Binding of a thromboxane A2 (TXA2)/Prostaglandin H2 (PGH2) receptor antagonist to rabbit and pig heart membrane protein

Abstract This study aimed at testing the hypothesis that the binding sites for TXA 2 PGH 2 are present and different in the heart as compared to platelets and blood vessels. Kinetic studies on the thromboxane binding to protein of membrane preparations from rabbit and pig hearts were carried out using [ 125 I]-PTA-OH, a potent specific thromboxane receptor antagonist. The following points are stressed: 1. 1. the binding sites to 125 I-PTA-OH were shown to be present in the heart membrane whatever the adopted experimental conditions were i.e. - the temperature (4 or 3o °C) used for incubation - the protein fractions under study: either the 105 000 g supernatant or the 200 000 g supernatant of the solubilized pellet (105 000 g) - the animal species: rabbit and pig 2. 2. the radioligand binding was rapid, saturable and reversible 3. 3. the kinetically determined K d 's were in the picomolar range − 11 and 14 pM for the rabbit and pig heart membrane preparation respectively — instead of the nanomolar range found in other tissues of the nanomolar range found in other tissues − 27–39 nM and 2 nM for human platelets and bovine artery endothelial cells respectively- using the same thromboxane receptor antagonist.