Abstract

The role of nitric oxide (NO) and thiol-containing compounds in glyceryl trinitrate (GTN)-induced vasodilation was investigated using the thiol-alkylating agent N-ethylmaleimide (NEM). Bovine pulmonary artery (BPA) rings were submaximally contracted with K⁺ and exposed to increasing concentrations of GTN after a 30-min incubation with 50 μM NEM. NEM decreased maximal relaxation (10 μM GTN) by 20%, compared with controls. Treatment with 5 mM L-cysteine for 30 min before incubation with 50 μM NEM (protection protocol) prevented this decrease in GTN-induced relaxation, but 5 mM D-cysteine did not. Treatment of BPA rings with 5 mM L-cysteine after NEM treatment (reversal protocol) did not reverse the effect of NEM to decrease relaxation inducible by GTN. NO production from 30 μM GTN (chemiluminescence-headspace gas method) in the presence of BPA strips was 46.7 ± 19.4 pmol NO/g tissue after 10 min of incubation and 76.4 ± 10.4 pmol NO/g tissue after 20 min. After a 30-min incubation with 50 μM NEM, NO was not detected at either time point. NO production from GTN by BPA strips, with either the protection or reversal protocol, was elevated approximately 2-fold at both time points, compared with controls. No increase in NO production from GTN was observed at either time point for tissues treated with 5 mM D-cysteine using the same protocols. These results are consistent with the concept that thiol compounds play a role in the mechanism of GTN-induced vasodilation, but they indicate that the mechanism of action of GTN and other organic nitrates is more complex than their acting as immediate prodrugs of NO.