Head‐up tilt suppressed sympathetic nerve activity synchronized with cardiac cycle to skin but increased that to muscle in passively warmed men

We have suggested that a component of skin sympathetic nerve activity (SSNA) synchronized with cardiac cycle might be an active cutaneous vasodilator. However, the possibility that the component involves signals of muscle sympathetic nerve activity (MSNA) was not completely excluded. Since head‐up tilt (HUT) suppresses cutaneous vasodilation in hyperthermia while increases heart rate and arterial pressure, we hypothesized that 30°HUT reduced the SSNA component while enhanced MSNA with reduced right atrial and carotid artery distensions. In 12 men (22–42yr), wearing a perfusion suit, we measured right atrial volume (V RA ) and carotid artery diameter (CAD; echocardiography), esophageal temperature (T es ), SSNA and MSNA (microneurography; the peroneal nerve; N=7) during supine and also 30°HUT while T es was increased by 0.7°C with 47‐°C water perfusion into the suit. We found that HUT reduced the SSNA component (P=0.003) while enhanced MSNA (P=0.007) with reduced right atrial and carotid artery distensions (both P<0.0001). The latency of the SSNA component from R‐wave was 1.0s, shorter than that of MSNA (1.2s; P<0.01). Moreover, the latency of the SSNA component from peak of V RA and that of MSNA from valley of CAD were almost constant, 0.7s and 1.2s, respectively, as reported previously. Thus, we successfully distinguished the SSNA component from MSNA, which might be an active cutaneous vasodilator.