title: Loss of Nav1.5 expression and function in murine atria containing the RyR2-P2328S gain-of-function mutation.
abstract: OBJECTIVE
  Recent studies reported slowed conduction velocity (CV) in murine hearts homozygous for the gain-of-function RyR2-P2328S mutation (RyR2(S/S)) and associated this with an increased incidence of atrial and ventricular arrhythmias. The present experiments determined mechanisms contributing to the reduced atrial CV.
  
  
  RESULTS
  The determinants of CV were investigated in murine RyR2(S/S) hearts and compared with those in wild-type (WT) and slow-conducting Scn5a(+/-) hearts. Picrosirius red staining demonstrated increased fibrosis only in Scn5a(+/-) hearts. Immunoblot assays showed similar expressions of Cx43 and Cx40 levels in the three genotypes. In contrast, Nav1.5 expression was reduced in both RyR2(S/S) and Scn5a(+/-) atria. These findings correlated with intracellular microelectrode and loose-patch-clamp studies. Microelectrode measurements showed reduced maximum rates of depolarization in Scn5a(+/-) and RyR2(S/S) atria compared with WT, despite similar diastolic membrane potentials. Loose-patch-clamp measurements demonstrated reduced peak Na(+) currents (INa) in the Scn5a(+/-) and RyR2(S/S) atria relative to WT, with similar normalized current-voltage relationships. In WT atria, reduction in INa could be produced by treatment with high extracellular Ca(2+), caffeine, or cyclopiazonic acid, each expected to produce an acute increase in [Ca(2+)]i.
  
  
  CONCLUSIONS
  RyR2(S/S) atria show reduced levels of Nav1.5 expression and Na(+) channel function. Reduced Na(+) channel function was also observed in WT atria, following acute increases in [Ca(2+)]i. Taken together, the results suggest that raised [Ca(2+)]i produces both acute and chronic inhibition of Na(+) channel function. These findings may help explain the relationship between altered Ca(2+) homeostasis, CV, and the maintenance of common arrhythmias such as atrial fibrillation.
authors.affiliation: Physiological Laboratory, University of Cambridge, UK.
authors.forename: James H
authors.initials: JH
authors.lastname: King
authors.forename: Chandu
authors.initials: C
authors.lastname: Wickramarachchi
authors.forename: Krystine
authors.initials: K
authors.lastname: Kua
authors.forename: Yuan
authors.initials: Y
authors.lastname: Du
authors.forename: Kamalan
authors.initials: K
authors.lastname: Jeevaratnam
authors.forename: Hugh R
authors.initials: HR
authors.lastname: Matthews
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Grace
authors.forename: Christopher L-H
authors.initials: CL
authors.lastname: Huang
authors.forename: James A
authors.initials: JA
authors.lastname: Fraser
mesh_terms: D000818:Animals; D001145:Arrhythmias, Cardiac / Q000209:etiology; D016275:Atrial Function / Q000502:physiology*; D002118:Calcium / Q000378:metabolism; D018031:Connexin 43 / Q000032:analysis; D017630:Connexins / Q000032:analysis; D006325:Heart Atria / Q000378:metabolism; D006329:Heart Conduction System / Q000502:physiology*; D051379:Mice; D009154:Mutation; D062554:NAV1.5 Voltage-Gated Sodium Channel / Q000502:physiology*; D019837:Ryanodine Receptor Calcium Release Channel / Q000235:genetics*; D000097003:Gap Junction alpha-5 Protein
chemical_list: D002118:Calcium; D018031:Connexin 43; D017630:Connexins; D062554:NAV1.5 Voltage-Gated Sodium Channel; D019837:Ryanodine Receptor Calcium Release Channel; D000097003:Gap Junction alpha-5 Protein; C568321:Scn5a protein, mouse
keywords: Atrial fibrillation; Calcium; Catecholaminergic polymorphic ventricular tachycardia; Conduction velocity; Ryanodine receptor