1. Left and right ventricular weights were determined in newborn, 6, 15 and 28 week-old spontaneously hypertensive and normotensive control rats.

2. Left and right ventricles were heavier in all spontaneously hypertensive rats than in age-matched control rats, but the left/right ventricular weight ratio was increased only in 15 and 28 week-old spontaneously hypertensive rats.

3. If the cardiomegaly in newborn spontaneously hypertensive rats, in which the two ventricles have worked in parallel until birth, is an adaptation to hypertension in utero, left ventricular weight would rapidly increase after birth as the left ventricle alone is then exposed to an increased pressure. As left/right ventricular weight ratio was not increased in 6 week-old spontaneously hypertensive rats, factors other than hypertension may stimulate cardiac growth in the foetal spontaneously hypertensive rat.

4. This implies an initial structural ‘resetting’ of the cardiovascular dimensions in spontaneously hypertensive rats and that true pressure-dependent adaptation further potentiates these alterations in cardiovascular structure once hypertension is initiated.

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