title: R4496C RyR2 mutation impairs atrial and ventricular contractility.
abstract: Ryanodine receptor (RyR2) is the major Ca(2+) channel of the cardiac sarcoplasmic reticulum (SR) and plays a crucial role in the generation of myocardial force. Changes in RyR2 gating properties and resulting increases in its open probability (Po) are associated with Ca(2+) leakage from the SR and arrhythmias; however, the effects of RyR2 dysfunction on myocardial contractility are unknown. Here, we investigated the possibility that a RyR2 mutation associated with catecholaminergic polymorphic ventricular tachycardia, R4496C, affects the contractile function of atrial and ventricular myocardium. We measured isometric twitch tension in left ventricular and atrial trabeculae from wild-type mice and heterozygous transgenic mice carrying the R4496C RyR2 mutation and found that twitch force was comparable under baseline conditions (30°C, 2 mM [Ca(2+)]o, 1 Hz). However, the positive inotropic responses to high stimulation frequency, 0.1 µM isoproterenol, and 5 mM [Ca(2+)]o were decreased in R4496C trabeculae, as was post-rest potentiation. We investigated the mechanisms underlying inotropic insufficiency in R4496C muscles in single ventricular myocytes. Under baseline conditions, the amplitude of the Ca(2+) transient was normal, despite the reduced SR Ca(2+) content. Under inotropic challenge, however, R4496C myocytes were unable to boost the amplitude of Ca(2+) transients because they are incapable of properly increasing the amount of Ca(2+) stored in the SR because of a larger SR Ca(2+) leakage. Recovery of force in response to premature stimuli was faster in R4496C myocardium, despite the unchanged rates of recovery of L-type Ca(2+) channel current (ICa-L) and SR Ca(2+) content in single myocytes. A faster recovery from inactivation of the mutant R4496C channels could explain this behavior. In conclusion, changes in RyR2 channel gating associated with the R4496C mutation could be directly responsible for the alterations in both ventricular and atrial contractility. The increased RyR2 Po and fractional Ca(2+) release from the SR induced by the R4496C mutation preserves baseline contractility despite a slight decrease in SR Ca(2+) content, but cannot compensate for the inability to increase SR Ca(2+) content during inotropic challenge.
authors.affiliation: Center for Molecular Medicine and Applied Biophysics, University of Florence, 50121 Florence, Italy cecilia.ferrantini@unifi.it.
authors.forename: Cecilia
authors.initials: C
authors.lastname: Ferrantini
authors.affiliation: Center for Molecular Medicine and Applied Biophysics, University of Florence, 50121 Florence, Italy.
authors.forename: Raffaele
authors.initials: R
authors.lastname: Coppini
authors.affiliation: Center for Molecular Medicine and Applied Biophysics, University of Florence, 50121 Florence, Italy.
authors.forename: Beatrice
authors.initials: B
authors.lastname: Scellini
authors.affiliation: Center for Molecular Medicine and Applied Biophysics, University of Florence, 50121 Florence, Italy.
authors.forename: Claudia
authors.initials: C
authors.lastname: Ferrara
authors.affiliation: Center for Molecular Medicine and Applied Biophysics, University of Florence, 50121 Florence, Italy.
authors.forename: Josè Manuel
authors.initials: JM
authors.lastname: Pioner
authors.affiliation: Center for Molecular Medicine and Applied Biophysics, University of Florence, 50121 Florence, Italy.
authors.forename: Luca
authors.initials: L
authors.lastname: Mazzoni
authors.affiliation: IRCCS Fondazione Salvatore Maugeri, 27100 Pavia, Italy.
authors.forename: Silvia
authors.initials: S
authors.lastname: Priori
authors.affiliation: Center for Molecular Medicine and Applied Biophysics, University of Florence, 50121 Florence, Italy.
authors.forename: Elisabetta
authors.initials: E
authors.lastname: Cerbai
authors.affiliation: Center for Molecular Medicine and Applied Biophysics, University of Florence, 50121 Florence, Italy.
authors.forename: Chiara
authors.initials: C
authors.lastname: Tesi
authors.affiliation: Center for Molecular Medicine and Applied Biophysics, University of Florence, 50121 Florence, Italy.
authors.forename: Corrado
authors.initials: C
authors.lastname: Poggesi
mesh_terms: D000818:Animals; D001145:Arrhythmias, Cardiac / Q000378:metabolism / Q000503:physiopathology; D002118:Calcium / Q000378:metabolism; D020746:Calcium Channels, L-Type / Q000378:metabolism; D020013:Calcium Signaling / Q000187:drug effects / Q000235:genetics / Q000502:physiology; D006325:Heart Atria / Q000187:drug effects / Q000378:metabolism / Q000503:physiopathology*; D006352:Heart Ventricles / Q000187:drug effects / Q000503:physiopathology*; D007545:Isoproterenol / Q000494:pharmacology; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic / Q000235:genetics; D009119:Muscle Contraction / Q000187:drug effects / Q000235:genetics* / Q000502:physiology*; D009154:Mutation / Q000187:drug effects / Q000235:genetics; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D019837:Ryanodine Receptor Calcium Release Channel / Q000235:genetics*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D017180:Tachycardia, Ventricular / Q000378:metabolism / Q000503:physiopathology; D000098850:Polymorphic Catecholaminergic Ventricular Tachycardia
chemical_list: D020746:Calcium Channels, L-Type; D019837:Ryanodine Receptor Calcium Release Channel; D007545:Isoproterenol; D002118:Calcium