GUANINE-NUCLEOTIDE IS ESSENTIAL AND CA-2+ IS A MODULATOR IN THE EXOCYTOTIC REACTION OF PERMEABILIZED RAT MAST-CELLS

被引:38
作者
LILLIE, THW
GOMPERTS, BD
机构
[1] Department of Physiology, University College London, London WC1E 6JJ, University Street
基金
英国惠康基金;
关键词
D O I
10.1042/bj2880181
中图分类号
Q5 [生物化学]; Q7 [分子生物学];
学科分类号
071010 ; 081704 ;
摘要
Exocytosis from metabolically depleted permeabilized rat mast cells was measured in response to provision of Ca2+ and guanine nucleotide [GTP or guanosine 5'-[gamma-thio]triphosphate (GTP[S])]. For cells permeabilized in simple salt solutions (NaCl), both of these effectors were required to induce secretion. Exclusion of Mg2+ caused an increase in both the sensitivity of the system to GTP and the extent of secretion elicited. while having no such effects on secretion induced by GTP[S]. The effect of Mg2+ depletion oil the ability of GTP to stimulate secretion is probably due to the dependence on Mg2+ of the GTPase activity of G(E) (a postulated GTP-binding protein which mediates exocytosis). This argues that a persistent stimulus to the G-protein is required to support secretion. Affinity for both GTP[S] and GTP is enhanced when the cells are permeabilized in zwitterionic electrolytes (glutamate, gamma-aminobutyric acid, glycine) instead of NaCl. Under these conditions, secretion occurs in response to provision of either GTP[S] [in the effective absence of Ca2+ (pCa 9)] or Ca2+ (in the absence of guanine nucleotide). Secretion induced by GTP[S] is strongly promoted by the presence of Mg2+ at concentrations in the millimolar range; this promotion by Mg2+ declines as the concentration of Ca2+ is elevated towards pCa 7. At pCa 6, Mg2+ is without effect. Ca2+-induced secretion requires the provision of MgATP. Since this is further enhanced by low concentrations (< 100 mum) and then inhibited by high concentrations of GDP, the essential role of ATP is likely to be in the maintenance of GTP via transphosphorylation by a nucleoside diphosphate kinase reaction. Thus, under conditions of high affinity (glutamate environment), GTP[S] alone is capable of inducing exocytosis. Ca2+ acts in concert with guanine nucleotides: it enhances the rate and extent of secretion and increases the affinity for Mg2+ and guanine nucleotides in the activation of the GTP-binding protein (G(E)) which regulates exocytosis.
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页码:181 / 187
页数:7
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