title: 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 [14C]serotonin were permeabilized, equilibrated at 0 degrees C with ATP and with various Ca2+ buffers and guanine nucleotides, and then incubated at 25 degrees 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 secretion to Ca2+ was greatly enhanced by guanine nucleotides [6-fold by 100 microM GTP, 100-fold by 100 microM guanyl-5'-yl imidodiphosphate and greater than 500-fold by 100 microM guanosine 5'-O-(3-thiotriphosphate)] or by stimulatory agonists (10-fold by 2 units thrombin/ml and 4-fold by 1 microM 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 secretion to Ca2+.
mesh_terms: D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology*; D002463:Cell Membrane Permeability; D019204:GTP-Binding Proteins; D006150:Guanine Nucleotides / Q000494:pharmacology*; D006160:Guanosine Triphosphate / Q000097:blood*; D006801:Humans; D007700:Kinetics; D011956:Receptors, Cell Surface / Q000097:blood*; D012701:Serotonin / Q000097:blood / Q000378:metabolism*; D013917:Thrombin / Q000502:physiology
chemical_list: D006150:Guanine Nucleotides; D011956:Receptors, Cell Surface; D012701:Serotonin; D006160:Guanosine Triphosphate; D013917:Thrombin; D019204:GTP-Binding Proteins; D002118:Calcium