THIOL AGENTS POTENTIATE GLYCERYL TRINITRATE MEDIATED RELAXATION OF RABBIT TAENIA-COLI - EVIDENCE FOR THIOL-DEPENDENT BIOTRANSFORMATION

In the present study, the role of thiols on glyceryl trinitrate (GTN) induced relaxation of rabbit taenia coli strips (RTCS) was investigate d. This study was designed to test the hypothesis that a deficiency in thiols is responsible for RTCS insensitivity to GTN, and thus thiols play a key role in the enzymatic activation of GTN. Isolated RTCS bath ed in normothermic, oxygenated Krebs solution were pretreated with the thiols L-cysteine (5 mM) and N-acetyl-L-cysteine (NAG, 5 mM) for 30 m in and washed. The effects of GTN were determined by changes in isomet ric tension of K+-precontracted RTCS. Both L-cysteine and NAC resulted in increased relaxations to GTN (0.1 nM - 10 mu M) as the GTN relaxat ion EC50 decreased compared with that of the untreated RTCS (L-cystein e, 0.06 +/- 0.12 mu M and NAG, 0.08 +/- 0.03 mu M versus control 0.25 +/- 0.08 mu M, n = 5, p < 0.05). In contrast, 5 mM D-cysteine had no s ignificant effects on the RTCS GTN relaxation EC50 (0.16 +/- 0.13 mu M , n = 5). Similarly, the thiol donor L-methionine significantly increa sed RTCS sensitivity to GTN, as the relaxation EC50 decreased from the control value of 0.25 +/- 0.08 mu M to 10 +/- 4 nM (n = 5, p < 0.001) , whereas the D-isomer did not. These results are consistent with the idea that thiols play a key stereospecific role in the metabolic activ ation of GTN in RTCS. However, RTCS treated with amino acids were stil l less sensitive to GTN compared with vascular tissue, and this sugges ts that RTCS may be deficient in some other enzyme(s) relative to vasc ular tissue that is (are) responsible for the activation of GTN.