Activation of muscarinic potassium currents by ATP gamma S in atrial cells

Regulation of muscarinic agonist-induced activation of phosphoinositidase C in electrically permeabilized SH-SY5Y human neuroblastoma cells by guanine nucleotides

myo-[3H]Inositol-labelled SH-SY5Y cells were permeabilized with electrical discharges. 3H-Inositol phosphate formation in cells shown to be fully permeable was stimulated by the muscarinic agonist carbachol, by guanosine 5'-(gamma-thio)triphosphate [GTP(S)], and by guanosine 5'-(beta gamma-imido)diphosphate (GppNHp). Synergism was observed on coincubation of these GTP analogues with carbachol. GTP was also stimulatory and guanosine 5'-(beta-thio)diphosphate was inhibitory in the presence of agonist. Atropine blocked the effects of carbachol. Stimulation by GTP(S) (0.1 mM) occurred after a 1-2-min lag, whereas Ca2+ (0.5 mM), carbachol (1 mM), and carbachol plus GTP(S) stimulated without delay. The effects of carbachol plus GTP(S) but not those of Ca2+ were inhibited by spermine (4 mM). Accumulation of 3H-inositol phosphates was enhanced by Li+ (4 mM) only in intact cells. In intact or permeabilized cells, the "partial" agonist arecoline was maximally 40-50% as efficacious as carbachol. In permeabilized cells, the maximal effects of carbachol and arecoline were enhanced 2.8- and 5.3-fold, respectively, by 0.1 mM GTP(S), but only the EC50 for carbachol was substantially reduced. The binding affinity of carbachol but not that of arecoline in permeabilized cells was significantly reduced by 0.1 mM GppNHp. These data indicate that a guanine nucleotide-binding regulatory protein is involved in coupling muscarinic receptors to phosphoinositidase C in SH-SY5Y cells and that the activity of this protein influences the relationship between receptor occupation and phosphoinositide response.

Beta-adrenergic inhibition of cardiac sodium channels by dual G-protein pathways

The signaling pathways by which beta-adrenergic agonists modulate voltage-dependent cardiac sodium currents are unknown, although it is likely that adenosine 3'5'-monophosphate (cAMP) is involved. Single-channel and whole-cell sodium currents were measured in cardiac myocytes and the signal transducing G protein Gs was found to couple beta-adrenergic receptors to sodium channels by both cytoplasmic (indirect) and membrane-delimited (direct) pathways. Hence, Gs can act on at least three effectors in the heart: sodium channels, calcium channels, and adenylyl cyclase. The effect on sodium currents was inhibitory and was enhanced by membrane depolarization. During myocardial ischemia the sodium currents of depolarized cells may be further inhibited by the accompanying increase in catecholamine levels.

Receptor-regulated formation of GTP[gamma S] with subsequent persistent Gs-protein activation in membranes of human platelets

Preincubation of human platelet membranes with the ATP analog ATP[gamma S] led to persistent adenylate cyclase activation. This stimulation was increased by copreincubation with PGE1 and obliterated by removing endogenous GDP by the NTP-regenerating system, creatine phosphate plus creatine kinase. PGE1 partially reversed the action of the regenerating system. Control formation of GTP[gamma S] from ATP[gamma S] and GDP in platelet membranes was apparently not stimulated by PGE1. In contrast, in the presence of creatine phosphate plus creatine kinase, which prevented formation of GTP[gamma S], PGE1 stimulated formation of this GTP analog, by partially reversing the action of the NTP-regenerating system. The data indicate that GTP[gamma S] can be formed by a membrane-associated nucleoside diphosphokinase from ATP[gamma S] and GDP, resulting in persistent Gs-protein activation, and that this process can be stimulated by an agonist-activated receptor.