Actin-like filaments in the endothelial cells of adult rat brain capillaries

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While studying glycerinated adult rat brain tissue by means of heavy meromyosin (HMM) labeling, the presence of actin-like filaments within the cytoplasm of capillary and postcapillary venule endothelial cells was observed. After incubation in cold glycerol solutions, rat brain tissue pieces were transferred into buffer, pH 7.0: (i) with HMM, (ii) without HMM, (iii) with HMM + 5 m m ATP, and (iv) with HMM + 2.5 m m Na + pyrophosphate. In the endothelial cells from untreated nervous tissue, we observed smooth-surfaced microfilaments, averaging 6 nm in diameter, branched and anastomosed, intertwining in loosely meshed networks, and anchored to the plasma membrane. After exposure to HMM, the microfilaments reacted with the latter molecules. They increased in width to 18 to 20 nm and were tightly packed in portions of the endothelial cells. When the HMM solutions contained adenosine 5′-triphosphate (ATP) or Na + pyrophosphate, the microfilaments were no longer coated with thick sidearms. Thus, we demonstrated that the microfilaments present within the endothelial cells of capillary and postcapillary venules in adult rat brain tissue are of actin-like nature. In addition, after incubation with ATP, the microfilaments were intermingled with, and converged onto the surfaces of, thick, tapered filaments, which we tentatively identified as of myosin-like nature. However, the number of contractile filaments within the endothelial cells was small compared to that observed in the pericyte of the same capillary and postcapillary venule. Several possible functions of the contractile filaments in the endothelial cells are presented.