Distribution of microspheres in the brain of hypertensive rats

The blood perfusion of different parts of the brain tissue was examined by means of microspheres 15 and 50 micron in diameter, in normotensive control rats and in animals with experimental renovascular hypertension. The microspheres were labelled with fluorescein isothiocyanate and their numbers in the tissue were determined in consecutive histological sections by UV microscopy. In the control rats, the incidence of wedged microspheres per 1 mm3 tissue was high in the cerebellum, cerebral cortex, subcortical tissue and pons Varolii, but low in the thalamic and hypothalamic regions, indicating that these parts were relatively poorly perfused with blood. The significantly greater accumulation of microspheres in the cortex and subcortical tissue of hypertensive rats seems to have been due to hypertensive narrowing of the arterioles. Conversely, the diminished incidence of microspheres in the thalamus and hypothalamus may have been due partly to microsphere trapping in the narrowed upstream blood vessels and partly to thinning of the capillary network. Total microsphere recovery in the brains of the control and the hypertensive rats was almost identical, implying that only the distribution of brain blood perfusion is altered in experimental hypertension.