Diminished cardiac fibrosis in heart failure is associated with altered ventricular arrhythmia phenotype

Abstract

Objectives: We sought to define the role of interstitial fibrosis in the proarrhythmic phenotype of failing ventricular myocardium.

Background: Multiple cellular events that occur during pathological remodeling of the failing ventricle are implicated in the genesis of ventricular tachycardia (VT), including interstitial fibrosis. Recent studies suggest that ventricular fibrosis is reversible, and current anti-remodeling therapies attenuate ventricular fibrosis. However, the role of interstitial fibrosis in the proarrhythmic phenotype of failing ventricular myocardium is currently not well defined.

Methods: Class II histone deacetylases (HDACs) have been implicated in promoting collagen biosynthesis. As these enzymes are inhibited by protein kinase D1 (PKD1), we studied mice with cardiomyocyte-specific transgenic over-expression of a constitutively active mutant of PKD1 (caPKD). caPKD mice were compared with animals in which cardiomyopathy was induced by severe thoracic aortic banding (sTAB). Hearts were analyzed by echocardiographic and electrocardiographic means. Interstitial fibrosis was assessed by histology and quantified biochemically. Ventricular arrhythmias were induced by closed-chest, intracardiac pacing.

Results: Similar degrees of hypertrophic growth, systolic dysfunction and mortality were observed in the two models. In sTAB mice, robust ventricular fibrosis was readily detected, but myocardial collagen content was significantly reduced in caPKD mice. As expected, VT was readily inducible by programmed stimulation in sTAB mice and VT was less inducible in caPKD mice. Surprisingly, episodes of VT manifested longer cycle lengths and longer duration in caPKD mice.

Conclusion: Attenuated ventricular fibrosis is associated with reduced VT inducibility, increased VT duration, and significantly longer arrhythmia cycle length.

Figure 1

(A) Representative histological heart sections from WT, caPKD, and sTAB-operated animals showed similar degrees of cardiac enlargement and dilation in sTAB-operated and caPKD transgenic animals. Echocardiographic assessment based on M-mode images showed significantly depressed left ventricular function in both the sTAB and caPKD mice (B) and comparable increases in the left ventricular systolic diameters (C). Assessment of heart mass revealed comparable increases in heart weight to body weight ratio in sTAB and caPKD mice (D). PR interval was slightly prolonged in caPKD mice as compared with WT and sTAB animals, and QRS interval was shorter in WT animals as compared with sTAB and caPKD animals. There were no significant differences between the groups in various other electrocardiographic intervals, under resting sedated conditions (E). Vertical bars represent S.E.M. * denotes P < 0.05. Bar of measure equals 2 mm.

Figure 2

(A) Representative action potentials recorded from LV myocytes in WT, CaPKD, and sTAB mice. (B) Summary of the resting and active membrane properties of LV myocytes in WT, CaPKD, and sTAB mice. * denotes P < 0.05, compared with WT.

Figure 3

(A) Representative Masson’s trichrome-stained histological heart sections from WT, Sham-operated, sTAB-operated, and caPKD transgenic animals. Histologically, caPKD hearts had diminished fibrosis as compared with sTAB hearts, whereas Wild type and Sham-operated animals showed no evidence of collagen deposition in the heart. (B) Quantification of collagen, using hydroxyproline assay, showed decreased amounts of collagen in caPKD hearts as compared with sTAB animals. Vertical bars represent S.E.M. * denotes P < 0.05. Bar of measure equals 200 µm.

Figure 4

(A) Shown is a representative image of induced ventricular tachycardia in sTAB and caPKD hearts. Electrophysiological evaluation, including comprehensive programmed electrical stimulation with and without isoproterenol, revealed significantly less amount of induced ventricular arrhythmias in caPKD mice as compared with sTAB animals (B). * denotes P < 0.05. Bar of measure equals 200 milliseconds.

Figure 5

(A) The duration of induced ventricular tachycardia was significantly longer in caPKD mice as compared with sTAB animals. (B) Cycle length analysis of induced arrhythmias revealed that ventricular tachycardia was slower in caPKD mice as compared with sTAB animals. Vertical bars represent SEM. * denotes P < 0.05.