Summary
Regulation of platelet protein synthesis by selected extracellular factors and events
have been examined. Protein synthetic function was assessed by the incorporation of
3H-leucine into trichloroacetic acid-precipitable protein and was characterized by:
a) incorporation of 0.1 μ moles leucine/hr/1088 platelets; b) sensitivity to puromycin but not actinomycin D; and c) synthesis of
multiple size species. Inhibition and potentiation of the rate of protein synthesis
by extracellular factors was demonstrable. Plasma and serum both inhibited protein
synthesis, and the inhibitory effect appeared to be dependent upon a cooperative effect
of two distinct plasma factors. The inhibitory effect may reflect a decrease in the
rate of protein synthesis and/or an increase in the rate of protein catabolism. Fibrinogen
and its plasmic cleavage products X, Y and E did not affect platelet protein synthesis;
but the D: E complex and D fragment produced approximately 50% increases in the rate
of synthesis. Protein synthesis and the release reaction appeared to be independent
cellular functions as ADP, thrombin, AMP, ATP, adenosine and dibutyryl derivatives
of cyclic AMP and GMP did not influence protein synthesis. Epinephrine at 1 mM inhibited
the initial rate of protein synthesis by 45%; however, this effect appeared to be
independent of the release reaction. It is concluded that platelet protein synthesis
in vitro is independent of selected platelet hemostatic functions such as the release reaction
but is susceptible to regulation by agents which occur in vivo.
