Progressive induction of left ventricular pressure overload in a large animal model elicits myocardial remodeling and a unique matrix signature

Abstract

Objective: Patients with severe left ventricular pressure overload secondary to aortic stenosis can present with signs and symptoms of heart failure despite normal left ventricular ejection fraction. This process occurs, at least in part, as a result of left ventricular pressure overload-induced extracellular matrix remodeling that promulgates increased left ventricular stiffness and impaired diastolic function. However, the determinants that drive extracellular matrix remodeling in this form of left ventricular pressure overload remain to be fully defined.

Methods: Left ventricular pressure overload was induced in mature pigs (n = 15) by progressive ascending aortic cuff inflation (once per week for 4 weeks), whereby left ventricular mass, left ventricular ejection fraction, and regional myocardial stiffness (rK(m)) were compared with referent controls (n = 12). Determinants of extracellular matrix remodeling were assessed by measuring levels of mRNA expression for fibrillar collagens, matrix metalloproteinases, and tissue inhibitors of matrix metalloproteinase 1 and 4.

Results: With left ventricular pressure overload, left ventricular mass and rK(m) increased by 2- and 3-fold, respectively, compared with control, with no change in left ventricular ejection fraction. Left ventricular myocardial collagen increased approximately 2-fold, which was accompanied by reduced solubility (ie, increased cross-linking) with left ventricular pressure overload, but mRNA expression for fibrillar collagen and matrix metalloproteinases remained relatively unchanged. In contrast, a robust increase in mRNA expression for tissue inhibitors of matrix metalloproteinase-1 and 4 occurred with left ventricular pressure overload.

Conclusions: In a progressive model of left ventricular pressure overload, which recapitulates the phenotype of aortic stenosis, increased extracellular matrix accumulation and subsequently increased myocardial stiffness were not due to increased fibrillar collagen expression but rather to determinants of post-translational control that included increased collagen stability (thereby resistant to matrix metalloproteinase degradation) and increased endogenous matrix metalloproteinase inhibition. Targeting these extracellular matrix post-translational events with left ventricular pressure overload may hold both diagnostic and therapeutic relevance.

Figure 1

(Top) Experimental design: progressive cuff inflation was used to recapitulate the LV phenotype of LVPO secondary to AS. (Bottom, left) Disengaged 12 mm inflatable silastic cuff connected to an implantable subcutaneous port accessed with an angled Huber needle. (Bottom, right) Intraoperative photograph demonstrates retraction of the main pulmonary artery (*) with fixation of a 12 mm inflatable silastic cuff around the supra-coronary ascending aorta (arrow).

Figure 2

(Left) LVPO was associated with a marked increase in the LV mass to body weight ratio. (Right) The LV ejection fraction remained unchanged. Thus, this gradual induction of LVPO caused significant LV hypertrophy without a compromise in LV ejection fraction. (* p<0.05 vs. Control)

Figure 3

(Top, left) Total collagen content was greater with LVPO. (Top, right) Total collagen content is determined by soluble and insoluble collagen components. The soluble:insoluble collagen ratio changed with LVPO and favored an increase in insoluble collagen content indicating increased collagen cross-linking and collagen stability. (Bottom, left) A relationship between myocardial collagen content and regional myocardial stiffness was evident. (Bottom, right) Collagen 1A1 and collagen 3A1 mRNA expression did not change with LVPO. Thus, the observed increase in collagen content was not due to an increase in fibrillar collagen expression.

Figure 4

Determinants of ECM remodeling and fibrosis in the setting of LVPO include the MMPs and TIMPs. The overall MMP:TIMP ratio favored reduced collagen degradation with LVPO.