsweat rate during whole-body heating in humans

1. Prior findings suggest that baroreflexes are capable of modulating skin blood flow, but the effects of baroreceptor loading/unloading on sweating a re less clear. Therefore, this project tested the hypothesis that pharmacol ogically induced alterations in arterial blood pressure in heated humans wo uld lead to baroreflex-mediated changes in both skin sympathetic, nerve act ivity (SSNA) and sweat rate. 2 In seven subjects mean arterial blood press-are was lowered (similar to8 mmHg) and then raised (similar to 13 mmHg) by bolus injections of sodium ni troprusside and phenylephrine, respectively. Moreover, in a separate protoc ol, arterial blood pressure was reduced via steady-state administration of sodium nitroprusside. In both normothermia and heat-stress conditions the f ollowing responses were monitored: sublingual and mean skin temperatures, h eart rate, beat-by-beat blood pressure, skin blood flow (laser-Doppler flow metry), local sweat rate and SSNA (microneurography from peroneal nerve). 3. Whole-body heating increased skin and sublingual temperatures, heart rat e, cutaneous blood flow, sweat rate and SSNA, but did not change arterial b lood pressure. Heart rate was significantly elevated (from 74 +/- 3 to 92 /- 4 beats min(-1); P < 0.001) during bolus sodium nitroprusside-induced re ductions in blood pressure, and significantly reduced (from 92 +/- 4 to 68 +/- 4 beats min(-1); P < 0.001) during bolus phenylephrine-induced elevatio ns in blood pressure, thereby demonstrating normal baroreflex function in t hese subjects. 4. Neither SSNA nor sweat rate was altered by rapid (bolus infusion) or sus tained (steady-state infusion) changes in blood pressure regardless of the thermal condition. 5. These data suggest that SSNA and sweat rate are not modulated by arteria l baroreflexes in normothermic or moderately heated individuals.