Summary
The platelet aggregating activity of extracts of different layers of the arterial
wall was compared to that of Achilles tendon. Arterial media and tendon extracts,
adjusted to equivalent protein content as an index of concentration, aggregated platelets
to the same extent but an arterial intima extract did not aggregate platelets. Platelet
aggregation induced by collagen could be inhibited by mixing with intima extract,
but only to a maximum of about 80%. Pre-mixing adenosine diphosphate (ADP) with intima
extracts diminished the platelet aggregation activity of the ADP. Depending on the
relationship between ADP and intima extract concentrations aggregating activity could
either be completely inhibited or inhibition abolished. Incubation of ADP with intima
extract and subsequent separation of degradation products by paper chromatography,
demonstrated a time-dependent breakdown of ADP with AMP, adenosine, inosine and hypoxanthine
as metabolic products; ADP removal was complete. Collagen, thrombin and adrenaline
aggregate platelets mainly by endogenous ADP of the release reaction. Results of experiments
comparing inhibition of aggregation caused by premixing aggregating agent with intima
extract, before exposure to platelets, and the sequential addition of first the intima
extract and then aggregating agent to platelets, suggest that the inhibitory effect
of intima extract results from ADP breakdown. It is suggested that this ADP degradation
by intima extract may play a protective role in vivo by limiting the size of platelet
aggregates forming at the site of minimal “wear and tear” vascular trauma.