ULTRASTRUCTURAL ASPECT OF LEFT VENTRICULAR HYPERTROPHY IN SPONTANEOUSLY HYPERTENSIVE RATS : A QUALITATIVE AND QUANTITATIVE STUDY

Abstract

The morphology of the left ventricular myocardium of spontaneously hypertensive rats (SHR) (Okamoto and Aoki) was studied with light and electron microscopes, with special reference to a quantitative analysis of subcellular organelles and a description of the characteristics of the capillaries in myocardium. Paired groups of SHR and control Wistar-Kyoto rats were sacrificed at 3, 5, 9, 11, 15 and 21 weeks and one year of age. In the left ventricular lateral wall of SHR, starting at the age of 11 weeks, the cardiocytes became progressively thicker as the blood pressure rose and exhibited various ultrastructural alterations. In these cardiocytes, the proliferation of Z-band materials was occasionally present in the myofibrils, and was closely associated with the subsarcolemmal area and intercalated discs. Convolutions and extensive foldings of the intercalated discs were prominent. Mitochondrial abnormalities consisted of fragmentation and stacks of cristae, aggregates of small mitochondria and intramitochondorial α-glycogen rosettes. In one-year-old SHR, malaligned cardiocytes as well as myofibrils were seen in certain areas of the myocardium. In some cardiocytes, streaming and clumping of Z-band material, myofibrillolysis and proliferation of sarcoplasmic reticulum appeared. In addition, there were clusters of spherical microparticles, tubular aggregates and intramitochondrial inclusions, suggestive of degenerative changes. Stereological analysis of subcellular organelles in the hypertrophied cardiocytes of SHR revealed an increase of myofibrillar volume fractions and a decrease of mitochondrial volume fractions, resulting in an increase in the ratio of myofibrils to mitochondria. These findings differed from those in the posterior papillary muscle of the left ventricle in the same animal hearts. Thus, it is apparent that myocardial hypertrophy varies in each area of the myocardium due to different stresses during the course of its development. In the hypertrophied myocardium of SHR, the capillaries were seen to proliferate with the appearance of a "tunnel capillary", while interstitial fibrosis and hypertensive vascular lesions progressed with aging. The capillary proliferation maintained a constant diffusion distance from the capillary to the cardiocyte, probably as one of adaptation in cardiac hypertrophy in SHR.