Guanine nucleotides decrease the free [Ca2+] required for secretion of serotonin from permeabilized blood platelets

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Abstract
Human platelets containing granule-bound [14C]serotonin were permeabilized, equilibrated at 0°C with ATP and with various Ca2+ buffers and guanine nucleotides, and then incubated at 25°C with or without a stimulatory agonist. Ca2+ alone induced the ATP-dependent secretion of [14C]serotonin (50% at a pCa of 5.1) but the sensitivity of secretrion to Ca2+ was greatly enhanced by guanine nucleotides [6-fold by 100μM GTP, 100-fold by 100 μM guanyl-5'-yl imidodiphosphate and > 500-fold by 100 μM guanosine 5'-O-(3-thiotriphosphate)] or by stimulatory agonists (10-fold by 2 units thrombin/ml and 4-fold by 1 μM 1-O-octadecyl-2-O-acetyl-sn-glyceryl-3-phosphorylcholine). When both GTP and a stimulatory agonist were added, they had synergistic effects on secretion. Cyclic GMP and GMP acted similarly to GTP. The effects of all these guanine nucleotides were inhibited by guanosine 5'-O-(2-thiodiphosphate), whereas those of stimulatory agonists were not. Our results demonstrate the presence in platelets of guanine nucleotide-dependent and independent mechanisms regulating the sensitivity of secretrion to Ca2+.