Guanine nucleotides decrease the free [Ca2+] required for secretion of serotonin from permeabilized blood platelets: Evidence of a role for a GTP‐binding protein in platelet activation

Abstract

Human platelets containing granule‐bound [14 C]serotonin were permeabilized, equilibrated at 0°C with ATP and with various Ca 2+ buffers and guanine nucleotides, and then incubated at 25°C with or without a stimulatory agonist. Ca 2+ alone induced the ATP‐dependent secretion of [14 C]serotonin (50% at a p Ca of 5.1) but the sensitivity of secretrion to Ca 2+ 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 Ca 2+.