CROSS-TOLERANCE BETWEEN NITROGLYCERIN AND NEURAL RELAXATION OF THE RABBIT SPHINCTER OF ODDI

We studied whether non-adrenergic, non-cholinergic (NANC) relaxation o f the rabbit sphincter of Oddi was influenced by tolerance to nitrogly cerin (NG) in vitro. Sphincter of Oddi (SO) muscle rings precontracted with EC50 concentrations of cholecystokinin octapeptide (CCK8) were e xposed to cumulative increases in NG concentrations and tested for rel axation by measurement of isometric tension. A separate group of six r ings was subjected to a preceding exposure to 275 mu M nitroglycerin o ver 60 min to induce in vitro tolerance to nitroglycerin. The rings (b oth tolerant and non-tolerant) were subjected to electrical field stim ulation (FS: 50 V, 0.1 ms, 20 Hz, 3 and 10 stimuli). The rings were th en preincubated with NANC solution: phentolamine, oxprenolol and atrop ine (all 1 mu M) for 20 min and FS was applied again. FS was repeated after additional incubation with N-G-nitro-L-arginine methyl ester (L- NAME), an inhibitor of NO synthase (30 mu M) and after a successive in cubation with 3 mM L-arginine (20 min). Maximum contractions produced by CCK8 in 'tolerant' and 'non-tolerant' sphincters were 29.9 +/- 5.8 and 28.3 +/- 5.2 mN, respectively. The sensitivity to CCK8 also was no t different between the two groups with EC50 (-log M) values of 8.5 +/ - 0.2 and 8.3 +/- 0.1, respectively. FS evoked twitchlike contraction followed by relaxation in the ampullary SO in both 'tolerant' and 'non -tolerant' preparations. Incubation in NANC solution resulted in monop hasic relaxations in response to FS in non-tolerant sphincters but not in tolerant ones. L-NAME (30 mu M) reversed NANC relaxation in non-to lerant muscle rings whereas it failed to modify NANC contractions in t he tolerant preparations. L-arginine (3 mM) reversed the inhibitory ef fect of L-NAME on NANC relaxation in the 'non-tolerant' rings and it w as without effect on FS-induced contractions in the 'tolerant' SO. As measured by radioimmunoassay, tolerance to NG was without any signific ant effect on tissue content of both cyclic adenosine 3':5' monophosph ate (cAMP) and cyclic guanosine 3':5' monophosphate (cGMP). FS signifi cantly increased tissue cAMP and cGMP content in 'non-tolerant' prepar ations. FS failed to increase the level of either cyclic nucleotide in 'tolerant' tissue. We conclude that NANC relaxation of the ampullary part of the rabbit SO is significantly impaired in the state of tolera nce to NG 'in vitro '. (C) 1998 The Italian Pharmacological Society.