Influence of different classes of NO synthase inhibitors in the pig gastric fundus

The inhibitory potency of different classes of nitric oxide synthase (NOS) inhibitors (amino acid-based substances, guanidines, isothioureas, imidazoles and indazoles) versus peripheral neuronal NOS in the pig gastric fundus was investigated by studying their influence on electrically induced relaxations in non-adrenergic non-cholinergic conditions. Circular muscle strips were mounted for isotonic registration in the presence of atropine and guanethidine, and tone was raised with 5-hydroxytryptamine. Electrical field stimulation (40V, 0.1ms, 4Hz, 10s) induced short-lasting relaxations. The inhibitory effect of 1-phenylimidazole could not be evaluated because it nearly abolished the 5-hydroxytryptamine-induced tone of the tissues. 7-Nitroindazole, imidazole, 2-iminobiotin and aminoguanidine did not inhibit the electrically induced relaxations, while the other 9 substances tested were able to do so. The influence of the incubation period was tested by studying the inhibitory effect after incubation for 10 up to 60min. For NG-nitro-L-arginine methyl ester (L-NAME), NG-nitro-L-arginine (L-NNA), L-N5-(1-iminoethyl)-ornithine (L-NIO), L-N6-(1-iminoethyl)-lysine (L-NIL), S-methyl-L-thiocitrulline and S-isopropyl isothiourea there was a moderate increase in the inhibitory effect up to 30min of incubation so that they were incubated for 30min to study their inhibitory potency. For L-thiocitrulline, S-methyl isothiourea and S-ethyl isothiourea, an incubation period of 60min was used. The 9 substances concentration-dependently inhibited the electrically induced relaxations with a maximal inhibitory effect of approximately 80% except for S-methyl isothiourea (E max of 53%). The over all order of potency was: S-isopropyl isothiourea > S-ethyl isothiourea ≥ S-methyl-L-thiocitrulline ≥ L-NNA > L-NIO > L-NAME > S-methyl isothiourea > L-thiocitrulline > L-NIL. While the potency for S-isopropyl isothiourea (EC50: 3.1×10–5M, n=6) to S-methyl isothiourea (EC50: 11.5×10–5M, n=5) was in the same range, the potency of L-thiocitrulline and L-NIL was clearly lower.