Atrial natriuretic peptide augments the blood-brain transfer of water but not leucine and glucose

Atrial natriuretic peptide inhibits osmotical glial cell swelling in the ischemic rat retina: Dependence on glutamatergic-purinergic signaling

Atrial natriuretic peptide (ANP) is a regulator of the water and electrolyte content in the brain which also mediates cell volume homeostasis. Here, we determined whether the expression of ANP in the retina of the rat undergoes changes during ischemia-reperfusion, and whether ANP affects the osmotic swelling of Müller glial cells in postischemic retinas under hypotonic conditions. Transient retinal ischemia was induced by elevation of the intraocular pressure above systolic blood pressure for 1h. At 1 and 3 days after reperfusion, there was an increased content of ANP protein in the retina, as determined by Western blotting. The increase of the retinal ANP content was markedly reduced when triamcinolone acetonide (10 mM in 2 microl vehicle) was intravitreally injected before ischemia. ANP inhibited the osmotic swelling of Müller cell somata in retinal slices. The effect of ANP was mediated by activation of NP receptors expressed by retinal neurons which evoked a release of glutamate. The stimulation of metabotropic glutamate receptors expressed by Müller cells evoked an autocrine purinergic signaling mechanism that resulted in the opening of K(+) and Cl(-) channels; the ion efflux counteracted the osmotic swelling of Müller cells. It is concluded that the expression of ANP is transiently upregulated in the postischemic retina of the rat. The increased expression of ANP may represent a part of the retinal response to transient ischemia and may inhibit cytotoxic glial cell swelling.

The influx of neutral amino acids into the porcine brain during development: a positron emission tomography study

Pigs of three different age groups (newborns, 1 week old, 6 weeks old) were used to study the transport of the large neutral amino acids 6-[18F]fluoro-L-DOPA ([18F]FDOPA) and 3-O-methyl-6-[18F]fluoro-L-DOPA ([18F]OMFD) across the blood-brain barrier (BBB) with positron emission tomography (PET). Compartmental modeling of PET data was used to calculate the blood-brain clearance (K1) and the rate constant for the brain-blood transfer (k2) of [18F]FDOPA and [18F]OMFD after i.v. injection. A 40-70% decrease of K1(OMFD), K1(FDOPA) and k2(OMFD) from newborns to juvenile pigs was found whereas k2(FDOPA) did not change. Generally, K1(OMFD) and k2(OMFD) are lower than K1(FDOPA) and k2(FDOPA) in all regions and age groups. The changes cannot be explained by differences in brain perfusion because the measured regional cerebral blood flow did not show major changes during the first 6 weeks after birth. In addition, alterations in plasma amino acids cannot account for the described transport changes. In newborn and juvenile pigs, HPLC measurements were performed. Despite significant changes of single amino acids (decrease: Met, Val, Leu; increase: Tyr), the sum of large neutral amino acids transported by LAT1 remained unchanged. Furthermore, treatment with a selective inhibitor of the LAT1 transporter (BCH) reduced the blood-brain transport of [18F]FDOPA and [18F]OMFD by 35% and 32%, respectively. Additional in-vitro studies using human LAT1 reveal a much lower affinity of FDOPA compared to OMFD or L-DOPA. The data indicate that the transport system(s) for neutral amino acids underlie(s) developmental changes after birth causing a decrease of the blood-brain barrier permeability for those amino acids during brain development. It is suggested that there is no tight coupling between brain amino acid supply and the demands of protein synthesis in the brain tissue.

Impaired cerebral glucose metabolism in eclampsia: a new finding in two cases

The pathophysiology of the reversible neurological manifestation in eclamptic women remains unclear. We report on 2 women with eclampsia who were repetitively examined by (1) transcranial Doppler (TCD), (2) magnetic resonance imaging (MRI) including T1- and T2-weighted images, fluid attenuated inversion recovery sequence, dynamic susceptibility-weighted perfusion imaging and magnetic resonance angiography (MRA), and (3) (18)fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET). In both cases repetitive TCD revealed no signs of vasospasm; the same was true for MRA. MRI perfusion imaging showed completely homogenous cerebral blood flow in both cases. In the initial phase T2-weighted images revealed hyperintensities in both patients (predominantly bilateral frontal and parietal in 1 and in the temporo-occipital subcortex and the basal ganglia in the other). FDG-PET showed inhomogeneous glucose metabolism (GM) in both patients. Primary increased glucose utilization in the hyperintense T2-weighted areas as well as an attenuated GM parieto-occipital were observed in the 1st case; a high GM was found bilaterally in the basal ganglia and an attenuated one in the occipital cortex in the 2nd. In both cases MRI, and FDG-PET normalized within 3 weeks. These case reports document an altered cerebral GM in the presence of homogenous perfusion in eclamptic women. The high GM may be explained by a decoupling of cerebral perfusion and GM, possibly indicating an increased neuronal activity. The attenuation of the GM is most probably due to a deafferentation of cortical neurons.

