Abstract 1088: Prevention of Connexin 43 (Cx43) Dephosphorylation is Associated with Reduced Ventricular Tachycardia in Cardiac-Specific Angiotensin Converting Enzyme (ACE) Overexpression Mice
Introduction: Connexin 43 (Cx43) is a major component of the ventricular gap junctional complex, which provides a low resistance pathway for electrical conduction between myocytes. Reduction of Cx43 amount of phosphorylation has been implicated in the pathogenesis of ventricular tachycardia (VT) and fibrillation. Cardiac-specific angiotensin converting enzyme overexpression mice (ACE 8/8) have slow conduction, show conduction blocks, are prone to VT and sudden death, and show profound reductions in Cx43 levels and phosphorylation. Therefore, we hypothesized that reversal of Cx43 changes, by preventing the renin-angiotensin system (RAS) activation, would reduce the arrhythmic risk in these animals.
Methods: Prevention of RAS activation was accomplished by one of three methods:
ACE 8/8 receiving an ACE inhibitor (captopril),
ACE 8/8 mice treated with a specific angiotensin receptor type-1 blocker (losartan), and
ACE 8/8 mice crossed with homozygote angiotensinogen null mice.
Pooled results were compared to untreated ACE 8/8 littermates. Surface electrocardiography and invasive electrophysiological study with programmed ventricular stimulation were performed on ten-week old mice, and the hearts were extracted for immunoblotting.
Results: VT was induced in 10 out of 12 untreated ACE 8/8 mice, but only in 3 out of 11 treated mice (p<0.02). Total Cx43 was not changed significantly in treated animals as compared to controls, but the ratio (P/NP) of phosphorylated Cx43 (bands P1 and P2 in immunoblot) to non-phosphorylated Cx43 (band P0) showed a trend toward increase in the treatment group (0.78±0.18 vs. 1.25±0.19; P=0.09). Dividing the mice into two groups, high-risk (inducible and/or spontaneous VT) versus low-risk (no VT), we observed a significant difference in P/NP ratio (0.75±0.12 in high-risk mice vs. 1.78±0.32 in low-risk, p<0.01).
Discussion: Unexpectedly, disruption of RAS signaling did not increase total Cx43 levels in ACE 8/8 mice. On the other hand, the treatments increased phospho-Cx43, suggesting that RAS controls Cx43 phosphorylation but not total amount. Even in the presence of significantly depressed levels of Cx43, small changes in Cx43 phosphorylation were associated with a significant reduction in VT inducibility.