title: Thiol agents potentiate glyceryl trinitrate mediated relaxation of rabbit taenia coli: evidence for thiol-dependent biotransformation.
abstract: In the present study, the role of thiols on glyceryl trinitrate (GTN) induced relaxation of rabbit taenia coli strips (RTCS) was investigated. 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 bathed in normothermic, oxygenated Krebs solution were pretreated with the thiols L-cysteine (5 mM) and N-acetyl-L-cysteine (NAC, 5 mM) for 30 min and washed. The effects of GTN were determined by changes in isometric tension of K(+)-precontracted RTCS. Both L-cysteine and NAC resulted in increased relaxations to GTN (0.1 nM-10 microM) as the GTN relaxation EC50 decreased compared with that of the untreated RTCS (L-cysteine, 0.06 +/- 0.12 microM and NAC, 0.08 +/- 0.03 microM versus control 0.25 +/- 0.08 microM, n = 5, p < 0.05). In contrast, 5 mM D-cysteine had no significant effects on the RTCS GTN relaxation EC50 (0.16 +/- 0.13 microM, n = 5). Similarly, the thiol donor L-methionine significantly increased RTCS sensitivity to GTN, as the relaxation EC50 decreased from the control value of 0.25 +/- 0.08 microM 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 activation of GTN in RTCS. However, RTCS treated with amino acids were still less sensitive to GTN compared with vascular tissue, and this suggests that RTCS may be deficient in some other enzyme(s) relative to vascular tissue that is (are) responsible for the activation of GTN.
authors.affiliation: Department of Pharmacology and Toxicology, Faculty of Medicine, Queen's University, Kingston, ON, Canada.
authors.forename: A S
authors.initials: AS
authors.lastname: Hussain
authors.forename: T N
authors.initials: TN
authors.lastname: Abu-Zahra
authors.forename: J F
authors.initials: JF
authors.lastname: Brien
authors.forename: G S
authors.initials: GS
authors.lastname: Marks
authors.forename: K
authors.initials: K
authors.lastname: Nakatsu
mesh_terms: D000818:Animals; D001711:Biotransformation; D003106:Colon / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology*; D004357:Drug Synergism; D066298:In Vitro Techniques; D008297:Male; D009126:Muscle Relaxation / Q000187:drug effects* / Q000502:physiology; D009569:Nitric Oxide / Q000378:metabolism / Q000502:physiology; D005996:Nitroglycerin / Q000493:pharmacokinetics / Q000494:pharmacology*; D011817:Rabbits; D013438:Sulfhydryl Compounds / Q000494:pharmacology*; D014665:Vasodilator Agents / Q000493:pharmacokinetics / Q000494:pharmacology*
chemical_list: D013438:Sulfhydryl Compounds; D014665:Vasodilator Agents; D009569:Nitric Oxide; D005996:Nitroglycerin