Cross tolerance between nitroglycerin and neural relaxation of the rabbit sphincter of Oddi

We studied whether non-adrenergic, non-cholinergic (NANC) relaxation of the rabbit sphincter of Oddi was influenced by tolerance to nitroglycerin (NG) in vitro. Sphincter of Oddi (SO) muscle rings precontracted with EC50 concentrations of cholecystokinin octapeptide (CCK8) were exposed to cumulative increases in NG concentrations and tested for relaxation by measurement of isometric tension. A separate group of six rings was subjected to a preceding exposure to 275 microM nitroglycerin over 60 min to induce in vitro tolerance to nitroglycerin. The rings (both tolerant and non-tolerant) were subjected to electrical field stimulation (FS: 50 V, 0.1 ms, 20 Hz, 3 and 10 stimuli). The rings were then preincubated with NANC solution: phentolamine, oxprenolol and atropine (all 1 microM) for 20 min and FS was applied again. FS was repeated after additional incubation with NG-nitro-L-arginine methyl ester (L-NAME), and inhibitor of NO synthase (30 microM) and after a successive incubation 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 not 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 monophasic relaxations in response to FS in non-tolerant sphincters but not in tolerant ones. L-NAME (30 microM) reversed NANC relaxation in non-tolerant muscle rings whereas it failed to modify NANC contractions in the tolerant preparations. L-arginine (3 mM) reversed the inhibitory effect of L-NAME on NANC relaxation in the 'non-tolerant' rings and it was without effect on FS-induced contractions in the 'tolerant' SO. As measured by radioimmunoassay, tolerance to NG was without any significant effect on tissue content of both cyclic adenosine 3':5' monophosphate (cAMP) and cyclic guanosine 3':5' monophosphate (cGMP). FS significantly increased tissue cAMP and cGMP content in 'non-tolerant' preparations. 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 tolerance to NG 'in vitro'.