Absolute morphometric study of myocardial hypertrophy in experimental hypertension. II. Ultrastructure of myocytes and interstitium.

The left ventricular myocardium of normal and hypertensive rats has been characterized morphometrically in the endocardial and epicardial zones. Compared to the epicardial regions, the normal endocardial regions contain 30 per cent more myocytes, 27 per cent less interstitial space, 48 per cent less capillary volume, 17 per cent less capillary surface, and the same capillary length per unit tissue volume. In terms of both the relative and absolute volumes and surface areas of their organelles, the cytoplasmic composition of normal endocardial and epicardial myocytes is nearly identical. After 14 weeks of hypertension, induced by constriction of the left renal artery, left ventricular weight is increased by 30 per cent, wall thickness by 42 per cent. The number of myocytes and the total length of capillaries remain constant. The epicardial region enlarged 37 per cent with proportional increases of myocyte and interstitial volumes. In contrast, the endocardial enlargement was only 26 per cent, comprised of 21 per cent hypertrophy of myocytes and a 55 per cent increase in interstitial components. Expansion of capillary lumina accounted for much of the interstitial enlargement throughout the myocardium. Hypertrophy of myocytes is 76 per cent greater in the epicardial region and is accompanied by a reduced mitochondria to myofibril ratio and disproportionately large increases (2- to 3-fold) in both smooth endoplasmic reticulum and T-system volume and surface area. On a cellular basis the absolute morphometric characteristics of myocytes from hypertensive rats are significantly different from normal, and significant differences occur between the inner and outer layers of the myocardium for practically every cytoplasmic component.