Ernö Dux

Protective effect of adenosine and a novel xanthine derivative propentofylline on the cell damage after bilateral carotid occlusion in the gerbil hippocampus

The role of adenosine in the development of ischemia induced pathological changes has been examined in Mongolian gerbils. A dramatic increase in the concentrations of adenosine, inosine and hypoxanthine was detected by microdialysis in the dorsal part of hippocampus and in the striatum immediately after 5 min bilateral occlusion of the carotid arteries. From a resting value of about 0.5 microM the concentration of adenosine increased to more than 10 microM. The adenosine levels became normalized within 30 min after ischemia. Inosine and hypoxanthine levels were higher and they increased and also returned towards control somewhat later than adenosine. A second occlusion resulted in a similar but somewhat smaller increase in purine levels. Carotid occlusion for up to 12 min had no major, lasting effect on the binding to adenosine A1-receptors in the CA-regions of the hippocampus, as determined by autoradiography. Neuronal and vascular changes (degeneration of neurons, mitochondrial destruction and ribosomal disaggregation, astroglial oedema) due to ischemia (3-12 min, followed by 48 h recirculation) was studied with light and electron microscopy in the selectively vulnerable CA1 area of hippocampus. In one series of experiments the adenosine antagonist theophylline (20 mg/kg i.p.), given 15 min prior to a 5 min occlusion, significantly enhanced the ischemia induced changes. In another experiment the adenosine uptake inhibitor propentofylline (HWA 285, 10 mg/kg), injected 15 min before a 12 min carotid occlusion, reduced the neuronal (90%) and astroglial changes (84%) due to ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Prostacyclin and prostagladin synthesis in isolated brain capillaries

The synthesis of prostacyclin and prostaglandins was examined in isolated blood-free brain capillaries of guinea-pigs and rats using 1-14C-arachidonic acid as a precursor. The main prostaglandins synthesized by guinea-pig microvessels were prostaglandin D2 and prostaglandin E2. Substantially less prostaglandin F2 alpha or the prostacyclin stable metabolite, 6-oxo-prostaglandin F1 alpha was synthesized. Rat capillary prostaglandin distribution differed substantially from that of the guinea-pigs although the principle prostaglandin was also PGD2. Total prostaglandin conversion was greater in guinea-pig capillaries than in the rat. Norepinephrine stimulated the prostaglandin forming capacity of blood free cerebral microvasculature of guinea-pigs. Prostacyclin and prostaglandins could be involved in the activity dependent regulation of regional cerebral blood flow and permeability.

Regulation of the macromolecular transport in the brain microvessels: The role of cyclic GMP

The possible effects of dibutyryl cyclic GMP (db-cGMP), the lipid-soluble derivative of cyclic GMP, on brain microvessels were studied by light microscopic detection of albumin and quantitative electron microscopy. Different concentrations (25, 50, 100 and 200 micrograms) of db-cGMP were given to adult rats by intracarotid infusion, while in the controls, the animals were infused either with Krebs-Ringer solution or with the same solution containing butyrate or cyclic GMP. In contrast to the controls, db-cGMP was found to be able to increase the permeability of brain microvessels to albumin in a dose-dependent manner. At the same time, the number of transport vesicles being indicative of the pinocytotic activity of the endothelial cells was also increased. These results indicate that, similarly to the cyclic AMP whose effects have been revealed earlier, the cyclic GMP-system may also be involved in the regulation of opening mechanisms of the blood-brain barrier.

Protective effect of antihistamines on cerebral oedema induced by experimental pneumothorax in newborn piglets

As a consequence of general hypoxaemia evoked experimentally by bilateral pneumothorax, brain oedema of vasogenic type developed in newborn piglets after 4 h survival. Histamine receptor antagonists, mepyramine (H1-receptor blocker), metiamide, cimetidine and ranitidine (H2-receptor antagonists) were administered either intraperitoneally or intrathecally to check to what extent the formation of brain oedema could be reduced. Mepyramine and ranitidine decreased the accumulation of water, sodium and albumin in the parietal cortex. By measuring the concentration of histamine, the presence of a histamine pool was demonstrated in the cerebral microvessels. The results suggest that histamine, if released upon hypoxic injury from the microvascular store, can take an important part in the development of vasogenic brain oedema.

The blood-brain barrier in hypoxia: Ultrastructural aspects and adenylate cyclase activity of brain capillaries

The ultrastructure of brain microvessels, their permeability to serum albumin, the activities of some endothelial enzymes and the effect of histamine were investigated in rats after a prolonged hypobaric-hypoxic treatment. After prolonged hypoxia, the permeation of serum albumin into endothelial cells increased together with the number of pinocytotic vesicles of the endothelium. Intracarotid histamine stimulated this process even further, and its effect was mediated by H2-histamine receptors. After hypoxia the specific activity of capillary alkaline phosphatase and gamma-glutamyl transpeptidase remained unchanged, while that of adenylate cyclase was greatly increased. Histamine did not modify the structure of tight junctions of isolated capillaries of normoxic animals. Both hypoxia- and histamine-induced modification of the brain microvessels were accompanied by an increase of pinocytosis, which may be stimulated by the activation of capillary adenylate cyclase.

