THIOL AGENTS SEPARATE NITRIC-OXIDE FORMATION FROM VASODILATION INDUCED BY GLYCERYL TRINITRATE

The role of nitric oxide (NO) and thiol-containing compounds in glycer yl trinitrate (GTN)-induced vasodilation was investigated using the th iol-alkylating agent N-ethylmaleimide (NEM), Bovine pulmonary artery ( BPA) rings were submaximally contracted with K+ and exposed to increas ing concentrations of GTN after a 30-min incubation with 50 mu M NEM, NEM decreased maximal relaxation (10 mu M GTN) by 20%, compared with c ontrols. Treatment with 5 mM L-cysteine for 30 min before incubation w ith 50 mu M NEM (protection protocol) prevented this decrease in GTN-i nduced 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 mu M GTN (chemiluminescence-headspace gas method) i n 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 mu M NEM, NO was not detected at eit her 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 acti on of GTN and other organic nitrates is more complex than their acting as immediate prodrugs of NO.