1. The effects of N(ω)-nitro-L-arginine (L-NOARG) and N(ω)-nitro-L-arginine methyl ester (L-NAME) on diaphragmatic microcirculation in male Sprague-Dawley rats were assessed under basal conditions and after acetylcholine (ACh) stimulation. In addition, L-arginine (L-arg) was used with the aim of preventing L-NOARG and L-NAME from inhibiting ACh-induced vasodilatation, in order to explore the possibility that L-NOARG is not only a nitric oxide (NO) synthase inhibitor but also a muscarinic receptor antagonist. 2. Male Sprague-Dawley rats were anaesthetized with urethane and mechanically ventilated. The left hemi-diaphragm of each rat was prepared and microvascular blood flow was recorded during continuous superfusion with bicarbonate-buffered prewarmed Ringer solution by using laser-Doppler flowmetry. The drugs were topically applied to the surface of the hemi-diaphragm. 3. Baseline microvascular blood flow was unaffected after 15 min superfusion with any one of the following agents: L-NOARG (0.1 mM), L-NAME (0.1 mM), L-arg (10 mM). 4. ACh (0.03 mM, 0.1 mM and 0.3 mM) elicited a significant increase of microvascular blood flow (171 ± 16%, 214 ± 55%, and 323 ± 68% of baseline values, respectively), via interaction with the muscarinic receptor, for the vasodilator response was severely inhibited by 15 min superfusion with atropine (0.3 mM). 5. Following 15 min superfusion with either of the L-arg analogues (0.1 mM), the ACh-induced vasodilator response was significantly inhibited. Pretreatment with L-arg (10 mM) for 5 min, followed by co-administration of L-arg (10 mM) and L-NOARG (0.1 mM) for another 15 min significantly prevented the inhibitory effect of L-NOARG on ACh-induced vasodilatation. However, a similar pretreatment schedule with L-arg failed to prevent L-NAME from exerting its inhibitory effect. 6. Neither of the L-arg analogues potentiated sodium nitroprusside (10 μM and 30 μM)-induced vasodilatation, However, adenosine (0.1 mM)-induced vasodilatation was slightly but significantly attenuated by either L-NOARG (0.1 mM) or L-NAME (0.1 mM), an effect which was prevented by L-arg (10 mM). 7. In conclusion, an increase in endothelium-dependent blood flow stimulated by ACh may occur in diaphragmatic microcirculation of anaesthetized rats independently of low baseline NO activity. The results also suggest that L-NAME has muscarinic receptor antagonist action in addition to its ability to inhibit NO synthase. Thus, we suggest that L-NAME should not be used as a specific NO synthase inhibitor in the rat diaphragm in situations in which there is potential for muscarinic receptors to be stimulated.
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