An electron microscopic study of the passage of colloidal particles from the blood vessels of the ciliary processes and choroid plexus of the rabbit

The thinnest areas of the capillaries of the choroid plexus and ciliary processes in the eye of the rabbit are characterized by the presence of fenestrae. When various colloidal particles opaque to the electron beam (thorotrast, gold sol, and saccharated iron oxide) were injected into the blood stream, none were found in fenestrae or in areas that might suggest their having passed through fenestrae. The passage of marker particles from the lumen to the surrounding connective tissue does take place on occasion in the areas of thicker walls in the capillaries and venules rather than in the attenuated and fenestrated endothelial walls. The pathway taken by these markers may be either through the cytoplasm of the endothelial cells via membrane-bounded vesicles and vacuoles or through the intercellular spaces of the vessels. An altered aqueous humor (cloudy and plasmoid) was produced by endotoxin injection or by making a draining fistula in rabbit cornea. Both methods gave rise to the same changes in the blood vessels of the ciliary processes. Under such conditions of inflammation the passage of colloidal particles through the thicker walls of the capillaries and venules was greatly increased and occurred primarily as an intercellular passage between the endothelial cells. The attenuated and fenestrated areas of the endothelium of the small capillaries remained unchanged with no particles passing through them. These results on the altered vessels of the ciliary processes parallel those of Majno and Palade (26) on the rat cremaster muscle.

ACTIVE TRANSPORT OF QUATERNARY AMMONIUM COMPOUNDS BY THE CHOROID PLEXUS IN VITRO

The quaternary ammonium compounds, hexamethonium, decamethonium, and N1-methylnicotinamide (NMN) are taken up by the rabbit choroid plexus in vitro (Krebs-Ringer phosphate glucose solution, pH 7.4, 37 C, oxygen) by a process showing all the characteristics of active transport. Uptake against a concentration gradient occurs by a saturable process that is inhibited by low temperature, by anaerobic conditions, and by low concentrations of ouabain, reserpine, and certain metabolic inhibitors. Decamethonium and NMN act as competitive inhibitors of hexamethonium uptake, suggesting that the three cations share a common transport process. Hexamethonium uptake is dependent on levels of Na, K, Mg, and phosphate in the incubation medium. Hexamethonium and decamethonium, but not NMN, are bound to homogenates of choroid plexus. The characteristics of the binding are such that binding would not account for the bulk of drug accumulation seen in the intact tissue. High concentrations of p-aminohippurate do not inhibit the uptake of hexamethonium. No evidence could be obtained for active uptake of hexamethonium by subcellular particles of plexus homogenates. The transport process for quaternary ammonium compounds appears to be present in the choroid plexus of the dog, cat, and guinea pig as well as in that of the rabbit.

NA-K ACTIVATED ADENOSINE TRIPHOSPHATASE FORMATION OF CEREBROSPINAL FLUID IN THE CAT

Digitalis-sensitive Na-K activated ATPase (Na-K ATPase), implicated in active cation transport, was shown to occur in cat choroid plexus. Cerebrospinal fluid (CSF) formation rate, measured by collection from cat cerebral aqueduct was inhibited 18% by intravenous desacetyl lanatoside C (0.2 mg/kg). Ouabain, placed intraventricularly, caused inhibition of flow, ranging from 100% by 10–5 mole to 0% by 5 x 10–10 mole. After 10–6 mole ouabain, Na-K ATPase activity in lateral ventricle choroid plexus was inhibited 69% without change in digitalis-insensitive, Mg activated ATPase and carbonic anhydrase activities. Ventriculocisternal perfusion with varying concentrations of ouabain, scillaren A, and hexahydroscillaren A gave CSF flow inhibition (inulin dilution technique) correlating quantitatively with in vitro Na-K ATPase inhibition. Other compounds studied were erythrophleine, cassaine, and l-norepinephrine. It is concluded that the choroid plexus Na-K ATPase system has a primary function in the formation of CSF in the cat, presumably through the active secretion of Na ions into the ventricle.

CYTOCHEMICAL EFFECTS OF HYPERVITAMINOSIS A

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SECRETION OF CEREBROSPINAL FLUID BY CHOROID PLEXUS OF THE RABBIT

Blood flow in the principal draining vein of the choroid plexus of the lateral ventricle of the rabbit, which drains approximately the anterior 70% of the plexus, was measured by the analysis of motion pictures made during the injection of spheruler of 1-octanol. The relative loss of volume from the plasma compartment during the transit of blood was measured by the comparison of relative cell volumes in aortic blood and in venous blood of the plexus. The blood flow of the plexus averaged 2.86 µl/mg min and the rate of production of cerebrospinal fluid .37 µl/mg min. The effect upon the relative rate of production of fluid of several inhibitors was tested and of these acetazolamide was effective both intravascularly and topically; cardiac glycosides were effective topically, but not when given intravenously; and cyanide and 2,4-dinitrophenol were ineffective topically.