Effects of guanine nucleotides on the properties of 5‐hydroxytryptamine secretion from electropermeabilised human platelets

Abstract
1. Guanosine 5''-[.gamma.-thio]triphosphate and guanosine 5''-[.beta.,.gamma.-imido]triphosphate enhance Ca2+-dependent 5-hydroxytryptamine secretion from electropermeabilised human platelets. GTP has little such effect except when the platelets are permeabilised, and incubated with this nucleotide, at 2.degree. C and pH 7.4. 2. The lag phase observed in the time course of 5-hydroxytryptamine secretion induced by addition of guanosine 5''-[.gamma.-thio]triphosphate is markedly longer than that characterising secretion induced by Ca2+ alone, by thrombin .+-. GTP or by guanosine 5''-[.gamma.-thio]triphosphate in the presence of thrombin. 3. GTP causes competitive inhibition of the enhancement of the Ca2+-dependent secretory response induced by guanosine 5''-[.gamma.-thio]triphosphate when both nucleotides are added simultaneously. The extent of inhibition is decreased if guanosine 5''-[.gamma.-thio]triphosphate is added prior to GTP. 4. GTP markedly enhances the effect of thrombin on Ca2+-dependent 5-hydroxytryptamine secretion by increasing the maximal extent of the response and decreasing the thrombin concentration required to give half-maximal response. A similar effect is observed on addition of guanosine 5''-[.gamma.-thio]triphosphate in the presence of thrombin at short incubation times. On more prolonged incubation the effects of thrombin and guanosine 5''-[.gamma.-thio]triphosphate are additive. 5. Guanosine 5''-[.beta.-thio]diphosphate completely inhibits the response induced by guanosine 5''-[.gamma.-thio]triphosphate or guanosine 5''-[.beta.,.gamma.-imido]triphosphate but has little effect on the response induced by Ca2+ when added alone or in the presence of thrombin. Partial inhibition is observed for the response induced by thrombin + GTP. 6. Cyclic-AMP effectively inhibits the response induced by thrombin + GTP but has little effect on that induced by guanosine 5''-[.gamma.-thio]triphosphate or guanosine 5''-[.beta.,.gamma.]imidotriphosphate. 7. The results provide further support for the proposal [Haslam, R.J. and Davidson, M. M. L. (1984) FEBS Lett. 170, 90-95], that receptor-phospholipase-C coupling in platelets is mediated in part by a guanosine-nucleotide-binding (Np) protein but that a coupling mechanism may also exist which is independent of such a protein. The properties of guanine-nucleotide-dependent coupling resemble those previously described for receptor-adenylate-cyclase coupling.