title: Remodeling of excitation-contraction coupling in transgenic mice expressing ATP-insensitive sarcolemmal KATP channels.
abstract: Reducing the ATP sensitivity of the sarcolemmal ATP-sensitive K(+) (K(ATP)) channel is predicted to lead to active channels in normal metabolic conditions and hence cause shortened ventricular action potentials and reduced myocardial inotropy. We generated transgenic (TG) mice that express an ATP-insensitive K(ATP) channel mutant [Kir6.2(deltaN2-30,K185Q)] under transcriptional control of the alpha-myosin heavy chain promoter. Strikingly, myocyte contraction amplitude was increased in TG myocytes (15.68 +/- 1.15% vs. 10.96 +/- 1.49%), even though K(ATP) channels in TG myocytes are very insensitive to inhibitory ATP. Under normal metabolic conditions, steady-state outward K(+) currents measured under whole cell voltage clamp were elevated in TG myocytes, consistent with threshold K(ATP) activation, but neither the monophasic action potential measured in isolated hearts nor transmembrane action potential measured in right ventricular muscle preparations were shortened at physiological pacing cycles. Taken together, these results suggest that there is a compensatory remodeling of excitation-contraction coupling in TG myocytes. Whereas there were no obvious differences in other K(+) conductances, peak L-type Ca(2+) current (I(Ca)) density (-16.42 +/- 2.37 pA/pF) in the TG was increased compared with the wild type (-8.43 +/- 1.01 pA/pF). Isoproterenol approximately doubled both I(Ca) and contraction amplitude in wild-type myocytes but failed to induce a significant increase in TG myocytes. Baseline and isoproterenol-stimulated cAMP concentrations were not different in wild-type and TG hearts, suggesting that the enhancement of I(Ca) in the latter does not result from elevated cAMP. Collectively, the data demonstrate that a compensatory increase in I(Ca) counteracts a mild activation of ATP-insensitive K(ATP) channels to maintain the action potential duration and elevate the inotropic state of TG hearts.
authors.affiliation: Department of Cell Biology and Physiology, Washington University School of Medicine, St Louis, MO 63110, USA.
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Flagg
authors.forename: Flavien
authors.initials: F
authors.lastname: Charpentier
authors.forename: Jocelyn
authors.initials: J
authors.lastname: Manning-Fox
authors.forename: Maria Sara
authors.initials: MS
authors.lastname: Remedi
authors.forename: Decha
authors.initials: D
authors.lastname: Enkvetchakul
authors.forename: Anatoli
authors.initials: A
authors.lastname: Lopatin
authors.forename: Joseph
authors.initials: J
authors.lastname: Koster
authors.forename: Colin
authors.initials: C
authors.lastname: Nichols
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000255:Adenosine Triphosphate / Q000502:physiology*; D000818:Animals; D020746:Calcium Channels, L-Type / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology; D002469:Cell Separation; D018076:DNA, Complementary / Q000235:genetics; D004558:Electric Stimulation; D004594:Electrophysiology; D051379:Mice; D008822:Mice, Transgenic; D016296:Mutagenesis; D009200:Myocardial Contraction / Q000235:genetics* / Q000502:physiology*; D032383:Myocytes, Cardiac / Q000502:physiology; D024661:Potassium Channels, Inwardly Rectifying / Q000235:genetics / Q000378:metabolism*; D011863:Radioimmunoassay; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D020746:Calcium Channels, L-Type; D018076:DNA, Complementary; D024661:Potassium Channels, Inwardly Rectifying; D000255:Adenosine Triphosphate