Abstract

The effects of in vitro nitroglycerin tolerance and methylene blue pretreatment on the ability of nitroglycerin and nitroprusside to promote relaxation and tissue accumulation of cyclic GMP were examined in the carbachol-contracted longitudinal smooth muscle of the guinea-pig ileum. Nitroglycerin and nitroprusside produced concentration-dependent increases in cyclic GMP levels. However, only nitroglycerin increased cyclic GMP levels before the onset of relaxation. Nitroglycerin tolerance produced approximately 200- and 5-fold shifts to the right of nitroglycerin and nitroprusside relaxation curves, respectively. Methylene blue pretreatment produced approximately 5-fold shifts to the right of both nitroglycerin and nitroprusside relaxation curves. Whenever there was an inhibition of nitroglycerin- or nitroprusside-induced relaxation, there was a corresponding reduction of cyclic GMP generation. Methylene blue inhibited the ability of 8-bromoguanosine 3',5'-monophosphoric acid to promote smooth muscle relaxation, suggesting that it also may impair the subsequent actions of cyclic GMP. This study provides the first demonstration of nitroglycerin tolerance in a nonvascular smooth muscle and provides evidence that cyclic GMP mediates the relaxant effects of nitroglycerin in the longitudinal smooth muscle of the guinea-pig ileum. However, because nitroprusside promoted tissue accumulation of cyclic GMP subsequent to relaxation, an exclusive role for cyclic GMP-mediated relaxation for this drug in the longitudinal smooth muscle appears unlikely.