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The effects of Ca2+ in the presence or absence of F VIII-VWF on blood platelet adherence to subendothelium of human arteries were investigated in an annular perfusion chamber according to Baumgartner, using a non-pulsatile perfusion system. The perfusates consisted of citrated blood or reconstituted fluids with washed red blood cells, 51Cr-labelled platelets, and the appropriate plasma replacements. F VIIIVWF was purified from cryoprecipitate by Sepharose CL-4B chromatography and radio-labelled with 125I by the lactoperoxidase tecjiijtique. Platelet adherence was measured by registration of 51Cr count or by morphometry. Free Ca2+ was assayed with a calcium electrode, with a minimum detection limit of 10 µM. A close correlation was observed between the Ca2+ concentration and the haematocrit in normal blood anticoagulated with l/lO vol 110mM Na3-citrate. The plasma concentration of citrate and Ca2+ had mean values of l8 mM and 65 yM respectively. Perfusion for 7 min at a vessel wall shear rate of 1000 s-1 with this blood, produced a monolayer of adherent platelets and only occasionally small platelet aggregates on the subendothelium2+but no thrombi &gt; 5 µM in height. At plasma citrate and Ca2+ concentrations of respectively 40 mM and 25 µM the platelet spreading on the subendothelius^ was impaired. At a citrate concentration of l60 mM and a Ca2+ concentration below the detection limit, the initial contact between platelets and subendothelium was abolished. In perfusion fluids in which plasma was substituted by a human albumin solution (HAS), the adherence at Ca2+ concentrations above 10 µM was higher in the presence of F VIII-VWF than in its absence. However, at a Ca concentration below 10 pM induced by addition of citrate (160 mM in HAS), the adherence yith and without F VIII-VWF was abolished. The binding of 125I F VIII-VWF to the subendothelium was unchanged at levels below 10 µM Ca2+.  These data indicate that the various steps of pikelet interaction with subendothelium require specific Ca2+ concentrations. The effect of F VIII-VWF is independent of this.

CA2+ Affects Platelet-Subendothelium Interaction Independent Of F VIII-Vwf