Involvement of α2-Adrenoreceptors and G Proteins in the Modulation of Platelet Secretion in Response to Streptococcus sanguis

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Abstract
In the presence of plasma, human platelets secrete the contents of their dense granules and then aggregate in response to certain strains of Streptococcus sanguis. After 2 to 5 min of incubation with streptococci, platelets from fast-responding donors will begin to aggregate. Slow responders aggregate after a longer delay. Platelets may secrete after a short (fast responder) or long (slow responder) delay because of differences in the basal levels or responses to potentiating catecholamines. To test this hypothesis in vitro, endogenous basal catecholamine levels in platelets and plasma from fast and slow responders were analyzed by HPLC with electrochemical detection. The total basal concentration of epinephrine in platelets plus plasma was fourfold higher in fast responders, with the platelet compartment showing the greatest difference. The basal affinity of α2-adrenoreceptors in platelets from both groups was similar when estimated using a specific antagonist, [3H]-yohimbine. Platelets from all donors showed decreased α2-adrenoreceptor affinity in the presence of low (2 n M), but not higher (10 n M), concentrations of added epinephrine. Platelets in the two groups were then compared for secretion of ATP. More ATP was secreted after a shorter delay from fast responding platelets, which was mimicked in slow responders by adding physiologically attainable levels (40 n M) of epinephrine. Addition of the α2-antagonist, phentolamine (10 μ M), to the platelets of slow and fast responders completely inhibited or reduced secretion by one third, respectively. Therefore, α2-adrenoreceptors modulate the secretory response of platelets to cells of S. sanguis. Modulation of secretion may involve coupling of the α2-adrenoreceptor and S. sanguis receptor via G proteins because secretion in response to S. sanguis was inhibitable by pertussis or cholera toxins, or GTPyS.

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