title: Alpha1-adrenoceptor-mediated breakdown of phosphatidylinositol 4,5-bisphosphate inhibits pinacidil-activated ATP-sensitive K+ currents in rat ventricular myocytes.
abstract: Phosphatidylinositol 4,5-bisphosphate (PIP2) stimulates ATP-sensitive K+ (K(ATP)) channel activity. Because phospholipase C (PLC) hydrolyzes membrane-bound PIP2, which in turn may potentially decrease K(ATP) channel activity, we investigated the effects of the alpha1-adrenoceptor-G(q)-PLC signal transduction axis on pinacidil-activated K(ATP) channel activity in adult rat and neonatal mouse ventricular myocytes. The alpha1-adrenoceptor agonist methoxamine (MTX) reversibly inhibited the pinacidil-activated K(ATP) current in a concentration-dependent manner (IC50 20.9+/-6.6 micromol/L). This inhibition did not occur when the specific alpha1-adrenoceptor antagonist, prazosin, was present. An involvement of G proteins is suggested by the ability of GDPbetaS to prevent this response. Blockade of PLC by U-73122 (2 micromol/L) or neomycin (2 mmol/L) attenuated the MTX-induced inhibition of K(ATP) channel activity. In contrast, the MTX response was unaffected by protein kinase C inhibition or stimulation by H-7 (100 micro mol/L) or phorbol 12,13-didecanoate. The MTX-induced inhibition became irreversible in the presence of wortmannin (20 micro mol/L), an inhibitor of phosphatidylinositol-4 kinase, which is expected to prevent membrane PIP2 replenishment. In excised inside-out patch membranes, pinacidil induced a significantly rightward shift of ATP sensitivity of the channel. This phenomenon was reversed by pretreatment of myocytes with MTX. Direct visualization of PIP2 subcellular distribution using a PLCdelta pleckstrin homology domain-green fluorescent protein fusion constructs revealed reversible translocation of green fluorescent protein fluorescence from the membrane to the cytosol after alpha1-adrenoceptor stimulation. Our data demonstrate that alpha1-adrenoceptor stimulation reduces the membrane PIP2 level, which in turn inhibits pinacidil-activated K(ATP) channels.
authors.affiliation: Department of Cardiovascular Medicine, Kyoto University Graduate School of Medicine, Kyoto, Japan.
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Haruna
authors.forename: Hidetada
authors.initials: H
authors.lastname: Yoshida
authors.forename: Tomoe Y
authors.initials: TY
authors.lastname: Nakamura
authors.forename: Lai-Hua
authors.initials: LH
authors.lastname: Xie
authors.forename: Hideo
authors.initials: H
authors.lastname: Otani
authors.forename: Tomonori
authors.initials: T
authors.lastname: Ninomiya
authors.forename: Makoto
authors.initials: M
authors.lastname: Takano
authors.forename: William A
authors.initials: WA
authors.lastname: Coetzee
authors.forename: Minoru
authors.initials: M
authors.lastname: Horie
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology*; D058646:Adrenergic alpha-1 Receptor Agonists; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000730:Androstadienes / Q000494:pharmacology; D000818:Animals; D000831:Animals, Newborn; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D004553:Electric Conductivity; D004791:Enzyme Inhibitors / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000502:physiology; D006352:Heart Ventricles / Q000187:drug effects / Q000378:metabolism; D008729:Methoxamine / Q000494:pharmacology; D051379:Mice; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000378:metabolism*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D020110:Pinacidil / Q000037:antagonists & inhibitors*; D015221:Potassium Channels / Q000502:physiology*; D011493:Protein Kinase C / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D018340:Receptors, Adrenergic, alpha-1 / Q000502:physiology*; D010738:Type C Phospholipases / Q000502:physiology; D016276:Ventricular Function; D000077191:Wortmannin
chemical_list: D058646:Adrenergic alpha-1 Receptor Agonists; D000316:Adrenergic alpha-Agonists; D000730:Androstadienes; D004791:Enzyme Inhibitors; D019269:Phosphatidylinositol 4,5-Diphosphate; D000081082:Phosphoinositide-3 Kinase Inhibitors; D015221:Potassium Channels; D018340:Receptors, Adrenergic, alpha-1; D020110:Pinacidil; D000255:Adenosine Triphosphate; D011493:Protein Kinase C; D010738:Type C Phospholipases; D019204:GTP-Binding Proteins; D008729:Methoxamine; D000077191:Wortmannin