Coupling to Gs and G(q/11) of histamine H2 receptors heterologously expressed in adult rat atrial myocytes.
The predominant histamine receptor subtype in the supraventricular and ventricular tissue of various mammalian species is the H2 receptor (H2-R) subtype, which is known to couple to stimulatory G proteins (Gs), i.e. the major effects of this autacoid are an increase in sinus rate and in force of contraction. To investigate histamine effects in H2-R-transfected rat atrial myocytes, endogenous GIRK currents and L-type Ca2+ currents were used as functional assays. In H2-R-transfected myocytes, exposure to His resulted in a reversible augmentation of L-type Ca2+ currents, consistent with the established coupling of this receptor to the Gs-cAMP-PKA signalling pathway. Mammalian K+ channels composed of GIRK (Kir3.x) subunits are directly controlled by interaction with betagamma subunits released from G proteins, which couple to seven-helix receptors. In mock-transfected atrial cardiomyocytes, activation of muscarinic K+ channels (IK(ACh)) was limited to Gi-coupled receptors (M2R, A1R). In H2-R-overexpressing cells, histamine activated IK(ACh) via Gs-derived betagamma subunits since the histamine-induced current was insensitive to pertussis toxin. These data indicate that overexpression of Gs-coupled H2-R results in a loss of target specificity due to an increased agonist-induced release of Gs-derived betagamma subunits. When IK(ACh) was maximally activated by GTP-gamma-S, histamine induced an irreversible inhibition of the inward current in a fraction of H2-R-transfected cells. This inhibition is supposed to be mediated via a G(q/11)-PLC-mediated depletion of PIP2, suggesting a partial coupling of overexpressed H2-R to G(q/11). Dual coupling of H2-Rs to Gs and Gq is demonstrated for the first time in cardiac myocytes. It represents a novel mechanism to augment positive inotropic effects by activating two different signalling pathways via one type of histamine receptor. Activation of the Gs-cAMP-PKA pathway promotes Ca2+ influx through phosphorylation of L-type Ca2+ channels. Simultaneous activation of Gq-signalling pathways might result in phosphoinositide turnover and Ca2+ release from intracellular stores, thereby augmenting H2-induced increases in [Ca2+]i.
Wellner-Kienitz
Marie Cécile
MC
Department of Physiology, Ruhr-University Bochum, D-44780 Bochum, Germany. marie-cecile.wellner-kienitz@ruhr-uni-bochum.de
Bender
Kirsten
K
Meyer
Thomas
T
Pott
Lutz
L
D000818:Animals; D020746:Calcium Channels, L-Type / Q000187:drug effects / Q000378:metabolism; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism; D017868:Cyclic AMP-Dependent Protein Kinases / Q000187:drug effects / Q000378:metabolism; D005260:Female; D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D043802:GTP-Binding Protein alpha Subunits, Gq-G11; D019205:GTP-Binding Protein alpha Subunits, Gs / Q000187:drug effects / Q000378:metabolism*; D006325:Heart Atria / Q000187:drug effects / Q000378:metabolism*; D020962:Heterotrimeric GTP-Binding Proteins / Q000187:drug effects / Q000378:metabolism*; D006632:Histamine / Q000378:metabolism / Q000494:pharmacology; D008297:Male; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism*; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000378:metabolism; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism; D024661:Potassium Channels, Inwardly Rectifying; D051381:Rats; D043585:Receptor, Muscarinic M2; D011970:Receptors, Histamine H2 / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D014162:Transfection
D020746:Calcium Channels, L-Type; D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D019269:Phosphatidylinositol 4,5-Diphosphate; D015221:Potassium Channels; D024661:Potassium Channels, Inwardly Rectifying; D043585:Receptor, Muscarinic M2; D011970:Receptors, Histamine H2; D011976:Receptors, Muscarinic; D006632:Histamine; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D043802:GTP-Binding Protein alpha Subunits, Gq-G11; D019205:GTP-Binding Protein alpha Subunits, Gs; D020962:Heterotrimeric GTP-Binding Proteins