SP stimulates platelet secretion and calcium-dependent signaling. (A) Dense granule secretion was measured using a 5-HT secretion assay at the concentrations of SP shown. Levels of [³H]-5HT secretion are shown as percentage of total cell content following subtraction of release under resting conditions. Values shown are mean percent of total [3H]-5HT released ± SEM (n = 5). *P < .005 with respect to basal; **P < .005 with respect to 2.5 and 5 μM. (B) The mobilization of calcium from intracellular stores was determined at increasing concentrations of SP as described in “Materials and methods.” (C) Platelets loaded with increasing concentrations of BAPTA-am or dimethyl sulfoxide (DMSO) control were stimulated with SP (20 μM) for 90 seconds (arrow indicates addition of agonist). Aggregation was monitored by turbidometric aggregometry at 37° C under stirring conditions. Traces are representative of 3 separate experiments.