An alternative pathway for metabolism of leukotriene D 4 : effects on contractions to cysteinyl‐leukotrienes in the guinea‐pig trachea

Contractions of guinea‐pig tracheal preparations to cysteinyl‐leukotrienes (LTC 4 , LTD 4 and LTE 4 ) were characterized in organ baths, and cysteinyl‐leukotriene metabolism was studied using radiolabelled agonists and RP‐HPLC separation. In the presence of S‐hexyl GSH (100 μ M ) the metabolism of [ 3 H]‐LTC 4 into [ 3 H]‐LTD 4 was inhibited and the LTC 4 ‐induced contractions were resistant to CysLT 1 receptor antagonism but inhibited by the dual CysLT 1 /CysLT 2 receptor antagonist BAY u9773 (0.3–3 μ M ) with a pA 2 ‐value of 6.8±0.2. In the presence of L ‐cysteine (5 m M ), the metabolism of [ 3 H]‐LTD 4 into [ 3 H]‐LTE 4 was inhibited and the LTD 4 ‐induced contractions were inhibited by the CysLT 1 receptor antagonist ICI 198,615 (1–10 n M ) with a pA 2 ‐value of 9.3±0.2. However, at higher concentrations of ICI 198,615 (30–300 n M ) a residual contraction to LTD 4 was unmasked, and this response was inhibited by BAY u9773 (1–3 μ M ). In the presence of the combination of S‐hexyl GSH with L ‐cysteine, the LTD 4 ‐induced contractions displayed the characteristics of the LTC 4 contractile responses, i.e. resistant to CysLT 1 receptor antagonism, increased maximal contractions and slower time‐course. This qualitative change of the LTD 4 ‐induced contraction was also observed in the presence of S‐decyl GSH (100 μ M ), GSH (10 m M ) and GSSG (10 m M ). S‐hexyl GSH, S‐decyl GSH, GSH and GSSG all stimulated a formation of [ 3 H]‐LTC 4 from [ 3 H]‐LTD 4 . In conclusion, GSH and GSH‐related compounds changed the pharmacology of the LTD 4 ‐induced contractions by stimulating the conversion of LTD 4 into LTC 4 . Moreover, the results indicate that, in addition to the metabolism of LTC 4 into LTD 4 and LTE 4 , also the formation of LTC 4 from LTD 4 may regulate cysteinyl‐leukotriene function.British Journal of Pharmacology (2001) 133 , 1134–1144; doi: 10.1038/sj.bjp.0704180