VARIABLE POTENCY OF NITRERGIC-NITROVASODILATOR RELAXATIONS OF THE MOUSE ANOCOCCYGEUS AGAINST DIFFERENT FORMS OF INDUCED TONE

1 U46619 (thromboxane A(2) receptors; 0.002-1 mu M), carbachol (muscarinic M(3) receptors; 0.1-100 mu M), cyclopiazonic acid (CPA; Ca2+-ATPase inhibitor; 0.1-30 mu M) and K+ (5-100 mM) produced concentration-dependent contractions of the mouse isolated anococcygeus muscle. Equi-effective, submaximal concentrations of each agent were used in further experiments (40 nM U46619; 5 mu M carbachol; 5 mu M CPA; 70 mM K+). 2 Nifedipine (1 mu M) totally abolished contractile responses to K+; those to U46619, carbachol and CPA were reduced by only 20-30% in the presence of nifedipine, but were greatly reduced (> 90%) by a combination of nifedipine and SKF 96365 (0.1-40 mu M). 3 In Ca2+-free medium, contractions to K+ and CPA were abolished. Small residual responses remained to both carbachol and U46619; those to carbachol were transient, could not be repeated in the continued absence of Ca2+ and were prevented by pre-incubation with CPA, but unaffected by SKF 96365; those to U46619 were sustained, could be repeated in the absence of Ca2+, and were resistant to CPA and SKF 96365. 4 Tone induced by all four agents could be relaxed by sodium nitroprusside (SNP), but with a clear order of potency. SNP (pIC(40)) was most effective against U46619 (7.92), less so against carbachol (6.80) and CPA (6.68), and least potent against K+ (5.94). A similar order of potency was observed with 8Br-cyclic GMP (50 mu M) and nitrergic field stimulation (1-20 Hz). 5 The relaxant potency of SNP was similar in normal Krebs solution and in high K+ (70 mM) Krebs containing 1 mu M nifedipine. 6 Inclusion of SNP (0.01-1 mu M) or 8Br-cyclic GMP (50 mu M) in the Ca2+-free medium inhibited the transient residual response to carbachol. Inclusion of similar concentrations of SNP or 8Br-cyclic GMP, during Ca2+ re-loading, increased the subsequent residual contraction to carbachol in Ca2+-free medium. 7 At higher concentrations, SNP (0.1-10 mu M) produced a partial relaxation of the sustained contraction to U46619 in Ca2+-free medium. 8 Thus, the relaxant potency of the nitrergic stimuli was dependent on the agent and mechanism used to induce tone in the preparation. Examination of the contractile/relaxant interactions suggests that altered Ca2+ sequestration and inhibition of contractile protein function may underlie nitrergic relaxations of this tissue.