Summary
Washed human platelets were stimulated with fibrillar collagen and platelet aggregation
was prevented by non-stirring conditions. In these samples, electron microscopy revealed
three fractions of platelets: 1) a majority without contacts to the collagen fibers,
2) with focal contacts to collagen, and 3) a small fraction of platelets with internalized
collagen. All platelets had undergone shape change, and exhibited an internal contraction
and granule release. However, only those with internalized collagen were completely
degranulated. The internalized collagen was found to be in close contact to the contractile
sphere in the platelet center, as it was demonstrable with computer assisted 3-D reconstruction
from serial sections. Aspirin inhibited neither the adhesion to collagen nor its internalization
by internal contraction. Also it did not impair the shape change and degranulation
in the platelet fractions that internalized collagen. However, aspirin blocked the
shape change and internal contraction of the other platelets and inhibited the [3H]serotonin release. Cytochalasin D 0.1 uM also suppressed the internalization of
collagen, the shape change, the formation of a contractile gel, the degranulation,
and the [3H]serotonin release in all platelets, whereas the number of platelets that adhered
to collagen remained unchanged. The same effects were produced by prostaglandin E1. If the platelets were stimulated with the TXA2 mimetic, U46619, cytochalasin D at 0.1 εM had no effect; but at 20 εM it strongly
enhanced the degranulation and the [3H]serotonin release, although the platelets remained discoid. It is concluded that
collagen triggers a focal activation of an adherent platelet at the site of its initial
contact to collagen. This subthreshold activation proceeds to degranulation and TXA2 formation via a feedback cascade which involves internal contraction, internalization
of collagen and multiplication of the collagen membrane contacts.
