Chronic pressure overload causes myocardial
hypertrophy, increased
fibrillar collagen content, and abnormal diastolic function. We hypothesized that one determinant of these pressure overload-induced changes is the extracellular processing of newly synthesized
procollagen into mature
collagen fibrils. We further hypothesized that secreted
protein acidic and rich in
cysteine (SPARC) plays a key role in post-synthetic
procollagen processing in normal and pressure-overloaded myocardium.
METHODS AND RESULTS: To determine whether pressure overload-induced changes in
collagen content and diastolic function are affected by the absence of SPARC, age-matched wild-type (WT) and SPARC-null mice underwent either transverse aortic constriction (TAC) for 4 weeks or served as nonoperated controls. Left ventricular (LV)
collagen content was measured histologically by
collagen volume fraction,
collagen composition was measured by
hydroxyproline assay as soluble
collagen (1 mol/L NaCl extractable) versus insoluble
collagen (mature cross-linked
collagen), and
collagen morphological structure was examined by scanning electron microscopy. SPARC expression was measured by immunoblot. LV, myocardial, and cardiomyocyte structure and function were assessed by echocardiographic, papillary muscle, and isolated cardiomyocyte studies. In WT mice, TAC increased LV mass, SPARC expression, myocardial diastolic stiffness,
fibrillar collagen content, and soluble and insoluble
collagen. In SPARC-null mice, TAC increased LV mass to an extent similar to WT mice. In addition, in SPARC-null mice, TAC increased
fibrillar collagen content, albeit significantly less than that seen in WT TAC mice. Furthermore, the proportion of LV
collagen that was insoluble was less in the SPARC-null TAC mice (86+/-2%) than in WT TAC mice (99+/-2%, P<0.05), and the proportion of
collagen that was soluble was greater in the SPARC-null TAC mice (14+/-2%) than in WT TAC mice (1+/-2%, P<0.05) As a result, myocardial diastolic stiffness was lower in SPARC-null TAC mice (0.075+/-0.005) than in WT TAC mice (0.045+/-0.005, P<0.05).
CONCLUSIONS: The absence of SPARC reduced pressure overload-induced alterations in extracellular matrix
fibrillar collagen and diastolic function. These data support the hypothesis that SPARC plays a key role in post-synthetic
procollagen processing and the development of mature cross-linked
collagen fibrils in normal and pressure-overloaded myocardium.