title: Effects of PP1/PP2A inhibitor calyculin A on the E-C coupling cascade in murine ventricular myocytes.
abstract: Calyculin A was used to examine the importance of phosphatases in the modulation of cardiac contractile magnitude in the absence of any neural or humoral stimulation. Protein phosphatase (PP)1 and PP2A activity, twitch contractions, intracellular Ca(2+) concentration ([Ca(2+)](i)) transients, action potentials, membrane currents, and myofilament Ca(2+) sensitivity were measured in isolated mouse ventricular myocytes. Calyculin A (125 nM) inhibited PP1 and PP2A by 50% and 85%, respectively, whereas it doubled the twitch magnitude and increased twitch duration by 50% in field-stimulated cells. Calyculin A-evoked increases in L-type Ca(2+) current (70%) and the resulting [Ca(2+)](i) transient (83%) explain the positive inotropic response. However, increases in twitch and action potential durations did not result from increased myofilament Ca(2+) sensitivity or K(+) current inhibition, respectively. Comparison of the effects of calyculin A and isoproterenol on [Ca(2+)](i) transients and twitch contractions revealed that calyculin A had a much smaller lusitropic effect than the beta-agonist, indicating that calyculin A did not significantly increase sarcoplasmic reticulum Ca(2+) reuptake. Thus while cardiac contractile magnitude is controlled by a steady-state kinase/phosphatase balance, this regulation is not equally operative at all of the steps in the excitation-contraction coupling pathway and may in fact be most important to the regulation of the L-type Ca(2+) channel.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, 108 N. Greene St., Baltimore, MD 21201, USA.
authors.forename: William H
authors.initials: WH
authors.lastname: duBell
authors.forename: Marisa S
authors.initials: MS
authors.lastname: Gigena
authors.forename: Silvia
authors.initials: S
authors.lastname: Guatimosim
authors.forename: Xilin
authors.initials: X
authors.lastname: Long
authors.forename: W J
authors.initials: WJ
authors.lastname: Lederer
authors.forename: Terry B
authors.initials: TB
authors.lastname: Rogers
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D020013:Calcium Signaling / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D006352:Heart Ventricles; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology; D007700:Kinetics; D008297:Male; D008387:Marine Toxins; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D051379:Mice; D009200:Myocardial Contraction / Q000187:drug effects / Q000502:physiology*; D009206:Myocardium / Q000166:cytology / Q000201:enzymology*; D010080:Oxazoles / Q000494:pharmacology*; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors*; D010762:Phosphorylase a / Q000378:metabolism; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology
chemical_list: D004791:Enzyme Inhibitors; D008387:Marine Toxins; D010080:Oxazoles; D015221:Potassium Channels; C059041:calyculin A; D010762:Phosphorylase a; D010749:Phosphoprotein Phosphatases; D007545:Isoproterenol; D002118:Calcium