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Independent changes in cardiac muscle fibres and connective tissue in rats with spontaneous hypertension, aortic constriction and hypoxia.

The collagen content (hydroxyproline concentration) and muscle fibre diameter were determined in hearts of 16 week old spontaneously hypertensive, aortic constricted and normotensive rats and spontaneously hypertensive and aortic constricted groups exposed to 6 weeks of hypobaric hypoxia (simulated altitude of 6100 m).

The early development of cardiomegaly associated with spontaneous hypertension is primarily caused by an increase in muscle mass with relatively less proliferation of the connective tissue elements. Although heart weight to body weight ratios and fibre diameter increase in the spontaneously hypertensive SHR compared with normotensive rats, the concentration of hydroxyproline decreases indicating that the connective tissue response in spontaneously hypertensive rats is less pronounced than the muscle response. Therefore, at 16 weeks of age, an increase in muscle mass is the primary contributor to cardiomegaly in spontaneously hypertensive rats. Hypobaric hypoxia induces increased collagen production in both the left and the right ventricles of spontaneously hypertensive rats.

Unlike the spontaneously hypertensive rats, the collagen response in aortic constricted rats parallels the muscle response since cardiomegaly and increased left ventricular fibre diameter are associated with the maintenance of hydroxyproline concentration similar to that in normotensive rats. Hypoxia failed to activate collagen production further in aortic constricted rats.

These findings suggest that the connective tissue response to cardiomegaly produced by aortic constriction is significantly different from the response produced by essential hypertension. The data also suggest that the factors governing connective tissue proliferation may be independent from those governing the muscle fibre hypertrophy.