In vitro simultaneous measurements of relaxation and nitric oxide concentration in rat superior mesenteric artery

Abstract

1. The relationship between nitric oxide (NO) concentration measured with an NO-specific microelectrode and endothelium-dependent relaxation was investigated in isolated rat superior mesenteric artery contracted with 1 μM noradrenaline.

2. Acetylcholine (10 μM) induced endothelium-dependent simultaneous increases in luminal NO concentration of 21 ± 6 nM, and relaxations with pD₂ values and maximum of 6.95 ± 0.32 and 97.5 ± 0.7 % (n = 7), respectively. An inhibitor of NO synthase, N^G-nitro-L-arginine (L-NOARG, 100 μM) inhibited the relaxations and increases in NO concentration induced by acetylcholine.

3. Oxyhaemoglobin (10 μM) reversed the relaxations and increases in NO concentrations induced by acetylcholine, S-nitroso-N-acetylpenicillamine (SNAP) and S-morpholino-sydnonimine (SIN-1), but not the relaxations induced with forskolin. Oxyhaemoglobin also decreased the NO concentration below baseline level.

4. In the presence of L-NOARG (100 μM), a small relaxation to acetylcholine (10 μM) of noradrenaline-contracted segments was still seen; oxyhaemogobin inhibited this relaxation and decreased the NO concentration by 14 ± 4 nM (n = 4).

5. The NO concentration-relaxation relationship for acetylcholine resembled that for SNAP and SIN-1 more than for authentic NO. Thus while 7-17 nM NO induced half-maximal relaxations in response to SNAP or SIN-1, 378 ± 129 nM NO (n = 4) was needed for half-maximal relaxation to authentic NO.

6. The present study provides direct evidence that the relaxation of the rat superior mesenteric artery with the endothelium-dependent vasodilator acetylcholine is correlated to the endogeneous release of NO. The study also suggests that NO mediates the L-NOARG-resistant relaxations in this artery, and that there is a basal NO release.