title: Transient-outward K+ channel inhibition facilitates L-type Ca2+ current in heart.
abstract: BACKGROUND
  Transient outward current (I(to)) and L-type calcium current (I(Ca)) are important repolarization currents in cardiac myocytes. These two currents often undergo disease-related remodeling while other currents are spared, suggesting a functional coupling between them. Here, we investigated the effects of I(to) channel blockers, 4-aminopyridine (4-AP) and heteropodatoxin-2 (HpTx2), on I(Ca) in cardiac ventricular myocytes.
  
  
  RESULTS
  I(Ca) was recorded in enzymatically dissociated mouse and guinea pig ventricular myocytes using the whole-cell voltage clamp method. In mouse ventricular myocytes, 4-AP (2 mM) significantly facilitated I(Ca) by increasing current amplitude and slowing inactivation. These effects were not voltage-dependent. Similar facilitating effects were seen when equimolar Ba2+ was substituted for external Ca2+, indicating that Ca2+ influx is not required. Measurements of Ca2+/calmodulin-dependent protein kinase (CaMKII) activity revealed significant increases in cells treated with 4-AP. Pretreatment of cells with 10 microM KN93, a specific inhibitor of CaMKII, abolished the effects of 4-AP on I(Ca.) To test the requirement of I(to), we studied guinea pig ventricular myocytes, which do not express I(to) channels. In these cells, 2 mM 4-AP had no effect on I(Ca) amplitude or kinetics. In both cell types, Ca2+-induced I(Ca) facilitation, a CaMKII-dependent process, was observed. However, 4-AP abolished Ca2+-induced I(Ca) facilitation exclusively in mouse ventricular myocytes.
  
  
  CONCLUSIONS
  4-AP, an I(to) blocker, facilitates L-type Ca2+ current through a mechanism involving the I(to) channel and CaMKII activation. These data indicate a functional association of I(Ca) and I(to) in cardiac myocytes.
authors.affiliation: Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas 75390, USA. yanggan.wang@utsouthwestern.edu
authors.forename: Yanggan
authors.initials: Y
authors.lastname: Wang
authors.forename: Jun
authors.initials: J
authors.lastname: Cheng
authors.forename: Samvit
authors.initials: S
authors.lastname: Tandan
authors.forename: Minjie
authors.initials: M
authors.lastname: Jiang
authors.forename: Diana T
authors.initials: DT
authors.lastname: McCloskey
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Hill
mesh_terms: D015761:4-Aminopyridine / Q000494:pharmacology; D000818:Animals; D020746:Calcium Channels, L-Type / Q000378:metabolism*; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002478:Cells, Cultured; D006168:Guinea Pigs; D006352:Heart Ventricles / Q000166:cytology* / Q000378:metabolism; D015640:Ion Channel Gating / Q000502:physiology; D051379:Mice; D009498:Neurotoxins / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D051672:Shal Potassium Channels / Q000037:antagonists & inhibitors*; D013111:Spider Venoms / Q000737:chemistry
chemical_list: D020746:Calcium Channels, L-Type; D009498:Neurotoxins; D051672:Shal Potassium Channels; D013111:Spider Venoms; D015761:4-Aminopyridine; D017871:Calcium-Calmodulin-Dependent Protein Kinases