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

Abstract

1. Prior findings suggest that baroreflexes are capable of modulating skin blood flow, but the effects of baroreceptor loading/unloading on sweating are less clear. Therefore, this project tested the hypothesis that pharmacologically induced alterations in arterial blood pressure in heated humans would lead to baroreflex-mediated changes in both skin sympathetic nerve activity (SSNA) and sweat rate. 2. In seven subjects mean arterial blood pressure was lowered (approximately 8 mmHg) and then raised (approximately 13 mmHg) by bolus injections of sodium nitroprusside and phenylephrine, respectively. Moreover, in a separate protocol, arterial blood pressure was reduced via steady-state administration of sodium nitroprusside. In both normothermia and heat-stress conditions the following responses were monitored: sublingual and mean skin temperatures, heart rate, beat-by-beat blood pressure, skin blood flow (laser-Doppler flowmetry), local sweat rate and SSNA (microneurography from peroneal nerve). 3. Whole-body heating increased skin and sublingual temperatures, heart rate, cutaneous blood flow, sweat rate and SSNA, but did not change arterial blood pressure. Heart rate was significantly elevated (from 74 +/- 3 to 92 +/- 4 beats x min(-1); P < 0.001) during bolus sodium nitroprusside-induced reductions in blood pressure, and significantly reduced (from 92 +/- 4 to 68 +/- 4 beats x min(-1); P < 0.001) during bolus phenylephrine-induced elevations in blood pressure, thereby demonstrating normal baroreflex function in these subjects. 4. Neither SSNA nor sweat rate was altered by rapid (bolus infusion) or sustained (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 arterial baroreflexes in normothermic or moderately heated individuals.

Figure 1. Effects of baroreceptor modulation via bolus sodium nitroprusside (NP) and phenylephrine (PE) administration on mean arterial blood pressure (MAP) and normalized skin sympathetic nerve activity (SSNA) during both normothermia and hyperthermia

Baseline (Base) prior to each thermal condition was used for normalization (i.e. 100 %). Regardless of the thermal conditions, SSNA was unaffected by baroreceptor unloading or loading. Values are expressed as means ±s.e.m.

Figure 2

Representative data from one subject during bolus sodium nitroprusside (NP) and phenylephrine (PE) infusions on skin sympathetic nerve activity (SSNA), heart rate (HR), and arterial blood pressure (BP) during normothermia (A) and during heat stress (B)

Figure 3

Representative data from one subject during whole-body cooling after the heat stress on mean skin temperature (T_sk), skin sympathetic nerve activity (SSNA), skin blood flow (SkBF) and sweat rate (SR). Sublingual temperature was not altered during the period represented by this figure.