Prostacyclin is a circulating hormone

Abstract

PROSTACYCLIN (PGI₂) is generated by vascular walls from arachidonic acid or prostaglandin endoperoxides¹. It is the most potent inhibitor of platelet aggregation so far described, and acts by increasing cyclic AMP in the platelets^2,3, presumably through adenyl cyclase stimulation. The enzyme which synthesises prostacyclin is concentrated in the endothelial lining of the vessels⁴; clearly, platelets could be prevented from clumping onto the walls of healthy vessels by the generation of prostacyclin⁵. Unlike prostaglandin E₂ or F_2α, prostacyclin is not inactivated as it passes through the pulmonary circulation^6,7, and in dog lungs, in vivo, prostacyclin is the major metabolite generated during an infusion of arachidonic acid⁸. In the preceding paper, Gryglewski et al.⁹ conclude that prostacyclin is continuously generated by and released from cat lungs in vivo. We report here that we have confirmed this conclusion in a different species (rabbits) by the use of an antiserum which cross-reacts with prostacyclin and neutralises its anti-aggregating activity¹⁰. This antiserum (PGI₂ antiserum) was raised in rabbits using a stable analogue of prostacyclin, 5,6-dihydro-prostacyclin (PGI₁), as the hapten, conjugated to bovine serum albumin¹⁰.