A sedentary lifestyle is associated with increased cardiovascular risk factors and reduced cardiac compliance when compared to a lifestyle that includes exercise training. Exercise training increases cardiac compliance in humans, but the mechanisms underlying this improvement are unknown. A major determinant of cardiac compliance is the compliance of the giant elastic protein titin. Experimentally reducing titin compliance in animal models reduces exercise tolerance, but it is not known whether sedentary versus chronic exercise conditions cause differences in titin isoform content. We hypothesized that sedentary conditions would be associated with a reduction in the content of the longer, more compliant N2BA isoform relative to the stiffer N2B isoform (yielding a reduced N2BA:N2B ratio) compared to age-matched exercising controls. We obtained left ventricles from 16-week old rats housed for 12 weeks in standard (sedentary) or voluntary running wheel (exercised) housing. The N2BA:N2B ratio was decreased in the hearts of sedentary versus active rats (p = 0.041). Gene expression of a titin mRNA splicing factor, RNA Binding Motif 20 protein (RBM20), correlated negatively with N2BA:N2B ratios (p = 0.006, r = −0.449), but was not different between groups, suggesting that RBM20 may be regulated post-transcriptionally. Total phosphorylation of cardiac titin was not different between the active and sedentary groups. This study is the first to demonstrate that sedentary rats exhibit reduced cardiac titin N2BA:N2B isoform ratios, which implies reduced cardiac compliance. These data suggest that a lack of exercise (running wheel) reduces cardiac compliance and that exercise itself increases cardiac compliance.

Aging and disease are associated with dysregulation of collagen deposition and collagen degradation and crosslinking. These modifications in collagen structure are key determinants of cardiac integrity and function, making it advantageous to monitor collagen’s state throughout disease. Although myocardial fibrosis often occurs alongside both cardiac and systemic disease, monitoring myocardial collagen typically requires expensive, invasive, and/or contraindicated methods. However, collagen content has been correlated to ultrasonic backscatter, a low-cost, non-invasive index. Preliminary studies in our lab suggested collagen also dominates the anisotropy of backscatter(variation in the brightness) in a left ventricular short-axis ultrasound. The purpose of the present study was to determine a relationship between myocardial collagen structure and anisotropy of ultrasonic backscatter while using collagenase to degrade myocardial collagen networks. Hearts were excised from Sprague Dawley rats and perfused with a collagenase-containing solution for either 10 (n=7) or 30 minutes (n=7) or control solution for 30 minutes (control n=8). Serial ultrasound images were acquired throughout collagenase digestion and ultrasonic backscatter was assessed where the collagen is primarily aligned perpendicular to the angle of insonification (bright on ultrasound image), and where collagen is primarily aligned parallel to the angle of insonification (darker on image). Our data suggested that collagenase digestion reduced backscatter anisotropy within the myocardium (p<0.001) with the lateral and septal walls (collagen parallel to ultrasound) showing the greatest change in backscatter intensity. Histology (Trichrome staining) and biochemistry (hydroxyproline assay) suggests that collagen remains present but its crosslinking is altered within 10 minutes (p<0.047). These data suggest that myocardial collagen fiber orientation and crosslinking of collagen correlates with the anisotropy of ultrasonic backscatter. Thus, anisotropy of ultrasonic backscatter could potentially be used to assess collagen crosslinking and deposition.

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