The effects of beta adrenoceptor blockade with atenolol on myocardial cellular and subcellular hypertrophy in spontaneously hypertensive rats.

In this study we investigated the effects of chronic beta adrenoreceptor blockade with atenolol on cellular and subcellular hypertrophy in spontaneously hypertensive rats (SHR). Atenolol was injected subcutaneously (20 mg/kg) twice daily commencing in four-week-old rats. The treated animals (SHR-A) were compared to their nontreated controls and normotensive, Wistar-Kyoto (WKY) controls at the age of 16 weeks. A group of atenolol-treated WKY was also studied. Chronic drug treatment was effective in attenuating the rise in systolic blood pressure characteristic of SHR, but did not normalize the values to those of WKY. Cardiac hypertrophy, characteristic of SHR, was modified by drug treatment as evidenced by left ventricular weights as well as myocardial cell size. The cells from the subendocardium underwent selective hypertrophy in SHR which was attenuated by about 50% after atenolol treatment. Stereological analysis of electron micrographs showed that while relative mitochondrial volume was not affected by treatment, relative myofibrillar volume (%) decreased in both subepicardium (SHR = 63.28 +/- 1.25; SHR-A = 56.72 +/- 1.37) and subendocardium (SHR = 66.53 +/- 1.27; SHR-A = 58.30 +/- 1.51). This change raised the mitochondrial/myofibrillar volume ratio, which is characteristically low in SHR compared to WKY. Sarcoplasm, which included all cell constituents except mitochondria, increased with atenolol treatment, but water concentration remained unchanged. The data suggest that attenuation of hypertrophy in SHR after beta blockade is associated with selective effects on the myocardial cell involving primarily the myofibrillar cell compartment.