Nitric oxide synthase inhibition does not alter the reactive hyperemic response in the cutaneous circulation

Abstract

Reactive hyperemia is the sudden rise in blood flow after release of an arterial occlusion. Currently, the mechanisms mediating this response in the cutaneous circulation are poorly understood. The purpose of this study was to 1). characterize the reactive hyperemic response in the cutaneous circulation and 2). determine the contribution of nitric oxide (NO) to reactive hyperemia. Using laser-Doppler flowmetry, we characterized reactive hyperemia after 3-, 5-, 10-, and 15-min arterial occlusions in 10 subjects. The total hyperemic response was calculated by taking the area under the curve (AUC) of the hyperemic response minus baseline skin blood flow (SkBF) [i.e., total hyperemic response = AUC - [baseline SkBF as %maximal cutaneous vascular conductance (CVC(max) x duration of hyperemic response in s]]. For the characterization protocol, the total hyperemic response significantly increased as the period of ischemia increased from 5 to 15 min (P < 0.05). However, the 3-min response was not significantly different from the 5-min response. In the NO contribution protocol, two microdialysis fibers were placed in the forearm skin of eight subjects. One site served as a control and was continuously perfused with Ringer solution. The second site was continuously perfused with 10 mM NG-nitro-l-arginine methyl ester (l-NAME) to inhibit NO synthase. CVC was calculated as flux/mean arterial pressure and normalized to maximal blood flow (28 mM sodium nitroprusside). The total hyperemic response in control sites was not significantly different from l-NAME sites after a 5-min occlusion (3261 +/- 890 vs. 2907 +/- 531% CVC(max. s). Similarly, total hyperemic responses in control sites were not different from l-NAME sites (9155 +/- 1121 vs. 9126 +/- 1088% CVC(max. s) after a 15-min arterial occlusion. These data suggest that NO does not directly mediate reactive hyperemia and that NO is not produced in response to an increase in shear stress in the cutaneous circulation.

Physiological hyperinsulinaemia increases intramuscular microvascular reactive hyperaemia and vasomotion in healthy volunteers

“…The mechanisms responsible for the increase in vascular perfusion after arterial occlusion are not fully understood. The accumulation of metabolic vasodilators, myogenic and/or neurogenic mechanisms may play a role [18]. It seems unlikely that hyperinsulinaemia decreases sensitivity to vasoconstrictive effects, because, both in the isolated rat arteriole and in the human forearm, hyperinsulinaemia has been shown to increase vasoconstrictive effects by inducing endothelin-1 activity [19,20].…”

“…Finally, we clamped at euglycaemia (i.e. at 5 mmol/l [18]) instead of at isoglycaemia, which resulted in slightly higher glucose levels during insulin than during saline infusion. It is not clear whether this difference affected our results, but because hyperglycaemia impairs microvascular function [47,48], we may, if anything, have somewhat underestimated insulin-mediated effects on microvascular function.…”

Assessment of Human Skin Microcirculation and Its Endothelial Function Using Laser Doppler Flowmetry

“…Yet, the results on the contribution of different endotehlial vasodilators are unequivocal (Bingelli et al, 2003;DalleAve et al, 2004;Durand et al, 2004;;Medow et al, 2007;Wong et al, 2003).…”

“…Indices that are assessed most commonly are: the maximal (peak) LDF response after occlusion release (LDF peak ), the time to reach the peak response (t peak ), the half time in which the LDF returns to 50% of the baseline, the duration of hyperemia (time to recovery, t rec ) and the area under the curve (AUC) (Fig.4). The response of the microvasculature strongly dependes on the occlusion time (Wong et al, 2003;Yvonne-Tee et al, 2008). PRH has also been used to induce or amplify the vasomotion in skin microcirculation (Rossi et al, 2008).…”

Methods for Evaluating Endothelial Function in Humans

“…Second, poor interassay and intra-assay reproducibility and lack of standardization (eg, site of the skin measurement) limit within-patient and across-studies comparisons. Third, and most important, recent insights into the mechanisms of the postocclusive hyperemia 72 and acetylcholinemediated dilatation 73 indicate that these phenomena are not primarily NO mediated, suggesting that they might represent a summation of complex, microvascular responses involving sensory nerves and metabolic and endothelial (independent from NO) vasodilators. Therefore, rather than representing specific markers of endothelial function, these tests provide a more global form of assessing microvascular function.…”