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 electri cally induced relaxations in non-adrenergic noncholinergic conditions. Circular muscle strips were mounted for isotonic registration in the presence of atropine and guanethidine, and tone was raised with 5-hydr oxytryptamine. Electrical field stimulation (40 V, 0.1 ms, 4 Hz, 10 s) induced short-lasting relaxations. The inhibitory effect of l-phenyli midazole could not be evaluated because it nearly abolished the 5-hydr oxytryptamine-induced tone of the tissues. 7-Nitroindazole, imidazole, 2-iminobiotin and aminoguanidine did not inhibit the electrically ind uced 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 60 min. For N-G-nitro- L-arginine methyl ester (L-NAME), N-G-nitro-L-arginine (L-NNA), L-N-5- (1-iminoethyl)-ornithine (L-NIO), L-N-6-(1-iminoethyl)-lysine (L-NIL), S-methyl-L-thiocitrulline and S-isopropyl isothiourea there was a mod erate increase in the inhibitory effect up to 30 min of incubation so that they were incubated for 30 min to study their inhibitory potency. For L-thiocitrulline, S-methyl isothiourea and S-ethyl isothiourea, a n incubation period of 60 min was used. The 9 substances concentration -dependently inhibited the electrically induced relaxations with a max imal inhibitory effect of approximately 80% except for S-methyl isothi ourea (E-max of 53%). The over all order of potency was: S-isopropyl i sothiourea > S-ethyl isothiourea greater than or equal to S-methyl-L-t hiocitrulline greater than or equal to L-NNA > L-NIO > L-NAME > S-meth yl isothiourea > L-thiocitrulline > L-NIL. While the potency for S-iso propyl isothiourea (EC50: 3.1x10(-5) M, n = 6) to S-methyl isothiourea (EC50: 11.5x10(-5) M, n = 5) was in the same range, the potency of L- thiocitrulline and L-NIL was clearly lower. This study showed several compounds to be potent inhibitors of peripheral neuronal NOS in the pi g gastric fundus while some compounds, that were reported to inhibit b rain neuronal NOS were not effective. The EC,, values found for the ef fective substrates in this functional study may be a guideline for the concentrations required to evaluate the role of NO in NANC neurotrans mission in gastrointestinal smooth muscle preparations.