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

Cutaneous hyperemia in response to rapid skin local heating to 42 degrees 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 similar 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 degrees C, 39 degrees C, and 42 degrees C, and three rates of heating: 0.1 degrees C/s, 0.1 degrees C/10 s, 0.1 degrees 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 degrees C at 0.1 degrees C/s (protocol 2; N = 10) or 42 degrees C at 0.1 degrees C/min (protocol 3; N = 8). Rapid heating to 39 degrees 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 degrees C, but at 42 degrees C only 32.7 +/- 7.8% of vasodilation was attributable to NO. TEA + L-NAME attenuated CVC beyond L-NAME at temperatures >40 degrees C (43.4 +/- 4.5%CVCmax at 42 degrees C, P < 0.001 vs. L-NAME), suggesting a role of EDHFs at higher temperatures. Our findings suggest local heating to 39 degrees C offers an improved approach for isolating NO-dependent dilation and/or assessing perturbations that may improve microvascular function.