1 New potent inhibitors of nitric oxide synthase (NOS) may be useful in the treatment of septic shock, a disorder characterized by a vascular hyporeactivity to catecholamines caused by an overproduction of nitric oxide (NO-). We examined the effects of L-thiocitrulline (L-TC) and S-methyl-L-thiocitrulline (L-SMTC), novel NOS inhibitors, on the constitutive and inducible NOS in rat aorta and compared those effects with inhibition due to N-G-methyl-L-arginine (L-NMA). 2 Phenylephrine evoked similar concentration-contraction curves in the control rings and in the rings treated with these different NOS inhibitors (10 mu M), whereas 100 mu M of L-NMA, L-TC or L-SMTC increased significantly, and to a similar extent, contractions evoked by phenylephrine in aortic rings with endothelium without significantly affecting the maximal responses. 3 Relaxations evoked by acetylcholine, adenosine triphosphate, or calcium ionophore were significantly inhibited in a dose-dependent manner by L-NMA, L-SMTC, or L-TC (10-100 mu M). The potencies of these inhibitors in reducing the relaxations of these vasodilators were not significantly different. 4 In endotoxin-treated preparations with endothelium, the three L-arginine analogues (10 mu M) significantly potentiated contractile responses to phenylephrine (pEC(50): 6.73+/-0.12 and 7.3+/-0.12, 7.34+/-0.13, or 7.22+/-0.14; in the absence and the presence of L-NMA, L-TC, or L-SMTC respectively) and increased maximal contractions from 1.53+/-0.15 g to 1.95+/-0.13 g, 2.08+/-0.12 g, and 2.03+/-0.13 g with L-NMA, L-TC, and L-SMTC respectively. A higher concentration of these NOS inhibitors (100 mu M) further increased contractions evoked by this alpha(1)-agonist without further enhancing the maximal contractions; however, contractions evoked by 10 nM phenylephrine were significantly greater in the presence of L-SMTC or L-TC than in the presence of L-NMA (100 mu M) (L-NMA: 0.4+/-0.11 g; L-TC: 0.78+/-0.14 g and L-SMTC: 0.82+/-0.17 g). The effects of these inhibitors on NO- synthesis induced by endotoxin were significantly reversed by addition of L-arginine (1 mM) but not by L-citrulline (1 mM). In LPS-treated rings with endothelium, all three NOS inhibitors (100 mu M) shifted the concentration-contraction curves evoked by phenylephrine significantly to the left (pEC(50): 7.19+/-0.03 and 7.79+/-0.08, 8.01+/-0.07, or 8.02+/-0.07, in the absence and the presence of L-NMA, L-TC, or L-SMTC, respectively) and increased significantly maximal contractions from 2.05+/-0.05 g to 2.38+/-0.14 g, 2.5+/-0.12 g, and 2.4+/-0.21 g with L-NMA, L-TC, and L-SMTC, respectively. L-TC and L-SMTC were significantly more potent than L-NMA in potentiating contractions evoked by 10 nM and 30 nM phenylephrine. 5 L-TC and L-SMTC produced dose-dependent increases in tone in LPS-treated aortic rings with and without endothelium. In LPS-treated rings with endothelium, L-NMA induced contractions but in preparations without endothelium low concentrations of L-NMA induced small contractions while high concentrations of this inhibitor evoked relaxations. In both preparations L-TC and L-SMTC were significantly more potent than L-NMA. in increasing vascular tone. 6 These results suggest that L-SMTC, L-TC and L-NMA were equipotent on basal and agonist-stimulated NO- synthesis produced by the constitutive isoform of NOS, whereas the two new L-arginine analogues were more potent than L-NMA in inhibiting the production of NO- induced by endotoxin in rat aorta.
