SECRETION OF CEREBROSPINAL FLUID BY CHOROID PLEXUS OF THE RABBIT

Regulation of cerebrospinal fluid potassium by the cat choroid plexus

1. The regulation of cerebrospinal fluid (c.s.f.) potassium concentration was studied using the cat choroid plexus isolated in a chamber in situ. 2. Hyperkalaemia (plasma potassium concentration greater than 6 m-equiv/l.) caused relatively small increases in c.s.f. potassium concentration. 3. Alterations in c.s.f. potassium concentration (c.s.f. K = 0-15 or 6-56 m-equiv/l.) were countered by changes in potassium concentration of the produced fluid or in the rate of potassium transport which returned c.s.f. potassium towards normal. 4. The data indicate that potassium concentration in c.s.f. secreted by the choroid plexus is actively regulated by the plexus whether the primary alteration in potassium occurs in plasma or c.s.f.

Recent observations on ependyma and subependymal basement membranes

Directly under the ependyma, and between the ependymal cells, there are basement membranes which form labyrinths connecting with the perivascular basement membranes of the subependymal vessels. The basement membranes exhibit differences in form, position, and distribution, and they can be distended by fluid absorption into large lacunae. The membranes can be identified as glycoprotein and glylcolipid substances. Electron microscopic studies have shown the differences between the basement membrane labyrinths of the human, the rabbit and the rat. In the human, the labyrinths contain isolated collagen fibrils. Basement membranes generally line all connective tissue spaces and form their interstitial borders. From this point of view, the subependymal basement membrane labyrinths can be seen as interstitial spaces near the ventricles, forming pathways between the ependyma and the subependymal capillaries or postcapillary veins.

Formation of cerebrospinal fluid. Relation of studies of isolated choroid plexus to the standing gradient hypothesis

After a brief summary of current views on the origin of cerebrospinal fluid (CSF) and the processes underlying its elaboration, the author discusses studies of isolated choroid plexus in extracorporeal perfusion systems and flux chambers. The results suggest that transependymal water flow is secondary to the electrically silent pumping of sodium. The author presents evidence in support of the standing gradient hypothesis as the structural basis of CSF secretion.

The third circulation revisited

The author reviews modern information concerning the formation, flow and functions of the cerebrospinal fluid. Particular attention is given to the lymphatic-like features of the third circulation and to its importance as an internal milieu for nervous tissue.

Cerebrospinal fluid formation in ventricles and spinal subarachnoid space of the rhesus monkey

The authors perfused rhesus monkeys from lateral ventricles to lumbar sac with an artificial cerebrospinal fluid (CSF) containing a Blue Dextran 2000 marker. Analysis of marker dilution at steady state showed ventricular CSF formation occurring at a rate of 28.3 plus or minus mu1/min. No significant CSF formation was found in the spinal subarachnoid space.

Effect of systemic arterial hypotension on the rate of cerebrospinal fluid formation in dogs

✓The rate of cerebrospinal fluid (CSF) production in dogs was measured by ventriculocisternal perfusion with artificial CSF containing inulin. In normotensive animals, the average CSF production was 36 ± 6 µl/min. When the mean arterial blood pressure was reduced to 62 ± 1 mm Hg, the CSF production fell to 22 ± 5 µl/min, a 39% reduction in fluid formation. The authors briefly discuss various hypotheses to explain this reduction.

Role of cations, anions and carbonic anhydrase in fluid transport across rabbit corneal endothelium

