Angiotensin II and atrial natriuretic factor receptor interactions at the blood-brain barrier

Natriuretic peptides in brain physiology

Natriuretic peptides (NPs) regulate salt and water homeostasis by inducing natriuresis and diuresis in the kidney. These actions in addition to those via the heart and vascular system play important roles in the regulation of blood pressure. In the central nervous system NPs play a significant role in neuronal development, synaptic transmission and neuroprotection. Currently, six different human NPs have been described: atrial natriuretic peptide (ANP), urodilatin (URO, renal natriuretic peptide), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) as well as guanylin and uroguanylin. ANP, URO and BNP activate the natriuretic peptide receptor A (NPR-A or guanylate cyclase A (GC-A)) while CNP activates natriuretic peptide receptor B (NPR-B or guanylate cyclase B (GC-B)). Guanylin and uroguanylin are known to activate guanylate cyclase C (GC-C). The receptors GC-A, GC-B, and GC-C are widely expressed in the human body. Currently, GC-B and CNP seems to have the highest expression in central nervous system compared to other NPs and their receptors. All known NPs generate intracellular cyclic GMP (cGMP) by activating their specific guanylate cyclase receptors. Subsequently, cGMP is able to activate protein kinase I or II (PKG I or II) and/or directly regulate transmembrane proteins such as ion channels, transporters and pumps. NPs also bind to the natriuretic peptide receptor C (also called clearance receptor NPR-C) which is a major pathway for the degradation of NPs and has no guanylate cyclase activity. In this review we will focus on new insights regarding the physiological effects of NPs in the brain, especially specific areas of their signaling pathways in neurons and glial cells.

Glial cells as sources and targets of natriuretic peptides

Natriuretic peptides and their receptors are widely expressed in mammalian CNS and increasing evidence implicates them in the regulation of neural development, synaptic transmission and processing of information, and neuroprotection. Although the peptides have been mainly localized in neuronal populations they are also produced in glial cells. Astroglia and microglia also express functional natriuretic peptide receptors that can regulate important physiological responses. In this article we review evidence on the localization of natriuretic peptides and their receptors in astroglial and microglial cells and summarize data supporting the participation of this signalling system in neuron-glia and glia-brain blood vessel communication relevant to CNS function.

Differential effects of atrial natriuretic peptide on the brain water and sodium after experimental cortical contusion in the rat

Atrial natriuretic peptide (ANP) plays an important role in the regulation of water and sodium in the body via cyclic GMP (cGMP) pathway. Although ANP has been shown to be protective in cerebral ischemia or intracerebral hemorrhage, its role in traumatic brain injury (TBI) has yet to be elucidated. We herein assessed ANP effects on brain water and sodium in TBI. Controlled cortical impact (3 mm depth, 6 m/sec) was used to induce an experimental cortical contusion in rats. Continuous administration of ANP 0.2 (n = 6) or 0.7 microg/kg/24 h (n = 6), cGMP analogue (8-Bromo-cGMP) 0.1 (n = 5) or 0.3 mg/kg/24 h (n = 5), or vehicle (n = 6) was begun 15 minutes after injury, using a mini-osmotic pump implanted into the peritoneal cavity. At 24 hours after injury, ANP significantly exacerbated brain edema in the injured hemisphere in a dose-dependent manner while it reduced brain sodium concentrations in both hemispheres. These ANP effects could be mimicked by a cGMP analogue. In the second series (n = 20), BBB integrity was assessed by evaluating the extravasation of Evans blue dye. ANP or cGMP analogue significantly worsened BBB disruption in the injured hemisphere at 24 hours after injury. These findings suggest that ANP administration exacerbates brain edema after the experimental cortical contusion in rats, possibly because of an increase in the BBB permeability via cGMP pathway, whereas it reduces brain sodium levels.

