Collagen content, the ratio of collagen types I and III and elastin content were measured in 5-6- and 10-12-week-old rabbits with and without right-ventricular pressure overload. Significant and equivalent hypertrophy occurred in both age groups. A 2-day pressure overload caused a fall in collagen concentration below control levels in right-ventricular tissue from the older animals, but no change in the younger ones. A 2-week pressure overload in the older animals resulted in a rise in collagen concentration, a decreased ratio of type III to type I plus III [III/(I + III)] collagens, a fall in desmosine concentration and a fall in the desmosine/hydroxyproline ratio in the right ventricle. None of these changes occurred in the younger age group. We hypothesize that the changes in connective-tissue proteins after overload in the older group may contribute to previously observed changes in mechanical performance. The divergent connective-tissue responses in the two groups suggest the importance of age in determining outcome, as well as the possibility of separate regulatory mechanisms for contractile and for architectural elements of the heart.

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