Sympathetic nerve activity and whole body heat stress in humans

Abstract

We and others have shown that moderate passive whole body heating (i.e., increased internal temperature ∼0.7°C) increases muscle (MSNA) and skin sympathetic nerve activity (SSNA). It is unknown, however, if MSNA and/or SSNA continue to increase with more severe passive whole body heating or whether these responses plateau following moderate heating. The aim of this investigation was to test the hypothesis that MSNA and SSNA continue to increase from a moderate to a more severe heat stress. Thirteen subjects, dressed in a water-perfused suit, underwent at least one passive heat stress that increased internal temperature ∼1.3°C, while either MSNA (n = 8) or SSNA (n = 8) was continuously recorded. Heat stress significantly increased mean skin temperature (Δ∼5°C, P < 0.001), internal temperature (Δ∼1.3°C, P < 0.001), mean body temperature (Δ∼2.0°C, P < 0.001), heart rate (Δ∼40 beats/min, P < 0.001), and cutaneous vascular conductance [Δ∼1.1 arbitrary units (AU)/mmHg, P < 0.001]. Mean arterial blood pressure was well maintained (P = 0.52). Relative to baseline, MSNA increased midway through heat stress (Δ core temperature 0.63 ± 0.01°C) when expressed as burst frequency (26 ± 14 to 45 ± 16 bursts/min, P = 0.001), burst incidence (39 ± 13 to 48 ± 14 bursts/100 cardiac cyles, P = 0.03), or total activity (317 ± 170 to 489 ± 150 units/min, P = 0.02) and continued to increase until the end of heat stress (burst frequency: 61 ± 15 bursts/min, P = 0.01; burst incidence: 56 ± 11 bursts/100 cardiac cyles, P = 0.04; total activity: 648 ± 158 units/min, P = 0.01) relative to the mid-heating stage. Similarly, SSNA (total activity) increased midway through the heat stress (normothermia; 1,486 ± 472 to mid heat stress 6,467 ± 5,256 units/min, P = 0.03) and continued to increase until the end of heat stress (11,217 ± 6,684 units/min, P = 0.002 vs. mid-heat stress). These results indicate that both MSNA and SSNA continue to increase as internal temperature is elevated above previously reported values.

Fig. 1.

Group (± SD) averaged skin sympathetic nerve activity (SSNA) data during normothermia, and mid and end of heat stress, expressed as total activity (arbitrary units) against increases in mean body temperature (T_b; A) or increases in core temperature (T_c; B). Regardless of how it is expressed, SSNA continues to increase throughout heat stress with no evidence of a plateau following the midlevel of heating. *P < 0.05 vs. normothermia, #P < 0.05 vs. mid-heat stress (n = 8).

Fig. 2.

Group (± SD) averaged muscle sympathetic nerve activity (MSNA) data, expressed as burst incidence (A), bursts per 100 cardiac cycles (B), and total activity (C), during normothermia (first data point) and at mid (second data point) and end (last data point) of heat stress. Regardless of how it is expressed, MSNA continues to increase throughout heat stress with no evidence of a plateau following the mid-level of heating. Delta T_c: Increase in core temperature measured from telemetry pill in the intestines. *P < 0.05 vs. normothermia, #P < 0.05 vs. mid heat stress (n = 8).