Effect of atrial natriuretic factor on permeability of the blood-cerebrospinal fluid barrier

The blood-cerebrospinal fluid barrier: structure and functional significance

The choroid plexus (CP) of the blood-CSF barrier (BCSFB) displays fundamentally different properties than blood-brain barrier (BBB). With brisk blood flow (10 × brain) and highly permeable capillaries, the human CP provides the CNS with a high turnover rate of fluid (∼400,000 μL/day) containing micronutrients, peptides, and hormones for neuronal networks. Renal-like basement membranes in microvessel walls and underneath the epithelium filter large proteins such as ferritin and immunoglobulins. Type IV collagen (α3, α4, and α5) in the subepithelial basement membrane confers kidney-like permselectivity. As in the glomerulus, so also in CP, the basolateral membrane utrophin A and colocalized dystrophin impart structural stability, transmembrane signaling, and ion/water homeostasis. Extensive infoldings of the plasma-facing basal labyrinth together with lush microvilli at the CSF-facing membrane afford surface area, as great as that at BBB, for epithelial solute and water exchange. CSF formation occurs by basolateral carrier-mediated uptake of Na+, Cl-, and HCO3-, followed by apical release via ion channel conductance and osmotic flow of water through AQP1 channels. Transcellular epithelial active transport and secretion are energized and channeled via a highly dense organelle network of mitochondria, endoplasmic reticulum, and Golgi; bleb formation occurs at the CSF surface. Claudin-2 in tight junctions helps to modulate the lower electrical resistance and greater permeability in CP than at BBB. Still, ratio analyses of influx coefficients (Kin) for radiolabeled solutes indicate that paracellular diffusion of small nonelectrolytes (e.g., urea and mannitol) through tight junctions is restricted; molecular sieving is proportional to solute size. Protein/peptide movement across BCSFB is greatly limited, occurring by paracellular leaks through incomplete tight junctions and low-capacity transcellular pinocytosis/exocytosis. Steady-state concentration ratios, CSF/plasma, ranging from 0.003 for IgG to 0.80 for urea, provide insight on plasma solute penetrability, barrier permeability, and CSF sink action to clear substances from CNS.

Atrial natriuretic peptides elevate cyclic GMP levels in primary cultures of rat ependymal cells

The aim of this study was to examine the effect of atrial natriuretic peptides on primary cultures of ependymal cells, as measured by changes in intracellular levels of cyclic GMP. Incubation of ependymal cells with rat atrial natriuretic peptide-(1-28) (rANP) elicited a 30-fold increase in ependymal cGMP content within 1 min and more than a 100-fold increase within 10 min to a plateau value of approximately 30 pmol/mg protein. The C-type natriuretic peptide (CNP) elicited a similar increase in cGMP levels; however the maximal effect was observed within 1 min and the levels subsequently dropped by 90% to a low plateau within 10 min. A comparison of the concentration-response curves for rANP, human ANP-(1-28) (hANP) and CNP showed that rANP, hANP and CNP had similar effects, with regards to elevation of cGMP levels at high concentrations, but with differing EC50 values. These results demonstrate the presence of a heterogenous population of functional ANP receptors i n cultured ependymalcells suggesting that ANP may regulate specific ependymal cell activity.

Effects of centrally administered arginine vasopressin and atrial natriuretic peptide on the development of brain edema in hyponatremic rats

OBJECTIVE

Centrally released arginine vasopressin (AVP) and atrial natriuretic peptide (ANP) have been shown to participate in brain volume regulation. The aim of the present study was to evaluate the effects of centrally administered AVP and ANP on the time course of development of brain edema in vivo in hyponatremic rats, using diffusion-weighted magnetic resonance imaging.

METHODS

We performed intracerebroventricular (ICV) administration of 120 microg AVP, 20 microg ANP, or physiological saline into the right lateral ventricle in 18 rats. Twenty-five minutes after the treatment, we induced systemic hyponatremia by the intraperitoneal administration of 140 mmol/L dextrose solution. Serial diffusion-weighted imaging scans were obtained up to 96 minutes after the start of the hyponatremia. Changes in the brain extra-to intracellular volume fraction ratio were estimated as changes in the apparent diffusion coefficient (ADC).

