Effect of calcium and calcium antagonists on [3H]-Paf-acether binding to washed human platelets.

[3H]-Platelet activating factor (Paf-acether, 1-O-octadecyl-2-acetyl-sn-glycero-3-phosphocholine) binds to washed human platelets in a specific, dose-dependent, and saturable manner. Scatchard analysis reveals a high affinity site with a KD value of 0.25 +/- 0.033 nM (245 +/- 30 sites per platelet), and a second low affinity site with a KD value of 9.22 +/- 1.17 nM (1616 +/- 165 sites per platelet). Binding to the high affinity site is independent of buffer calcium concentration, inhibited on an equimolar basis by unlabelled 1-O-octadecyl-Paf-acether, but remains unchanged in the presence of 1-O-octadecyl-lyso-Paf-acether. The relative inhibitory effect of four calcium antagonists on [3H]-Paf-acether high affinity binding correlates closely with their respective anti-aggregatory activity against Paf-acether induced responses in human PRP; order of potency being (+)-cis diltiazem greater than (+/-)-verapamil greater than (-)-cis diltiazem greater than nifedipine. In the case of (+)-cis diltiazem, the effect is competitive, stereo-specific and progressively reversed by addition of calcium (1.0 mM and 5.0 mM). A close spatial relationship may thus exist between the Paf-acether receptor and membrane calcium channels in the human platelet.