title: Kinetic characterization of guanine-nucleotide-induced exocytosis from permeabilized rat mast cells.
abstract: 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.
authors.affiliation: Department of Physiology, University College London, U.K.
authors.forename: T H
authors.initials: TH
authors.lastname: Lillie
authors.forename: B D
authors.initials: BD
authors.lastname: Gomperts
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D005089:Exocytosis; D019204:GTP-Binding Proteins / Q000378:metabolism; D006150:Guanine Nucleotides / Q000494:pharmacology*; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D008297:Male; D008407:Mast Cells / Q000378:metabolism*; D010539:Permeability; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D006150:Guanine Nucleotides; D019204:GTP-Binding Proteins; D008274:Magnesium; D002118:Calcium