Remodelling of cardiac gap junction connexin 43 and arrhythmogenesis.

BACKGROUND: In cardiac muscle, the gap junction plays a pivotal role in electrical cell-to-cell coupling and impulse propagation between cells. The function of the gap junction depends on the regulation of connexin in the gap junction channel. A dysfunction of the gap junction is possibly caused by the downregulation of connexin or one of arrhythmogenic factors. The mechanisms of ventricular fibrillation, a lethal tachyarrhythmia, have been studied in relation to the remodelling of connexin.
OBJECTIVES: To determine what type of connexin 43 (Cx43) remodelling contributes to the generation of ventricular fibrillation and what factors induce the modelling of Cx43.
METHODS: Aconitine-induced ventricular fibrillation was induced in hearts isolated from adult rats. Alterations in the electrical activity, the phosphorylation of Cx43 and the expression of Cx43 were evaluated by both intracellular and extracellular recording of the action potentials, Western blotting and immunohistochemistry, respectively. Flutter activity after the application of aconitine shifted spontaneously to fibrillation, showing an electrical interaction between neighbouring cells in close proximity to one another. The facility of the shift from flutter to fibrillation was evaluated as a susceptibility of the heart to fibrillation in relation to gap junction function. The effects of phorbol 12-myristate 13-acetate, angiotensin II (AII) analogues, AII antagonists, the diabetic state, protein kinase A (PKA) activator, cyclic AMP analogues, d-sotalol (class III antiarrhythmic drug) and PKA inhibitors on the susceptibility of the heart to fibrillation were examined.
RESULTS: Pathological hearts with heterogeneous expression of Cx43 at the gap junction, such as phorbol 12-myristate 13-acetate-and AII analogue-treated hearts, as well as diabetic hearts, showed a significantly higher susceptibility to fibrillation. On the other hand, hearts with augmentative expression of Cx43 at the gap junction, such as hearts pretreated with a PKA activator, a cyclic AMP analogue (8-bromo-cyclic AMP) or d-sotalol, showed a significantly lower susceptibility to fibrillation. At the beginning of fibrillation, an increase in the cardiac tissue AII level, an augmentation of the protein kinase C (PKC)-epsilon activity, the presence of PKC-mediated hyperphosphorylation, a suppression of the PKA-mediated phosphorylation of Cx43 and a reduction in the expression of Cx43 at the gap junction were observed. These alterations in Cx43 expression were also observed to increase as the fibrillation advanced.
CONCLUSIONS: Augmentation of PKC-mediated phosphorylation and suppression of PKA-mediated phosphorylation induces the downward remodelling of Cx43. Such remodelling of Cx43 induces asynchronous electrical activities and makes the ventricular tissue susceptible to fibrillation. PKC is activated by AII. The fibrillation itself remodels Cx43, thereby causing a vicious cycle. As a result, PKC inhibitors, AII antagonists and PKA activators are considered to possibly have a protective effect against the initiation or advancement of ventricular fibrillation.