Platelet prostacyclin binding in smokers

Binding capacity (Bmax) and dissociation constant (KD) of platelet prostacyclin (PGI2) receptors in 23 male smokers and 14 nonsmokers have been determined by direct binding studies to investigate whether platelet prostacyclin binding is altered in smokers. In addition, the inhibitory effect of PGI2 (IC50) on adenosine diphosphate (ADP)-induced platelet aggregation was measured. As confirmed by discriminant analysis, 69% of the smokers had significantly different Bmax and KD. Binding capacity was increased in 12 smokers (Bmax + 74%), with a concomitant decrease in affinity (KD + 94%). In these volunteers, the postreceptor responses (PGI2-induced cyclic adenosine monophosphate [AMP] accumulation in platelet-rich plasma as well as IC50) did not differ from those of controls. In contrast, four smokers with considerably reduced PGI2 binding capacity (Bmax - 65%) exhibited a lower antiaggregatory effect (IC50 + 74%), although the affinity was slightly increased (KD - 61%). Nicotine, L-epinephrine, and prostaglandin E2 did not significantly compete with the binding of 9-3H-PGI2 sodium salt. The antiaggregatory effect of PGI2 on ADP-induced platelet aggregation, however, was inhibited by L-epinephrine (Ki 26 nmol/L) and prostaglandin E2 (Ki 230 nmol/L). Our data suggest that with respect to platelet PGI2 binding and in vitro responsiveness to PGI2, smokers are a heterogeneous population. Although increased binding capacity was observed in 50% of the smokers investigated, our data provide no evidence that a biologically relevant upregulation, for example with concomitant enhanced postreceptor reaction, occurs in smokers.