Comparison of vascular and platelet thromboxane A2/prostaglandin H2 receptors in the pig.

We compared the properties of vascular and platelet thromboxane A2/prostaglandin H2 receptors in the pig. The binding profiles of U46619, several prostaglandins and thromboxane A2/prostaglandin H2 receptor antagonists to the aorta and platelet receptors were almost the same irrespective of whether the agonist ([3H]U46619) or the antagonist ([3H]SQ29,548) radioligand was used, except that the receptor density in the latter was 4 times higher than that in the aorta. The antagonists suppressed U46619-induced contraction of pig coronary artery and secondary aggregation of platelets at potencies comparable to their KI values in the binding experiments. On the other hand, the responses of the artery specimens to U46619 and the prostaglandins differed from those of the platelets. Thus, the binding sites in the vascular and platelet receptors seem to be the same or quite similar, but there seem to be different mechanism(s) leading the agonistic binding signal to final responses.