Cholinergic neurogenic vasodilatation is mediated by nitric oxide in the dog hindlimb

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Abstract
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 α 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 [ΔAP: control = −21(SEM 3) mm Hg v NOLA treated = −9(3) mm Hg, p< 0.005] and the increase in femoral blood flow [ΔFBF: control = 43(7) ml·min−1 v NOLA treated = 17(4) ml·min−1, p< 0.005]. NOLA also significantly inhibited femoral vasodilator responses to acetylcholine [ΔFBF: control = 47(6) ml·min−1 v NOLA treated = 35(6) ml·min−1, p < 0.05] whereas responses to glyceryl trinitrate were enhanced [FBF: control = 54(9) ml·min−1 v 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 [ΔFBF: NOLA treated = 14(4) ml·min−1 v L-arginine treated = 35(8) ml·min−1, n = 8; greater than NOLA treated, pConclusions: 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.

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