PhD

dissertationHeparin antagonism of prostacyclin-induced inhibition of human platelet aggregation (PA) has been reported. To elucidate if the heparin proaggregatory effect involved a direct interaction with prostacyclin (PGI(,2)), heparin and PGI(,2) were preincubated alone or in combination prior to the addition of platelet-rich plasma (PRP) and ADP, collagen, or arachidonic acid. Heparin-PGI(,2) preincubation did not alter the heparin proaggregatory effect. Also, the removal of heparin from heparin-PGI(,2) mixtures did not alter the PGI(,2) concentration. The data from these experiments did not support the possibility of a direct interaction between heparin and PGI(,2). The structural requirements for the heparin proaggregatory activity were examined. This effect of heparin extends to two other glycosaminoglycans (GAGs), N-acetylated-O-sulfated heparin and a highly sulfated heparan sulfate. The observation that two other heparan sulfate fractions and chondroitin sulfate have no proaggregatory activity indicates a requirement for a highly O-sulfated GAG. Verapamil and trifluoperazine reduced PA to 18% and 21% respectively. The antiaggregatory effect of verapamil or trifluoperazine was reversed by heparin to 60% and 66%, respectively. Generally, heparin also opposed the inhibition of platelet secretion of 5-hydroxytryptamine induced by PGI(,2) or verapamil, but not the antisecretory activity of trifluoperazine. The heparin effect on platelet adenylate cyclase activity was also determined. Heparin did not affect basal platelet cAMP levels. Also, heparin did not suppress the elevation of cAMP promoted by PGI(,2) (10 nM), forskolin (10 (mu)M), or forskolin (10 (mu)M) plus PGI(,2) (1 nM). In conclusion, heparin promotes PA when a maximal stimulus is partially inhibited by an agent such as PGI(,2), verapamil, or trifluoperazine. This property of heparin requires a predominance of P-sulfate. The mechanism through which heparin exerts this effect does not involve a direct interaction with PGI(,2), the PGI(,2)-receptor, or an inhibition of cAMP synthesis