Maternal cerebral hemodynamics in pregnancy-related hypertension. A prospective transcranial Doppler study

AIM

To compare maternal cerebral hemodynamics, as assessed by transcranial Doppler studies, with the clinical and radiological findings in different types of pregnancy-related hypertension and to determine their pathophysiology.

METHODS

A prospective study of 66 consecutive pregnant women with hypertensive disorders (eclampsia, n = 3; pre-eclampsia, n = 41; isolated hemolysis, elevated liver enzymes, and low platelet count (HELLP)-syndrome, n = 12; pre-eclampsia superimposed on chronic hypertension, n = 5; chronic hypertension, n = 5) and 21 women with uncomplicated pregnancies. Mean blood flow velocities (Vmean) were assessed serially by means of transcranial Doppler in all basal arteries and correlated with changes in mean arterial blood pressure (MABP) and the clinical course.

RESULTS

Patients with the pre-eclampsia/eclampsia syndrome showed significantly elevated Vmean values as compared to controls. In the course of the illness Vmean over the whole length of all insonated basal arteries rose simultaneously. The three eclamptic patients showed the highest Vmean values (156, 182, 192 cm/s, respectively), of the middle cerebral artery (MCA) while MABP was 135, 135, and 150 mmHg, respectively. In pre-eclamptic patients the maximal Vmean MCA ranged from 80 (67, 93) to 145 (114, 151) cm/s [median (25th, 75th percentile)] depending on the severity of clinical presentation. In patients with isolated HELLP-syndrome changes in Vmean were either mild (5/12 cases) or absent (7/12 cases).

CONCLUSIONS

Considerable differences in cerebral hemodynamics were observed in the various types of pregnancy-related hypertensive disorders examined in this study. Our findings in patients with pre-eclampsia/eclampsia syndrome suggest a breakdown of autoregulation with hyperperfusion and vasogenic edema being the most probable pathophysiological mechanism.

Peripartal temporal course of endothelin 1, angiotensin II, and atrial natriuretic peptide in pre-eclampsia and normotensive pregnancy

OBJECTIVE

To investigate the temporal peripartal course of plasma endothelin 1 (ET-1), angiotensin II (ANG II), and atrial natriuretic peptide (ANP) in patients with pre-eclampsia as compared to women with normotensive pregnancy.

METHODS

Levels of ET-1, ANG II, and ANP were measured by means of radio-immunoassay twice a week in 17 patients with pre-eclampsia and in 17 women with normotensive uncomplicated pregnancy during four different periods: (a) from admission to 1 week before delivery, (b) 1st week before delivery, (c) 1st week after delivery, and (d) 2nd week after delivery.

RESULTS

ET-1 levels were not statistically different between the two groups. ANG II concentrations were lower in the pre-eclampsia group than in the control group with statistical significance in period c (2.8 +/- 3.0 vs. 7.1 +/- 5.9 pg/ml, mean +/- SD). ANP levels were higher in pre-eclamptic women than in the control group, the difference being statistically significant in period a (54 +/- 46 vs. 11 +/- 16 pg/ml) and in period c (122 +/- 134 vs. 39 +/- 22 pg/ml).

CONCLUSIONS

There are considerable differences in ANP and ANG II concentrations during the peripartal period between patients with pre-eclampsia and normotensive pregnant women. Decreased ANG II and elevated ANP levels in the 1st week postpartum may reflect the clinical observation that some women deteriorate in this period.

Cross talk among cyclic AMP, cyclic GMP, and Ca(2+)-dependent intracellular signalling mechanisms in brain capillary endothelial cells

C-type natriuretic peptide and sodium nitroprusside, a nitric oxide donor molecule, induced large increases in cyclic GMP formation in cultured rat brain capillary endothelial cells. Isoproterenol, a potent agonist of adenylate cyclase, potentiated the actions of C-type natriuretic peptide and of sodium nitroprusside. These actions were not observed in the presence of isobutylmethylxanthine and were mimicked by forskolin. Endothelin-1 had no action on basal cyclic GMP levels. It reduced cyclic GMP formation induced by C-type natriuretic peptide and sodium nitroprusside by about 50%. These actions involved an ETA receptor subtype and a Ca(2+)-dependent and protein kinase C-independent mechanism. Finally, increasing cyclic GMP slightly prolonged intracellular Ca2+ transients induced by endothelin-1. The results suggest the presence of extensive cross talk among cyclic AMP, cyclic GMP, and Ca(2+)-dependent mechanisms in endothelial cells of brain microvessels. The relevance of the results to the regulation of the blood-brain barrier permeability is discussed.

