Myocardial collagen concentration and nuclear magnetic resonance relaxation times in the spontaneously hypertensive rat.

In order to test the hypothesis that the increased myocardial collagen concentration in the older, spontaneously hypertensive (SH) rat is associated with altered T2 and T1, we performed in vitro studies of 70 left ventricles from 8-, 22-, and 33-week-old SH and Wistar-Kyoto (WKY) rats. We also measured the left ventricle/body weight (LV/BW) ratio (as a measure of hypertrophy), left ventricular water and fat content, and hydroxyproline concentration (as a measure of collagen). The LV/BW ration was not significantly different between 8-week-old SH rats and WKY rats but was significantly greater in SH rats than in WKY rats at 22 and 33 weeks of age. Comparing SH rats with WKY rats at 22 weeks of age, no significant difference existed in T1, T2, water content, or hydroxyproline concentration. However, at 33 weeks of age in SH rats compared with WKY rats, hydroxyproline concentration was significantly greater (4.3 +/- 0.6 mg/g, respectively; P less than .0005), water content was significantly greater (77.1% +/- 0.3% vs. 76.2% +/- 0.3%, respectively; P less than .0001), and T2 and T1 were significantly longer (T2: 52.6 +/- 2.1 msec vs. 48.6 +/- 2.2 msec, respectively; P less than .0001; T1: 656 +/- 14 msec vs. 619 +/- 12 msec, respectively; P less than .0001). In all SH rats combined, T2 and hydroxyproline concentration were significantly correlated (r = .63; P less than .0001). Thus, in SH rats, myocardial hypertrophy precedes increased collagen deposition. These data suggest that estimation of magnetic resonance relaxation times may permit noninvasive identification of increased myocardial collagen deposition independent from changes in myocardial hypertrophy.