Thromboxane A 2 analogues inhibit the metabolism of thromboxane B 2 in perfused guinea-pig lung

Abstract

The effect of four thromboxane A 2-like analogues as inhibitors of thromboxane B 2 uptake and metabolism to 13,14-dihydro-15-keto-thromboxane B 2 was studied in the perfused guinea-pig lung. We used 5-min infusions containing 1 μCi [ 3H]thromboxane B 2 (10 ng/ml) and measured uptake/accumulation (as tissue to medium ratio) and metabolism to 13,14-dihydro-15-ketothromboxane B 2 by radio-TLC. The results showed that thromboxane B 2 metabolism is saturable and exhibits substantial dose-dependent inhibition of both processes by U46619 and U44069 endoperoxide analogues (50% inhibition, ID 50, in the range 0.5-0.9 μM), pinane thromboxane A 2 (a thromboxane A 2 partial agonist, ID 50 against metabolism, 0.7 μM) and the thromboxane A 2 mimetic EPO11 (ID 50 against metabolism, 2.6 μM). These agents affected uptake and enzyme transformation steps differentially, thus strengthening the evidence that thromboxane B 2 metabolism is a multi-step, uptake-dependent process in this tissue. U46619 did not affect prostaglandin F 2α metabolism, nor did prostaglandin F 2α inhibit thromboxane B 2 metabolism, confirming that thromboxane B 2 uptake/ metabolism is distinct from the process which handles prostaglandins. Of the four analogues, only pinane thromboxane was a significant substrate for 15-hydroxv prostaglandin dehydrogenase and it was also the best inhibitor of 15-hydroxyprostaglandin dehydrogenase in purified enzyme preparations. These results advance our understanding of the inactivation in lung of thromboxane B 2 and invite study of thromboxane A 2 itself.