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We investigated the effect of hypoxia on acetylcholine (ACh) stimulated, endothelium-derived relaxing factor/nitric oxide (EDRF/NO)-dependent relaxation, and on basal tension in rat aortic rings. ACh (10(-9)-10(-6) M)-mediated relaxation at high [95%, Emax -76.2 +/- 4.5% of phenylephrine (PE)-induced constriction] and normal (20%, Emax -81.2 +/- 3.6%) O2 levels was inhibited by hypoxia (5%, Emax -36.2 +/- 7.2%); residual hypoxic relaxation was blocked by the K+ channel antagonist glibenclamide. To address whether O2 influenced EDRF/NO and K+ channel contributions to basal tone, the effect of stepwise reduction of available O2 (95, 20, 5, and 0%) was studied in intact and endothelial cell (EC)-denuded rings. The effects in these rings were compared with results of the same progressive reduction in O2 in the presence of the NO-synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) (10(-4) M) or glibenclamide (10(-4) M). EC-intact and EC-denuded rings constricted to 0.80 +/- 0.10 and 1.41 +/- 0.15 g, respectively. Reducing O2 to 20% had no significant effect on vascular tension, but 5% caused constriction (p < 0.05) in EC-intact rings (0.90 +/- 0.15 g). This hypoxic vasoconstriction was blocked by L-NAME, but not by glibenclamide, suggesting that hypoxic vasoconstriction was mediated by withdrawal of EDRF/NO. In contrast, EC-denuded rings showed a significant relaxant response at 5% O2. When O2 was then reduced further (95% N2/5% CO2), both EC-intact and EC-denuded rings relaxed, and this relaxation reached baseline tension (0.10 +/- 0.1 g).(ABSTRACT TRUNCATED AT 250 WORDS)

We investigated the responses of canine coronary rings to endothelium-derived relaxing factor-nitric oxide- (EDRF-NO) dependent agonists and NO synthase (NOS) inhibitors 3 h after endotoxic shock was induced in dogs by lipopolysaccharide infusion (LPS; 2 mg/kg). EDRF-NO-dependent relaxation to thrombin [control maximum response produced after administration of thrombin (Emax) was -85.2 +/- 7.0% of the constrictor response produced by the thromboxane analogue U-46619], acetylcholine (control Emax -88.4 +/- 3.4%), or bradykinin (control Emax -80.5 +/- 2.2%) was not inhibited by LPS (Emax thrombin -75.9 +/- 9.5%; Emax acetylcholine -90.2 +/- 2.4%; Emax bradykinin -91.6 +/- 3.4%). The NOS inhibitor NG-monomethyl-L-arginine (L-NMMA) (10(-6)-3 x 10(-4) M) caused constriction of rings with endothelium (Emax 36.3 +/- 5.6%), an effect that was greater after LPS (Emax 59.2 +/- 4.1%; P < 0.05). D-NMMA had no effect in control, but it increased tension after LPS (Emax 20.8 +/- 9.7%). Contrary to expectations, L- and D-NMMA relaxed endothelium-denuded rings (-30.4 +/- 8.7% L-NMMA; -45.1 +/- 11.7% D-NMMA; P < 0.05). However, neither agent caused relaxation after in vivo LPS (10.2 +/- 3.4% L-NMMA; 8.9 +/- 5.2% D-NMMA). N omega-nitro-L-arginine-methylester (L-NAME) and nitro-L-arginine (10(-6)-3 x 10(-4) M) increased tension (Emax 82.3 +/- 23.9 and 73.1 +/- 8.8%, respectively) but only when endothelium was present, and the increases were no greater in LPS-treated groups than in controls (with LPS: Emax L-NAME 87.3 +/- 16.5%; Emax nitro-L-arginine 65.7 +/- 3.3%).(ABSTRACT TRUNCATED AT 250 WORDS)

The actions of NG-monomethyl-L-arginine (L-NMMA) and N omega-nitro L-arginine methyl ester (L-NAME) on canine coronary arterial rings were compared with effects on rat aortic rings. After incubation of rat aortic rings with indomethacin (5 x 10(-6) M) and preconstriction with phenylephrine (10(-7) M), L-NMMA (2.5 x 10(-4) M) caused an increase in tension when endothelium remained intact (+1.1 +/- 0.2 g). L-NMMA had no effect when endothelium was absent. After incubation of canine coronary arterial rings with indomethacin and preconstriction with prostaglandin F2 alpha (10(-6) M), L-NMMA (2.5 x 10(-4) M) increased tension (+39.9 +/- 7.9% of PGF2 alpha-induced constriction) when endothelium was intact, but L-NMMA caused a significant reduction in tension when endothelium was removed (Emax -52.2 +/- 10.3%; P < .05). The reduction in tension after L-NMMA was greater in the absence of indomethacin (Emax -79.8 +/- 4.1%; P < .05). It has been suggested previously that biotransformation of L-NMMA to L-arginine may have contributed to vasorelaxation; L-arginine is the endogenous substrate of nitric oxide synthase. However L-arginine (10(-3) M) did not affect the fall in tension produced by L-NMMA (Emax -69.0 +/- 14.2% in the absence of indomethacin). We also found that incubation with the protein synthesis inhibitor, cycloheximide, did not block the L-NMMA-induced fall in vascular tension; in fact, it increased the magnitude of the relaxant effect (-95.4 +/- 2.5%, experiments performed without indomethacin).(ABSTRACT TRUNCATED AT 250 WORDS)