Secretion of serotonin from platelets induced by platelet-activating factor (PAF) derived from antigen-stimulated, IgE-sensitized rabbit basophils was studied to further characterize the biochemical requirements. Inhibition of secretion with diisopropylphosphofluoridate (DFP) was observed if the DFP was present during the reaction, but not if platelets or PAF were pretreated with the inhibitor. This suggested a role for an activatable serine protease in the secretion. Supporting evidence came from the observation that other protease inhibitors and a variety of low molecular weight amino acid esters were also inhibitory. TAMe was most effective, and AGLMe and LeuMe were inactive, indicating a specificity for different esters. Secretion was reduced by agents that increased intracellular cyclic AMP (cAMP), but enhanced by alpha-adrenergic stimulation, which reduced the levels of cAMP. Concurrent with PAF-induced secretion, a reduction in cAMP levels was observed. No effect of cyclic GMP or cholinergic stimulation was found. Secretion was inhibited by colchicine and enhanced by cytochalasin B, suggesting a role for microfilaments and microtubules. The effects of these three systems on PAF-induced secretion indicate the basic uniformity of the secretory process in platelets (and other cells) whatever the stimulus. The uniqueness of the reaction apparently lies in the stimulus-receptor interaction and the nature of the serine protease which is activated.

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