CHOLINERGIC NEUROGENIC VASODILATATION IS MEDIATED BY NITRIC-OXIDE IN THE DOG HINDLIMB

Objective: The aim was to investigate the role of nitric oxide (NO) in cholinergic neurogenic vasodilatation in the dog hindlimb using the NO synthase inhibitor, N-nitro-L-arginine (NOLA), and the NO precursor, L-arginine. Methods: 20 dogs were anaesthetised with thiopentone and alpha chloralose and experiments were performed in the presence of noradrenergic neurone blockade with guanethidine (15 mg.kg(-1) subcutaneously). Using stereotaxic procedures, specific sites in the hypothalamus were electrically stimulated (HS) to produce depressor and hindlimb vasodilator responses. In each experiment, responses to intra-arterial (ia) injections of acetylcholine and glyceryl trinitrate produced increases in femoral blood flow similar to those caused by HS. Results: Vasodilator responses to HS and acetylcholine but not glyceryl trinitrate were reduced by the muscarinic receptor antagonists tropicamide (3-12 mg ia) or atropine (0.5 mg.kg(-1) intravenously, iv). Administration of NOLA (5-15 mg.kg(-1) ia) significantly attenuated the HS induced decrease in arterial pressure [Delta AP: control =-21(SEM 3) mm Hg nu NOLA treated=-9(3) mm Hg, p < 0.005] and the increase in femoral blood flow [Delta FBF: control = 43(7) ml.min(-1) nu NOLA treated = 17(4) ml.min(-1), p < 0.005]. NOLA also significantly inhibited femoral vasodilator responses to acetylcholine [Delta FBF: control = 47(6) ml.min(-1) nu NOLA treated = 35(6) ml.min(-1), p<0.05] whereas responses to glyceryl trinitrate were enhanced [Delta FBF: control = 54(9) ml.min(-1) nu NOLA treated = 69(9) ml.min(-1), p < 0.005]. In addition L-arginine (150-300 mg.kg(-1) iv), but not D-arginine (150 mg.kg(-1) iv), reversed the inhibitory effect of NOLA on HS induced dilator responses [Delta FBF: NOLA treated = 14(4) ml.min(-1) nu L-arginine treated = 35(8) ml.min(-1), n = 8; greater than NOLA treated, p < 0.05]. Conclusions: Vasodilatation in the dog hindlimb evoked by activation of cholinergic nerves involves the synthesis of NO; however the source of this NO remains to be determined.