Extracellular Factors Influencing the In Vitro Protein Synthesis of Platelets

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Abstract
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.

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