DISTRIBUTION OF INTRAMEMBRANOUS PARTICLES (IMP) ON PLATELETS IN CLOTS UNDER ISOMETRIC TENSION

Abstract

Development of isometric tension in platelet-rich clots is a manifestation offibrin binding to the cells, as well as to platelet contractile activity. However, the nature of the platelet-fibrin association through which the force of contraction is transmitted remains unknown. A previous report suggested that clustering of the intramembranous particles (IMP) visible in replicas of freeze-fractured platelets in clots might represent sites of fibrin attachment and force transduction across the platelet membrane. In the present study platelet clots were prepared at various stages during development of isometric tension. Clots fixed in glutaraldehyde were combined with osmic acid and further processed for thin sections, or with glycerol, frozen in liquid nitrogen and freeze-fractured in a Balzer's device. Thin sections revealed the longitudinal orientation of platelets and fibrin strands in the long axis of tension. Close associations between fibrin strands and extended portions of platelets were readily identified. However, no clearly discernible repetitive associations could be identified. Replicas of freeze fractured platelet clots under isometric tension revealed aspects of fibrin strands substructure and the platelet secretory pathway. However, no specific associations between fibrin and IMP were apparent. The usual 2 to 1 ratio of IMP on the inside of the outer layer (E face) compared to the outside of the interior layer (P face) of resting platelets was preserved on thrombin activated platelets in isometric clots. No evidence of IMP clustering could be identified. Experiments to identify the origin of IMP clustering revealed it occurred only on severely damaged platelets. Thus, clustering of IMP in replicas of freeze fractured platelets does not appear related to fibrin binding to platelets or to transduction of contractile force from the interior of the platelet to fibrin strands and the development of tension in isometric clots.