Cold-induced sympathetic tone modifies the impact of endothelium-dependent vasodilation in the finger pulp

In thermoneutral and cold subjects, the sympathetic nervous system regulates skin blood flow by adjusting frequency of the tonic vasoconstrictor impulses. However, the way these thermoregulatory impulses influence the vascular endothelium is not well known. We studied how the sympathetic nervous system influences endothelium-dependent vasodilation (EDV) caused by shear stress in skin containing arteriovenous anastomoses (AVAs) and arterioles in healthy subjects.

Thirteen healthy subjects were exposed to thermoneutral (29°C) and cold (22°C) ambient temperatures on separate days. EDV was induced by releasing suprasystolic pressure cuff applied to the forearm or third finger after 4min. Bilateral laser Doppler flux from the finger pulp, dorsal finger and dorsal wrist was measured together with ultrasound Doppler from the right radial artery. Absolute EDV response (EDV peak minus baseline) and normalized relative EDV response (ratio EDV peak/baseline) were calculated (median, 95% confidence interval). The relative EDV response reflect the size of EDV response independent of the baseline level and is thus used to compare the EDV responses in the finger pulp and wrist skin in the two temperature conditions.

In finger pulp (dominated by AVAs), the absolute EDV response (flux, au) in thermoneutral (137.8 (67.5, 168.8)) and cold (130.3 (97.2, 154.9)) was the same (p=0.85), whereas the relative EDV response was significantly higher in cold (3.6 (2.5, 5.9)) than in thermoneutral (1.4 (1.1, 1.6), p=0.002). The same patterns were found in the radial artery. In the dorsal wrist (dominated by arterioles) the absolute EDV response (flux, au) was smaller in cold (30.9 (15.91, 38.0)) than in thermoneutral (52.1 (38.4, 57.8), p=0.04), whereas the relative EDV responses in cold (3.5 (2.3, 4.2)), and thermoneutral (2.3 (1.6, 2.7)) were equal (p=0.16).

The relative EDV responses show that the impact of EDV on skin perfusion in cold conditions is significantly greater in the finger pulp than in wrist skin. However, the absolute EDV responses indicate that vascular smooth muscle relaxation during EDV is probably not affected by higher mild cold-induced sympathetic activity either in AVAs or in arterioles.