Cutaneous vasoconstrictor responsiveness may be impaired by substance(s) directly or indirectly responsible for cutaneous active vasodilatation. In this study, we tested the hypothesis that endogenous nitric oxide (NO) attenuates the reduction in cutaneous vascular conductance (CVC) during an orthostatic challenge combined with whole-body heating, as well as during whole-body cooling. In protocol 1, healthy subjects were pretreated with an intradermal injection of botulinum toxin A (BTX) to block the release of neurotransmitters from nerves responsible for cutaneous active vasodilatation. On the experimental day, a microdialysis probe was placed at the BTX-treated site as well as at two adjacent untreated sites. NG-nitro-L-arginine methyl ester (L-NAME, 10 mM) was perfused through the probe placed at the BTX-treated site and at one untreated site. After confirmation of the absence of cutaneous vasodilatation at the BTX site during whole-body heating, adenosine was infused through the microdialysis probe at this site to increase skin blood flow to a level similar to that at the untreated site. Subsequently, 30 and 40 mmHg lower-body negative pressures (LBNPs) were applied. The reduction in CVC to LBNP was greatest at the BTX-treated site (15.0 ± 2.4% of the maximum level (% max)), followed by the L-NAME-treated site (11.3 ± 2.6% max), and then the untreated site (3.8 ± 3.0% max; P < 0.05 for all comparisons). In protocol 2, two microdialysis membranes were inserted in the dermal space of one forearm. Adenosine alone was infused at one site while the other site received adenosine and L-NAME. The reduction in CVC in response to whole-body cooling was significantly greater at the L-NAME-treated site than at the adjacent adenosine alone site. These results suggest that endogenous NO is capable of attenuating cutaneous vasoconstrictor responsiveness.
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