Immunocytochemical localization of factor VIII/von Willebrand factor antigen in human platelets

Studies on the platelet surface-associated von Willebrand factor

The contribution of platelets to the prevention and control of bleeding depends not only on the plasma and vessel wall von Willebrand (vW) factor, but also on the vW factor originating from platelets. Although platelet vW factor is mainly stored in the alpha-granules, small amounts of vW factor have been detected on the platelet surface under conditions without direct stimulation. This communication focuses on the small amount of vW factor associated with the surface of platelets. EDTA-washed platelets in a calcium-free medium containing prostaglandin E1 (PGE1) were exposed to anti-vW factor F(ab')2 fragments. The platelet-associated anti-vW factor antibodies were separated from the free fragments by gel chromatography. Normal platelets were compared to platelets from patients with Bernard Soulier syndrome, Glanzmann's thrombasthenia, and Hermansky-Pudlak syndrome. These experiments showed that, in the absence of extracellular calcium, vW factor can be detected on the surface of washed platelets independent of glycoproteins Ib and IIb-IIIa, and despite markedly reduced intracellular calcium content. This particular platelet surface expression of vW factor is probably the result of disturbing the platelet during the in vitro manipulation. von Willebrand factor is an extremely important element in the multiple molecular interactions required to stop and anchor the platelet in areas of high flow rates. Mechanical perturbation of platelets leading to surface expression of the vW factor helps increase its availability for such critical interactions.

Voltage-gated Ca entry in isolated bovine capillary endothelial cells: evidence of a new type of BAY K 8644-sensitive channel

Isolated bovine capillary endothelial cells have been examined for voltage-dependent Ca entry. All cells displayed a low threshold activity, with the main characteristics of a T-type transient current, when examined using whole-cell recording for activation and inactivation and cell-attached conditions or inside-out patches for the elementary conductance (8 pS). 25% of the cells displayed an additional sustained current in 5 mM CaCl2 above -40 mV, which was enhanced by application of BAY K 8644, but almost insensitive to superfusion with nicardipine. Two types of channels (2.8 and 21 pS, in 110 mM BaCl2) were shown to have a BAY K 8644 sensitivity. The large conductance channels were L-type channels. The smaller events were elicited at more hyperpolarized potentials (by some 30 mV). Their mean open time was 16 ms in control conditions. In presence of BAY K 8644, additional long open times were observed (up to 100 ms as compared to 7.8 ms for the time constants of the slow mode of the L-type channel). We refer to these channels as SB channels: of small conductance and sensitive to BAY K 8644. In the presence of nicardipine, SB channels are not noticeably modified, in contrast to the L-type openings which are abolished. Also, SB open times are close to control values when nicardipine is added after a BAY K 8644 application. We suggest that, at physiological concentrations of divalent ions, an SB-type activity is elicited above -40 mV which generates the low threshold sustained current.