New approach to measure cutaneous microvascular function: an improved test of NO-mediated vasodilation by thermal hyperemia

Abstract

Cutaneous hyperemia in response to rapid skin local heating to 42°C has been used extensively to assess microvascular function. However, the response is dependent on both nitric oxide (NO) and endothelial-derived hyperpolarizing factors (EDHFs), and increases cutaneous vascular conductance (CVC) to ∼90-95% maximum in healthy subjects, preventing the study of potential means to improve cutaneous function. We sought to identify an improved protocol for isolating NO-dependent dilation. We compared nine heating protocols (combinations of three target temperatures: 36°C, 39°C, and 42°C, and three rates of heating: 0.1°C/s, 0.1°C/10 s, 0.1°C/min) in order to select two protocols to study in more depth (protocol 1; N = 6). Then, CVC was measured at four microdialysis sites receiving: 1) lactated Ringer solution (Control), 2) 50-mM tetraethylammonium (TEA) to inhibit EDHFs, 3) 20-mM nitro-L-arginine methyl ester (L-NAME) to inhibit NO synthase, and 4) TEA+L-NAME, in response to local heating either to 39°C at 0.1°C/s (protocol 2; N = 10) or 42°C at 0.1°C/min (protocol 3; N = 8). Rapid heating to 39°C increased CVC to 43.1 ± 5.2%CVCmax (Control), which was attenuated by L-NAME (11.4 ± 2.8%CVCmax; P < 0.001) such that 82.8 ± 4.2% of the plateau was attributable to NO. During gradual heating, 81.5 ± 3.3% of vasodilation was attributable to NO at 40°C, but at 42°C only 32.7 ± 7.8% of vasodilation was attributable to NO. TEA+L-NAME attenuated CVC beyond L-NAME at temperatures >40°C (43.4 ± 4.5%CVCmax at 42°C, P < 0.001 vs. L-NAME), suggesting a role of EDHFs at higher temperatures. Our findings suggest local heating to 39°C offers an improved approach for isolating NO-dependent dilation and/or assessing perturbations that may improve microvascular function.

Keywords: axon reflex; endothelial function; endothelial-derived hyperpolarizing factors; laser-doppler flowmetry; nitric oxide.

Copyright © 2014 the American Physiological Society.

Figures

Fig. 1.

Plateau cutaneous vascular conductance across all nine local heating protocols. Protocols are combinations of three different rates of heating (0.1°C/s, 0.1°C/10 s, and 0.1°C/min) and three different target temperatures (36°C, 39°C, and 42°C). The heating protocol that has been used most commonly is indicated as the “standard protocol” (heating to 42°C at a rate of 0.1°C/s). Data are means ± SE. *P < 0.05 from all three protocols reaching 42°C, including the standard protocol; †P < 0.05 from all three protocols reaching 36°C. There were no significant differences between protocols at the same target temperature.

Fig. 2.

A: Representative response from one subject to rapid local heating to 39°C at a rate of 0.1°C/s; B: Average initial peak; and C: plateau cutaneous vascular conductance across the four microdialysis sites. Drugs include tetraethylammonium (TEA) and nitro-L-arginine methyl ester (L-NAME). Average baseline pooled across all four sites is represented by the dotted line. Data are means ± SE. *P < 0.05 from the Control site; †P < 0.05 from the TEA site; ‡P < 0.05 from the L-NAME site.

Fig. 3.

A: Representative response from one subject to gradual local heating to 42°C at a rate of 0.1°C/min. B: Average cutaneous vascular conductance at each 1°C increment in local temperature throughout the time period of gradual heating and during the prolonged plateau. Drugs include TEA and L-NAME. Data are means ± SE. *P < 0.05 from baseline (33°C) within sites.