Effects of acute hypoxia on the adenylate cyclase and evans blue transport of brain microvessels

Kinetic parameters of the albumin transport were measured during and after an acute hypoxic insult evoked in newborn piglets by experimental bilateral pneumothorax. Adenylate cyclase activity was determined in the cerebral microvessels isolated by ultracentrifugation from different stages of brain damage. A decrease of the adenylate cyclase activity was observed in the cerebral microvessels of animals with acute hypoxic condition. However, the adenylate cyclase activity was found to be increased significantly in the microvessels during recirculation. The activation of adenylate cyclase in the microvessel wall may be of pathogenetic importance in the development of vasogenic brain oedema.

Dexamethasone treatment attenuates the development of ischaemic brain oedema in gerbils

Transient global forebrain ischaemia was produced in Mongolian gerbils by occluding both common carotid arteries for 10 min followed by 48 h recirculation. Dexamethasone, 5 mg/kg i.p., was given 5 h before the occlusion and every 12 h thereafter. After occlusion an increase in water, sodium and calcium content was found in the parietal cortex and hippocampus, while the concentration of potassium decreased. Exudation of plasma albumin was not found in the brain. The activity of Na+, K(+)-ATPase decreased in the hippocampus. Morphological signs of cerebral oedema were also observed, both in the CA1 region of the hippocampus and in the cortex. Dexamethasone treatment prevented the accumulation of water, sodium and calcium in the ischaemic brain. It also attenuated the oedematous morphological changes of the blood-brain barrier. Thus dexamethasone treatment may also have therapeutic relevance in the acute, high-risk phase of patients suffering from repetitive, transitoric cerebral ischaemia.

EFFECT OF HISTAMINE ON THE PERMEABILITY, FINE STRUCTURE AND ADENYLATE CYCLASE ACTIVITY OF BRAIN CAPILLARIES

Publisher Summary This chapter discusses the effect of histamine on the permeability, fine structure, and adenylate cyclase activity of brain capillaries. Histamine has a definite role in the regulation of permeability in the peripheral microvessels. Although several studies have been performed in capillary-enriched fraction of brain tissues to throw some light on the possible cellular mechanisms regulating the permeability of the blood-brain barrier, the possible role of histamine in these processes has not been clarified yet. To the major part of vascular wall, histamine could also be detected in appreciable amount in the capillary-rich fraction of brain tissue. In a study described in the chapter, a capillary-enriched fraction was used to study the effect of histamine on adenylate cyclase in vitro and in vivo studies; the effect of histamine on macromolecular transport and on the fine structure of brain vessels has been determined. For the biochemical investigations, the capillary enriched fraction was prepared according to the micromethod. From the results, it seems that histamine can increase the transport processes in the cerebral capillaries. Thus, histamine—like in peripheral tissues—may take part in certain oedematous alterations of the brain.

Basic Molecular Events Underlying Transendothelial Transport in Brain Capillaries

Endothelial cells which are joined by tight junctions in the brain form a peculiar barrier restricting the movements of macro-molecules and many other solutes between blood and brain. So far. several enzymes including cholinesterases6,7, DOPA decarboxylase2 γ -glutamyl transpeptidase17. Na-K-activated adenosine triphospha-tase4,1, and other ATPases8,12 have been reported not only to be present in brain capillaries, but also to be involved in the regulation of blood-brain barrier permeability. Since the discovery of the importance of cyclic nucleotides in the mediation of the hormonal influences on different cellular functions, we have been trying to elucidate the possible role of cAMP and cGMP in the regulation of permeability of brain microvessels. Up to now, the following data have been collected: n n ndibutyryl (dibu) cAMP increases permeability and pinocytosis in brain capillaries9, n n nadenylate cyclase could be found in brain microvessels histochemically11, n n nhistamine receptors — mainly of H2-type — are linked to the capillary adenylate cyclase as detected by biochemical measurements14, n n nthe presence of guanylate cyclase was found in brain capillaries by means of histochemistry and biochemical measurements15.

Reverse pinocytosis induced in cerebral endothelial cells by injection of histamine into the cerebral ventricle.

SummaryHistamine dihydrochloride (10 μg of 500 μg/ml) was infused during 1 min into the lateral cerebral ventricle of rats, which resulted in a significant stimulation of pinocytosis in the endothelial cells. Systemic injections of mepyramine or metiamide could not prevent this activation. In contrast, ranitidine, injected with histamine was able to inhibit the stimulation of pinocytosis. Albumin exudation from the blood was not found. There was also no change in water and electrolyte contents of the brain tissue. The results suggest that histamine reaching the abluminal membrane can activate the pinocytosis in the cerebral endothelial cells in the reverse direction, i.e., from brain to blood, without opening the blood-brain barrier.