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Guanine nucleotide is essential and Ca2+ is a modulator in the exocytotic reaction of permeabilized rat mast cells.

ABSTRACT: 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 on the ability of GTP to stimulate secretion is probably due to the dependence on Mg2+ of the GTPase activity of GE (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. Ca(2+)-induced secretion requires the provision of MgATP. Since this is further enhanced by low concentrations (< 100 microM) 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 (GE) which regulates exocytosis.

SUBMITTER: Lillie TH

PROVIDER: S-EPMC1132097 | biostudies-other | 1992 Nov

REPOSITORIES: biostudies-other

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Project description:We have measured the time course of secretion of hexosaminidase from rat mast cells permeabilized (in simple buffered NaCl solutions) in response to guanine nucleotides [GTP or guanosine 5'-[gamma-thio]triphosphate (GTP[S])] and Ca2+. In these experiments, ATP was excluded from the system (and the cells were pretreated with metabolic inhibitors). For cells permeabilized in the absence of Mg2+ but in the presence of Ca2+, secretion commences promptly in response to addition of GTP; when Mg2+ (2 mM) is provided, secretion commences after an extended delay, much higher concentrations of GTP are required, and the final extent of secretion is decreased. Ongoing secretion due to GTP and Ca2+ is abruptly terminated by addition of Mg2+ to cells initially stimulated in its absence. In contrast, although Mg2+ has no effect on the sensitivity to the non-hydrolysable analogue GTP[S], its absence does nevertheless cause delays in the onset of secretion triggered by the addition of GTP[S] to cells initially permeabilized in the presence of Ca2+ (micromolar range, again in the absence of ATP). However, exocytosis from cells triggered with Ca2+ after permeabilization in the presence of high concentrations of GTP[S] is instantaneous. The delays due to triggering by GTP[S] have GTP[S]-concentration-dependent and -independent components. The guanine-nucleotide-concentration-dependent component is expressed as an extended duration of delay as the concentration of GTP[S] is decreased, and may reflect the binding of GTP[S] to GE. The concentration-independent component is manifested as a limiting delay which cannot be further diminished by increasing the guanine nucleotide concentration. The duration of the limiting delay is sensitive to the identity of the stimulating nucleotide (GTP < GTP[S] < p[NH]ppG) and may reflect the time taken for an activating conformational change to occur after binding. Since both components of the delays are abolished by the presence of Mg2+, both the binding of guanine nucleotide and the activation of GE appear to be Mg(2+)-dependent. We therefore conclude that nucleotide binding, activation and the GTPase activity of GE are strongly dependent on Mg2+, in common with the same three processes in Gs and Gi.

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Guanine nucleotide is essential and Ca2+ is a modulator in the exocytotic reaction of permeabilized rat mast cells.

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