1. Portal veins, vena cavae and pulmonary arteries from spontaneously hypertensive (SH) rats were found to undergo medial smooth-muscle hypertrophy when compared with corresponding blood vessels from age- and sex-matched Wister—Kyoto (WKy) normotensive rats. There was an increase in the density of the mucopolysaccharide and glycoprotein staining in veins, pulmonary arteries vena venorum and ventricular myocardium of SH rats.

2. Electron-microscopic examination of the blood vessels and myocardium of SH rats suggested enhanced protein synthesis and hypertrophy both in the smooth muscle and endothelial cells. Central venous, portal venous and right ventricular pressures were measured in SH rats and were not elevated when compared with corresponding values for WKy rats. Veins and pulmonary arteries from SH rats developed more tension when challenged with vasoconstrictor stimuli than corresponding vessels obtained from WKy rats.

3. Veins and pulmonary arteries obtained from SH rats demonstrated a greater uptake of [14C]glycosamine than corresponding blood vessels from WKy rats.

4. These findings demonstrate the existence of pressure-independent hypertrophy of smooth muscle in blood vessels obtained from SH rats, associated with an increase in glycoprotein synthesis and an enhanced contractile activity of the muscle. The data suggest that the enhanced contractility of blood vessels from SH rats reflects the enhanced muscle mass. It may represent an aberrant humoral or cellular mechanism which results in the hypertension.

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