Functional significance of mobile receptors on human platelets

Mobile receptors on surface- and suspension-activated platelets bind a variety of specific and nonspecific antigens and particulates and clear them from the plasma membrane to channels of the open canalicular system (OCS). The present study examined the interaction of platelets with latex spherules of increasing size to identify the role of mobile receptors and binding sites in hemostatic physiology. Small latex spherules 0.09 to 3.13 microns in average diameter were cleared from peripheral margins to central zones and the OCS on surface-activated platelets and from the surface membrane to the OCS of platelets in suspension. Larger spheres, 6.4 microns in diameter, could not be moved across the membranes of solid phase- and fluid phase-activated platelets. Instead, platelets moved their surface membranes through the contact sites fixed on the surface of immobile spheres, bringing interior membranes of the OCS channels to the latex. This change, in which mobile membranes move through fixed contact sites rather than mobile receptor complexes moving across plasma membranes, results in evagination of almost all OCS channels on large latex spherules. A similar mechanism may be involved in the interaction of platelets with relatively flat surfaces, such as denuded subendothelium.