Plasma concentrations of atrial and brain natriuretic peptides in a case with hypertensive encephalopathy

Hemodynamic mechanism for brain edema forrmation in patients with hypertensive encephalopathy is unclear. Potential roles of natriuretic peptides in the pathogenesis of hypertensive encephalopathy are discussed. A 32-year-old man presented with slight left hemiparesis. He was slightly confused, and his blood pressure was extremely high. Cranial plain computerized tomography scans revealed diffuse brain edema mainly in the supratentorial white matter region. Blood examination revealed that plasma concentrations of atrial and brain natriuretic peptides were significantly high. His left hemiparesis disappeared within a day, but he tended to be agitated. His altered mental status, however, resolved with control of blood pressure. Serial magnetic resonance imagings demonstrated that the magnitude of brain edema was attenuated in proportion to decline in plasma concentrations of natriuretic peptides. This case suggests that significant elevation of plasma concentrations of natriuretic peptides may contribute to an acute rise in blood pressure, and that these peptides potentially play an important role in development of brain edema in hypertensive encephalopathy.

The role of guanylyl cyclases in the permeability response to inflammatory mediators in pial venular capillaries in the rat

Inflammatory mediators have a role in the formation of cerebral oedema and there is evidence that cGMP is an important signal in vascular permeability increase. We have investigated the role and the source of cGMP in mediating the permeability response to acutely applied bradykinin and the histamine H(2) agonist dimaprit on single cerebral venular capillaries, by using the single vessel occlusion technique. We found that 8-bromo-cGMP applied acutely resulted in a small and reversible permeability increase with a log EC(50) -7.2 +/- 0.15 M. KT 5823, the inhibitor of cGMP-dependent protein kinase, abolished the permeability responses to both bradykinin and dimaprit, while zaprinast, an inhibitor of type 5 phosphodiesterase, potentiated the response to bradykinin. On the other hand, L-NMMA blocked the response to dimaprit, but not that to bradykinin. Inhibitors of soluble guanylyl cyclase, LY 85353 and methylene blue, also inhibited the permeability response to dimaprit, but not bradykinin. The permeability responses to the natriuretic peptides ANP and CNP were of similar magnitude to that of bradykinin with log EC(50) -10.0 +/- 0.33 M and -8.7 +/- 0.23 M, respectively. The natriuretic peptide receptor antagonist HS-142-1 blocked permeability responses to bradykinin as well as to ANP, and leukotriene D(4) blocked the responses to CNP and bradykinin, but not to dimaprit. In conclusion, the histamine H(2) receptor appears to signal via cGMP that is generated by a NO and soluble guanylyl cyclase, while bradykinin B(2) receptor also signals via cGMP but through particulate guanylyl cyclase.

Effects of cyclic GMP on microvascular permeability of the cerebral cortex

This study was performed to test the hypothesis that a direct application of cyclic guanosine monophosphate (cGMP) to the cortex would increase blood-brain barrier (BBB) permeability. Rats were anesthetized with 1.4% isoflurane and were mechanically ventilated. Two cranial windows (3 mm in diameter) were made on each side of the rat's skull (a total of four windows on each rat) to expose the cerebral cortex. A patch of normal saline, 10(-5) M, 10(-4) M, or 10(-3) M 8-bromo-cGMP was applied to each cranial window. The patches were changed every 5 min. Ten minutes after applying the patches, BBB permeability was determined by measuring the transfer coefficient (Ki) of [alpha-14C]aminoisobutyric acid. Vital signs were not changed after applying 8-bromo-cGMP. Blood gases were within normal limits. In the cortex, 10(-5) M 8-bromo-cGMP did not significantly affect the Ki; 10(-4) M 8-bromo-cGMP increased the Ki by 115%; 10(-3) M 8-bromo-cGMP increased the Ki by 124%. However, there was no statistical difference in the Ki between the doses of 10(-4) M and 10(-3) M 8-bromo-cGMP. In the pons where no patch was applied, the Ki was similar to that of the cortical area where a normal saline patch was applied. Our data demonstrated that a direct application of cGMP to the cerebral cortex significantly increased the permeability of the BBB.

