Greater Number of Capillary Endothelial Cell Mitochondria in Brain Than in Muscle

Abstract

The selective permeability of the brain mircrocirculatory bed (the blood-brain barrier-BBB) is widely believed to have its locus at the brain capillary endothelial cell. Whereas capillaries in other tissue are nonspecifically permeable to all small molecules by virtue of open clefts between endothelial cells, cellular fenestrae and pino-cytosis, these nonspecific routes of exchange are virtually absent in brain, and transcapil-lary exchange probably takes place through the membranous walls and the cytoplasm of endothelial cells. Although most organic molecular exchange is probably bidirectional and net flux is determined by concentration gradients, recent studies have suggested a unidirectional flux out of the brain for potassium, iodide and organic anions (1-3). Such uphill unidirectional transport would require a source of energy in the brain capillary endothelial cell.

If the brain capillary wall performs an energy-dependent excretory function as a substantial part of its total metabolic work, it might be expected that brain capillary endothelial cells would contain more mitochondria than a tissue with nonspecifically permeable capillaries, such as skeletal muscle.

Materials and Methods. To test this hypothesis, rat brain and skeletal muscle capillaries were examined by electron microscopy and counts made of the number of mitochondria encountered in random sections. A 175-g male Wistar rat on routine diet and in apparently good health, was sacrificed, routinely perfused with glutaral-dehyde, embedded, and sectioned for electron microscopy. Anesthesia was induced with 35% chloral hydrate and respiration supported with 95% O₂ and 5% CO₂. Two tenths milliliter heparin and 0.8 ml 1 % NaNO₂ were injected intracardiac, and a cannula was placed in the ascending aorta. The right atrium was cut, and the rat perfused with 500 ml 1.25% glutaraldehyde and 1% paraformaldehyde in 0.135 M phosphate buffer at pH 7.2. The animal was cooled for 4 hr in a plastic bag at 4°. Six blocks were taken from cerebellar vermis and six blocks from the vastus medialis muscle; washed in dextrose buffer, secondarily fixed with 1 % buffered osmium tetroxide, dehydrated, and embedded in epon resin 812. Thick sections were used to select capillaries; thin sections were stained with both uranium and lead.