Repeated warm water baths decrease sympathetic activity in humans

Abstract

Acute whole body heat stress evokes sympathetic activation. However, the chronic effects of repeated moderate heat exposure (RMHE) on muscle sympathetic nerve activity (MSNA) in healthy individuals remain unclear. We performed RMHE with 4 wk (5 days/wk) of warm baths (∼40°C, for 30 min) in nine healthy older (59 ± 2 yr) volunteers. Hemodynamic variables and MSNA were examined before, 1 day after, and 1 wk following 4 wk of RMHE in a laboratory at ∼23°C. Cold pressor test (CPT) and handgrip (HG) exercise were performed during the tests. Under normothermic condition, the resting MSNA burst rate (prior, post, post 1-wk: 31.6 ± 2.0, 25.2 ± 2.0, and 27.7 ± 1.7 bursts/min; P < 0.001) and burst incidence (P < 0.001) significantly decreased after RMHE. Moreover, the resting heart rate significantly decreased after RMHE (62 ± 2, 60 ± 2, and 58 ± 2 beats/min, P = 0.031). The sensitivity of baroreflex control of MSNA and heart rate were not altered by RMHE, although the operating points were reset. The MSNA and hemodynamic responses (i.e., changes) to handgrip exercise or cold pressor test were not significantly altered. These data suggest that the RMHE evoked by warm baths decreases resting sympathetic activity and heart rate, which can be considered beneficial effects. The mechanism(s) should be examined in future studies.NEW & NOTEWORTHY To our knowledge, this is the first study to observe the effects of repeated warm baths on sympathetic nerve activity during rest and stress in healthy middle age and older individuals. The data suggest that the repeated warm baths decreased resting sympathetic activity and heart rate, which can be considered beneficial effects. This study also provides the first evidence that the repeated warm baths did not alter the baroreflex sensitivity and the sympathetic responses to stress.

Keywords: autonomic; heart rate variability; heat therapy; stress; sympathetic.

Graphical abstract

Figure 1.

Mean and individual muscle sympathetic nerve activity (MSNA), mean arterial pressure (MAP), and heart rate (HR) under resting condition during pre-repeated moderate heat exposure (RMHE) (Pre), post-RMHE (Post), and post-1wk (Post-1wk) visits. The small symbols represent the individual data. P: P value of one-way repeated-measures ANOVA. Post hoc vs. Pre, ^aP = 0.040; ^bP = 0.045; ^cP < 0.001; ^dP = 0.001; ^eP < 0.001; and ^fP = 0.018.

Figure 2.

Mean and individual mean arterial pressure (MAP), heart rate (HR), muscle sympathetic nerve activity (MSNA) burst rate and incidence before and after the time control. The small symbols represent the individual data. P: P value of paired t test.

Figure 3.

Mean and individual spontaneous baroreflex sensitivity under resting conditions during the three visits. The small symbols represent the individual data. CBRS, cardiac baroreflex sensitivity. MBRS BI (bursts/100 beats/mmHg), MBRS area (U/mmHg), MBRS total (units/beats/mmHg): the sensitivity of baroreflex control of muscle sympathetic nerve activity (MSNA) burst incidence, burst area, and total activity. The sequence number detected for CBRS [Pre-repeated moderate heat exposure (RMHE), Post-RMHE, Post-1wk: 113 ± 23, 110 ± 17, and 119 ± 21, P = 0.730]. P: P value of one-way repeated-measures ANOVA. There were no significant differences among the three visits.

Figure 4.

Mean and individual responses of mean arterial pressure (MAP), heart rate (HR), and muscle sympathetic nerve activity (MSNA) burst rate and incidence to cold pressor test (CPT). The small symbols represent the individual data. PreCPT, PostCPT, Post-1wkCPT: the response by CPT during pre-repeated moderate heat exposure (RMHE), post-RMHE, and post-1wk visits. There were no significant differences among the three visits. P: P value of one-way repeated-measures ANOVA.

Figure 5.

Mean and individual responses of mean arterial pressure (MAP), heart rate (HR), and muscle sympathetic nerve activity (MSNA) burst rate and incidence to handgrip exercise and postexercise muscle ischemia (PECO) in the three visits. The small symbols represent the individual data. PreHG, PostHG, Post-1wkHG: the response by handgrip exercise during pre-repeated moderate heat exposure (RMHE), post-RMHE, and post-1wk visits. P: P value of one-way repeated-measures ANOVA. There were no significant differences during the three visits.