Developmental changes of enzymes involved in peptide degradation in isolated rat brain microvessels

The specific activities of aminopeptidase A (APA), aminopeptidase M (APM), and dipeptidyl-aminopeptidase IV (DP IV) were determined in isolated brain microvessels and in brain homogenate of rats with different ages (between 1 and 8 weeks old). In addition, the blood-brain barrier (BBB)-specific enzymes gamma-glutamyltranspeptidase (gamma-GT) and alkaline phosphatase (ALP) were measured. As similarly described by others, gamma-GT activity increased during this time period by fourfold, whereas ALP increased between weeks 1 and 2 and declined thereafter. DP IV activity increased fivefold during the first 8 weeks after birth and APM activity increased by twofold. A decrease of APA activity was found between weeks 1 and 2 after birth followed by an increase thereafter. The development of aminopeptidase activities responsible for the processing of specific neuropeptides acting on brain microvessels may be important in the development of regulation processes for cerebral blood flow and BBB permeability in the maturing animal.

The blood-brain barrier in vitro: the second decade

Ever since the discovery of Paul Ehrlich (1885 Das Sauerstoff-bedürfnis des Organismus: Hirschwald, Berlin) about the restricted material exchange, existing between the blood and the brain, the ultimate goal of subsequent studies has been mainly directed towards the elucidation of relative importance of different cellular compartments in the peculiar penetration barrier consisting the structural basis of the blood-brain barrier (BBB). It is now generally agreed that, in most vertebrates, the endothelial cells of the central nervous system (CNS) are responsible for the unique penetration barrier, which restricts the free passage of nutrients, hormones, immunologically relevant molecules and drugs to the brain. After an era of studying with endogenous or exogenous tracers the unique permeability properties of cerebral endothelial cells in vivo, the next generation, i.e. the in vitro blood-brain barrier model system was introduced in 1973. Recent advances in our knowledge of the BBB have in part been made by studying the properties and function of cerebral endothelial cells (CEC) with this in vitro approach. This review summarizes the results obtained on isolated brain microvessels in the second decade of its advent.

Peptide receptors of the blood-brain barrier and substrate transport into the brain

The BBB is a target for some peptide signals, as demonstrated by our group for arginine-vasopressin (AVP) and atriopeptin (ANP). Peptide molecules contacting the luminal surface of endothelial cells interact with specific high-affinity binding sites. The minimal simple diffusion of peptide molecules across the layer of endothelial cells which are connected by tight junctions is most probably without any significance under physiological conditions, although that question should be checked for brain regions like the olfactory bulb in which some leakiness of the BBB can be demonstrated. The AVP- and ANP-receptors at least partly localized at the luminal surface of the endothelial cells are heterogeneously distributed in the vessels of the brain. The number of AVP receptors is up-regulated by ligand deficiency, which induces furthermore a decrease in the receptor affinity. At physiological concentrations AVP and ANP do not affect the tightness of the BBB, but regulate the transcellular transfer of essential substances from blood to brain. AVP decreases the Km and Vmax of the transporter of large neutral amino acids, and ANP alters the water permeability of the endothelial cell layer. The phenomenon that the cells of the tight epithelium representing the BBB need information from blood-borne peptide signals for the regulation of intercompartmental transport processes seems to be only a special case of a general principle concerning tight epithelial cell layers which separate compartments containing fluids of different composition; amino acid transport across the intestine is regulated by specific peptides contacting that barrier, the casomorphins.

On the blood-brain barrier to peptides: specific binding of atrial natriuretic peptide in vivo and in vitro

Using the intracarotid bolus injection technique, a saturable binding of [125I]atrial natriuretic peptide (ANP) was found in 8 blood-brain barrier (BBB)-protected rat brain regions as well as in the pineal gland, choroid plexus, neurointermediate and anterior lobes of the pituitary, i.e. structures lacking a BBB. The presence of specific ANP binding on the BBB, here shown for the first time by an in vivo approach, was evidenced concomitantly in vitro by incubation of isolated microvessels. A single-class high affinity binding without regional differences was obtained with Kd = 0.23 nM and Bmax = 120 fmol/mg protein. From that a density of 1,400 binding sites per endothelial cell was calculated, thought to be localized predominantly in the luminal membranes. In the in vivo study, the portion of the extracted peptide that, under the conditions used, may have crossed the BBB by passive diffusion amounted to less than 0.4% of the labeled ANP administered. ANP itself did not change the tightness of the BBB to the non-diffusible reference molecule [14C]inulin. In the BBB-free areas, ANP enhanced the inulin space by nearly 50%.