Microtubules and angiotensin II receptors contribute to modulation of repolarization induced by ventricular pacing

BackgroundLeft ventricular pacing (LVP) in canine heart alters ventricular activation, leading to reduced transient outward potassium current (Ito), loss of the epicardial action potential notch, and T-wave vector displacement. These repolarization changes, referred to as cardiac memory, are initiated by locally increased angiotensin II (AngII) levels. In HEK293 cells in which Kv4.3 and KChIP2, the channel subunits contributing to Ito, are overexpressed with the AngII receptor 1 (AT1R), AngII induces a decrease in Ito as the result of internalization of a Kv4.3/KChIP2/AT1R macromolecular complex.ObjectiveTo test the hypothesis that in canine heart in situ, 2h LVP-induced decreases in membrane KChIP2, AT1R, and Ito are prevented by blocking subunit trafficking.MethodsWe used standard electrophysiological, biophysical, and biochemical methods to study 4 groups of dogs: (1) Sham, (2) 2h LVP, (3) LVP + colchicine (microtubule-disrupting agent), and (4) LVP + losartan (AT1R blocker).ResultsThe T-wave vector displacement was significantly greater in LVP than in Sham and was inhibited by colchicine or losartan. Epicardial biopsies showed significant decreases in KChIP2 and AT1R proteins in the membrane fraction after LVP but not after sham treatment, and these decreases were prevented by colchicine or losartan. Colchicine but not losartan significantly reduced microtubular polymerization. In isolated ventricular myocytes, AngII-induced Ito reduction and loss of action potential notch were blocked by colchicine.ConclusionsLVP-induced reduction of KChIP2 in plasma light membranes depends on an AngII-mediated pathway and intact microtubular status. Loss of Ito and the action potential notch appear to derive from AngII-initiated trafficking of channel subunits.