Immunolocalization of endothelin in the traumatized spinal cord: relationship to blood-spinal cord barrier breakdown

The purpose of this study was to determine the relationship between endothelin-1 (ET-1), a prominent vasoactive agent, and the breakdown of the blood-spinal cord barrier along the axis of the cord after a moderate spinal cord injury. In the first study rats (n = 10) were euthanized 24 h after spinal cord injury and compared to sham (n = 5) and unoperated (n = 10) controls. Endothelin and immunoglobulins (IgG) were immunolocalized in adjacent sections of spinal cord using semiquantitative immunocytochemical techniques. In the second study animals were pretreated with the endothelin antagonist Bosentan (n = 6) or vehicle (n = 6) prior to spinal cord injury. Animals were euthanized at 24 h postinjury. Ten minutes prior to euthanasia animals were given horseradish peroxidase (HRP) intravenously. After perfusion fixation sections of cord were prepared for quantitative HRP histochemistry. After spinal cord injury there was enhanced staining for endothelin along the axis of the cord that correlated with the anatomical pattern of barrier breakdown to IgG. In those animals that were pretreated with Bosentan, there was a significant reduction in barrier breakdown along the axis of the injured cord as compared to those animals that received vehicle only. Taken together, this data implicate involvement of endothelin in the axial pattern of barrier breakdown after spinal cord injury.

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.

Comparative effects of angiotensin-(1-7) and angiotensin II on piglet pial arterioles

BACKGROUND AND PURPOSE

Recent investigations indicated that degradation fragments of angiotensins could be involved in the regulation of the cerebral circulation and that their effects might be mediated by prostaglandins. The present study was designed to examine the effect of angiotensin-(1-7), a major endogenous heptapeptide fragment, on cerebral arteriolar diameter and compare it with the octapeptide angiotensin II, and further to determine whether prostaglandins mediate their effects.

METHODS

Newborn, anesthetized pigs were equipped with a closed cranial window, and the diameter of one pial arteriole was measured using intravital microscopy.

RESULTS

Topical application of angiotensin-(1-7) (n = 9) increased the diameter by 6.8 +/- 5.3% (mean +/- SEM), 10.4 +/- 5.2%, 14.3 +/- 5.9%, and 17.5 +/- 7.7% (P < .05) at 10(-7), 10(-6), 10(-5), and 10(-4) mol/L, respectively (baseline, 94 +/- 3 microns). Topical application of angiotensin II (n = 8) increased the diameter by 9.6 +/- 7.0%, 9.6 +/- 7.6%, 11.3 +/- 8.4% (P < .05), and 5.5 +/- 7.9% at 10(-7), 10(-6), 10(-5), and 10(-4) mol/L, respectively (baseline, 94 +/- 5 microns). After administration of indomethacin (5 mg/kg IV), which did not significantly change the baseline arteriolar diameter, neither angiotensin-(1-7) at 10(-4) mol/L nor angiotensin II at 10(-5) mol/L caused significant vasodilation.

CONCLUSIONS

The results indicate that angiotensin-(1-7) is a modest dilator in the cerebral circulation, as is angiotensin II, and that prostaglandins may mediate responses.

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.

Effect of angiotensin II and atrial natriuretic factor on neurons in the subfornical organ of ducks and rats in vitro

Atrial natriuretic factor (ANF) antagonizes many angiotensin II (ANGII)-induced effects on osmoregulatory relevant parameters in vivo. In this study ANF analogues decreased the spontaneous and the ANGII-induced electrical activity of subfornical organ (SFO) neurons in rats, but had no effect on ANGII sensitive or insensitive SFO neurons in ducks. These results suggest a more distinct functional separation for the responsiveness to ANGII and ANF in birds compared to mammals.