Effect of age on myocardial adaptation to volume overload in the rat.

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Abstract
To test the hypothesis that the capacity for left ventricular (LV) adaptation to volume overload might diminish with age, we examined the hemodynamics and degree of myocardial hypertrophy in response to aortic insufficiency in young adult (9 mo) and old (18 or 22 mo) Fischer rats. Before, immediately after, and at 2 and 4 wk after creating aortic insufficiency, LV and aortic pressures were measured using a catheterization technique. 4 wk after surgery, we measured aortic flow, and estimated the LV passive pressure-volume relationship and the degree of LV hypertrophy after killing. Immediately after the surgical creation of aortic insufficiency, both young and old rats showed similar elevation of LV end-diastolic pressure (from 4.8 +/- 0.6 to 12.0 +/- 1.5 mmHg in the young rats, P less than 0.01; from 4.9 +/- 0.4 to 11.0 +/- 0.7 mmHg in the old rats, P less than 0.01). In the young rats LV, end-diastolic pressure decreased to 8.0 +/- 1.0 and to 8.5 +/- 0.9 mmHg at 2 and 4 wk (P less than 0.05). In contrast, LV end-diastolic pressure at 2 (16.9 +/- 3.1 mmHg) and 4 wk (16.1 +/- 2.7 mmHg) in the old rats was even higher, compared with the values measured immediately after aortic insufficiency. At 4 wk, LV end-diastolic meridional wall stress (calculated from the in vivo LV end-diastolic pressure, and the pressure-volume relationship and muscle mass obtained after killing) was higher in the old rats than in the young rats. In the young rats, the diastolic pressure-volume relationship at 4 wk shifted to the right (P less than 0.01), and LV dry weight, LV dry weight/tibial length, and protein content of the LV myocardium increased by 26% (P less than 0.01), 24% (P less than 0.01), and 33% (P less than 0.01), respectively. However, old rats with aortic insufficiency did not show a significant change in the pressure-volume relationship, dry weight, or protein content at 4 wk. These results suggest that advanced age diminishes the capacity for LV hypertrophy in response to a volume overload, and this reduced LV hypertrophic response in the old rats resulted in persistent elevation of LV end-diastolic pressure and wall stress.