On passage through the lung vascular bed, prostaglandins are removed from the circulation by a transport carrier and subsequently inactivated by intracellular enzymes. However, PGI2 is not inactivated by the lung in vivo. Although PGI2 is an excellent substrate for the intracellular enzymes in vitro, PGI2 is not a substrate for the carrier system. Thus, the transport carrier determines which ci...