RESULTS

No change in the ADC was observed after the ICV injection of saline or AVP. The onset of hyponatremia induced a rapid and marked ADC reduction in both groups, indicating an increased intracellular space. However, the ADC decrease became significantly more pronounced in the ICV AVP group (83.3+/-4.7% of baseline level, mean +/- standard deviation) than in the saline group (93.7+/-3.3% of baseline, P < 0.001) after 78 minutes of hyponatremia. The ICV injection of ANP induced a prompt ADC increase to 111.5+/-10.0% (P < 0.05) of the baseline level, indicating a rapid reduction in the intracellular compartment. In the initial phase of hyponatremia, the ADC values in the ANP group were consistently higher than those in the saline group, decreasing finally to 86.9+/-9.6% after 96 minutes of hyponatremia.

CONCLUSION

Our findings demonstrate the opposite effects of AVP and ANP on the intracellular volume fraction of the brain during the development of cellular brain edema, with an immediate effect on ANP and a delayed effect on AVP. The results emphasize the direct effects of these hormones on the cellular volume regulatory mechanisms in the brain during the development of cerebral edema.

Atrial natriuretic peptide (ANP) attenuates brain oedema accompanying experimental subarachnoid haemorrhage

The effects of centrally administered atrial natriuretic peptide (ANP) on the brain water and electrolyte contents were investigated in a rodent subarachnoid haemorrhage (SAH) model. SAH caused statistically significant increases in the brain sodium and water contents, while the potassium content did not change significantly, indicating that the brain oedema could be classified as having a primarily vasogenic component. Two micrograms or 5 micrograms of rat ANP administered into the lateral ventricle at the time of SAH induction statistically significantly decreased the water and sodium accumulation measured 90 minutes following SAH. The same treatment did not inhibit development of brain oedema measured 3 hours following SAH. However, when 5 micrograms of ANP was administered intraventricularly at the time of SAH induction and also 90 minutes later, the brain oedema 3 hours following SAH was again reduced statistically significantly. These effects of ANP were found not to be mediated by primary changes in serum osmolality and electrolyte concentrations. The present results confirm that centrally administered ANP may act directly on the central nervous system to inhibit brain water and sodium accumulation in SAH-induced brain oedema. The potentials of influencing the central neuro-endocrine system as a novel way of the treatment of brain oedema are discussed.

Effects of arginine vasopressin and atriopeptin on glial cell volume measured as 3-MG space

This study evaluates the hypothesis that arginine vasopressin (AVP) and atriopeptin, peptide hormones synthesized and released within the brain, are regulators of brain cell volume using cultured astroglial cells derived from newborn rats. Cell water content, regarded as volume, was measured in defined, serum-free medium as the 3-O-methylglucose (3-MG) space. Initial experiments established conditions such that glucose, which competes with 3-MG for the glucose carrier, would not interfere with the measurement of the 3-MG space. AVP increased the 3-MG space of glial cells by an average of 25% between 30 and 120 min of exposure, whereas atriopeptin decreased it by 32%. The 3-MG space remained close to normal after coadministration of both peptides. The AVP-dependent increase in 3-MG space was blocked both by the V1 antagonist d(CH2)5Tyr(Me)AVP (Manning compound) and by the cotransport inhibitor, bumetanide. Results are consistent with a role for AVP and atriopeptin in the homeostasis of atroglial cell volume.

Volume regulation of the brain tissue--a survey

Though the brain bulk has been considered to be constant in several pressure homeostasis studies, the central nervous tissue may be responsible for the accommodation of extracerebral masses exceeding the volume regulation capacity of the cerebral blood and cerebrospinal fluid. Volume buffering of the nervous tissue may even be functioning in parallel, in conjunction with the "fluid" compartments. Of the existing volume regulatory models, the following are discussed: osmotic feedback (buffering) preventing major fluid shifts in osmotic or pressure disequilibrium at the blood brain barrier (BBB), and the 4-compartment model, which under steady-state conditions can be regarded as an analogue of systemic tissue volume regulation, i.e. secretion of fluid at the BBB, bulk flow of interstitial space fluid (ISF) in the brain and absorption via the cerebrospinal fluid (CSF). The most recent data are presented, confirming that accommodation of space occupation by the nervous tissue is achieved via shrinkage of the extracerebral fluid (ECF), while the cell volume remains relatively constant. These findings confirm Hakim's classical hypothesis, based on biomechanical considerations, that the brain behaves like a sponge. The data presented in this survey point to a more general hypothesis: the brain interstitial space can vary in volume according to physiological and pathological stress, within certain bounds this being a reversible process which does not affect brain function. The potential role of the central neuro-endocrine system in brain volume regulation is discussed. Vasopressin (AVP) and atriopeptin (ANP) probably, function within the brain via a paracrine mechanism, as physiological regulators of brain cell and ISF volume.(ABSTRACT TRUNCATED AT 250 WORDS)