1. A small electrical potential difference (541 +/- 48 muV, aqueous side negative) across rabbit corneal endothelium has been recently found. Its dependence on ambient [Na(+)], [K(+)], [H(+)] and metabolic and specific inhibitors was examined.2. Changes in concentration of the ions above either were known or were presently shown to affect the rate of fluid transport across this preparation (normal value: 5.2 +/- 0.4 mul./hr.cm(2)). Ionic concentration changes were also found here to influence potential difference in the same way as fluid transport. In the cases tested, the effects on both fluid transport and potential difference were reversible.3. Fluid transport and potential difference were both decreased or abolished in absence of Na(+), K(+) and HCO(3) (-), and when [H(+)] was decreased. Fluid transport and potential difference were saturable functions of [HCO(3) (-)] and half-saturation occurred in both cases at about 13 mM-HCO(3) (-). The potential difference was also a saturable function of [Na(+)] (half-saturation around 15 mM). There was a pH optimum for potential difference in the range 7.4-7.6. Lower pH values decreases the potential difference and the fluid transport, and a small (-100 muV) reversed potential was observed in the range of 5.3-5.5.4. Total replacement of Cl(-) by HCO(3) (-) or SO(4) (2-) produced no impairment on either fluid transport or potential difference.5. Carbonic anhydrase inhibitors (ethoxyzolamide 10(-5) or 10(-4)M and benzolamide 10(-3)M) produced a 40-60% decrease in the rate of fluid pumping. In contrast, ethoxyzolamide 10(-4)M or acetazolamide 10(-3)M did not produce any change in the potential difference. NaCN and Na iodoacetate (both 2 mM) eliminated the potential difference in 1-1.5 hr while in controls it lasted for 5-6 hr.6. Ouabain (10(-5)M) abolished the potential difference in less than 10 sec when added to the aqueous side, which suggests the existence of an electrogenic pump. This extremely fast time transient can be accounted for by the accessibility and simple geometry of the present monocellular layer. Ouabain abolished also the reversed potential difference observed at low pH.7. The data are interpreted in terms of a scheme similar to that advanced for other epithelia and in which (a) H(+) would be pumped into the intercellular spaces, while Na(+) and CO(2) would enter into the cells, and (b) Na(+) would be subsequently pumped into the aqueous humour, producing as a result the fluid movement observed. The actual origin of the potential difference is further discussed in terms of two contrasting possibilities: (i) one or more electrogenic pumps, and (ii) a neutral pump which would create a diffusion potential across ;leaky' intercellular junctions.

Composition of fluid obtained from choroid plexus tissue isolated in a chamber in situ

1. A method was developed for isolating a segment of the choroid plexus of the lateral ventricle of the cat brain in a chamber in situ.2. A comparison of electrolyte and protein concentrations in serum, ultrafiltrate of serum, cisterna magna fluid, fluid accumulated in the chamber and fluid collected from the choroid plexus by another technique, demonstrates that the chamber fluid is a secretory product of the choroid plexus.3. The rate of fluid formation in the chamber was 0.4 mul. min(-1) mg(-1) of tissue, a value in good agreement with reports in the literature.4. The observation that the concentration of K(+) in choroid plexus fluid was lower than that in the serum ultrafiltrate suggests that K(+) is regulated by an active transport process at the choroid plexus.5. Significant correlation was found between electrolyte values and the protein content of the chamber fluid. This suggests that plasma is the probable source of the protein in the chamber fluid and that K(+) and probably Ca(2+) and Mg(2+) in c.s.f. are subject to active homoeostatic regulation by the choroid plexus.6. The technique described provides a new opportunity to study in detail the functional characteristics of the choroid plexus.

The effect of intraventricular air on the rate of cerebrospinal fluid formation in dogs

✓ Experiments were performed on dogs to estimate the effect of intraventricular air on the rate of cerebrospinal fluid formation. The rate of formation was measured satisfactorily in eight beagle dogs before and after the introduction of air using ventriculocisternal perfusion of artificial cerebrospinal fluid containing 14C-labelled inulin. Statistical analysis of rates of cerebrospinal fluid formation estimated half hourly indicated that there was little or no change after the introduction of air.

Cerebrospinal fluid production by the choroid plexus and brain

The production of cerebrospinal fluid and the transport of (24)Na from the blood to the cerebrospinal fluid were studied simultaneously in normal and choroid plexectomized rhesus monkeys. Choroid plexectomy reduced the production of cerebrospinal fluid by an average of 33 to 40 percent and the rate of appearance of (24)Na in the cerebrospinal fluid and its final concentration were proportionately reduced. In both normal and plexectomized animals, (24)Na levels were found to be markedly greater in the gray matter surrounding the ventricles and in the gray matter bordering the subarachnoid space. That sodium exchanges in these two general areas of the brain may be linked to the formation of the cerebrospinal fluid is discussed here.

Choroid Plexus and Cerebrospinal Fluid Production

Acute progressive hydrocephalus occurred in 76 rhesus monkeys undergoing ventricular obstruction 2 to 6 months after choroid plexectomy and was only slightly less pronounced than in 73 nonplexectomized animals with similar ventricular obstructions. The composition of cerebrospinal fluid after choroid plexectomy was essentially normal. Experiments with ventricular perfusion techniques indicate only a slight reduction in cerebrospinal fluid production after choroid plexectomy. Thus, the choroid plexus is probably not the sole or even the major source of cerebrospinal fluid within the primate ventricular system.