Nitric oxide requirement for vasomotor nerve‐induced vasodilation and modulation of resting blood flow in muscle microcirculation

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Abstract
Intravital microscopy of rabbit tenuissimus muscle was used for studies of endogenous nitric oxide as a microvascular regulator in vivo. Derivatives of arginine were administered in order to modulate the formation of nitric oxide from L-arginine. N omega-nitro-L-arginine methylester (L-NAME) (1-100 mg kg-1 i.v.) dose-dependently reduced microvascular diameters. A concomitant blood pressure increase and a decrease in heart rate was observed. The blood pressure increase induced by L-NAME (30 mg kg-1) was reversed by L-arginine (1 g kg-1) but not D-arginine. Vasodilation in response to topical acetylcholine (0.03-3 microM) was significantly inhibited by L-NAME (30 mg kg-1), whereas vasodilation by sodium nitroprusside (300 nM) was not affected. Vasomotor nerve-induced vasodilatation, induced by stimulation of the tenuissimus nerve after neuromuscular blockade by pancuronium in animals pretreated with guanethidine, was significantly attenuated by L-NAME, an effect also reversed by L-arginine. The vasodilatation in response to active contractions of the muscle induced by motor nerve stimulation as well as the vasodilator response elicited by graded perfusion pressure reductions were unaffected by L-NAME or NG-monomethyl-L-arginine (L-NMMA, 10(-4) M) administered topically. Our results indicate that endogenous nitric oxide formed from L-arginine is a modulator of microvascular tone in vivo. Furthermore, the results suggest that endogenous nitric oxide is required for vasomotor nerve-induced vasodilatation, whereas it does not appear to play a role in myogenic vasodilatation or functional hyperaemia in this tissue.