What’s new in the renin-angiotensin system?

Virtually all existing evidence on the function of angiotensin II (Ang II) in the regulation of tissue homeostasis and blood pressure regulation bears on the more restricted question of what other mechanisms or systems may amplify or inhibit the actions of this important peptide. Whereas there is evidence that Ang II may potentiate the effects of catecholamines, various cytokines and also growth factors, the repertoire of substances which may inhibit the actions of Ang II is more limited and has been restricted primarily to prostacyclin, bradykinin and nitric oxide. Advances in receptor pharmacology and introduction of selective antagonists to two of the receptor subtypes at which Ang II binds permitted a more critical examination of the functions of the renin angiotensin system in physiological and pathophysiological conditions, as well as uncovering the previously unsuspected possibility that within the biochemical pathways leading to the formation of the peptide the renin angiotensin system could process either its immediate precursor (angiotensin I) or the actual Ang II peptide into an alternative form, angiotensin-(1-7) [Ang-(1-7)], the function of which was to antagonize the effects of Ang II. We review here the biological actions of Ang-(1-7) and discuss how this discovery may change altogether the perception of how the renin angiotensin system functions in the regulation of tissue perfusion pressure and the regulation of salt and water metabolism.     

Angiotensin-converting enzyme 2 in acute respiratory distress syndrome

Angiotensin-converting enzyme (ACE) and ACE2 are highly homologous metalloproteases that provide essential catalytic functions in the renin-angiotensin system (RAS). Angiotensin II is one key effector peptide of the RAS, inducing vasoconstriction and exerting multiple biological functions. ACE cleaves angiotensin I to generate angiotensin II, whereas ACE2 reduces angiotensin II levels. Accumulating evidence has demonstrated a physiological and pathological role of ACE2 in the cardiovascular systems. Intriguingly, the SARS coronavirus, the cause of severe acute respiratory syndrome (SARS), utilizes ACE2 as an essential receptor for cell fusion and in vivo infections. Moreover, recent studies have demonstrated that ACE2 protects murine lungs from acute lung injury as well as SARS-Spike protein-mediated lung injury, suggesting a dual role of ACE2 in SARS infections and protection from ARDS. Received 18 May 2006; received after revision 12 March 2007; accepted 24 April 2007     

The plasma level of some amino acids and physical and mental fatigue

Tryptophan is converted to 5-hydroxytryptamine (5-HT) in the brain and evidence is presented that an increase in the concentration of 5-HT can result in physical and mental fatigue during prolonged exercise. The entry of tryptophan in the brain is influenced by the plasma level of free tryptophan (that not bound to albumin) and, from competition for entry into brain, by the plasma level of branched chain amino acids. Hence, oral administration of branched chain amino acids could, theoretically, prevent the increase in 5-HT level during exercise and therefore delay physical and mental fatigue. Evidence in support of this hypothesis is presented.     

Hypertension mediated by the activation of the rat brain 5-hydroxytryptamine receptor sites

Summary 5-Hydroxytryptamine (5-HT) administered intraventricularly (i.vent.) in rats produced hypertension without considerable changes in heart rate. After transsection of the spinal cord or i.vent. administration of methysergide, 5-HT failed to produce the pressor effect. Thus, the hypertension results from the activation of 5-HT receptor sites of the rat brain.This work was supported by Union of Scientific Medical Institutions of Serbia (Grant No. 85).     

Tryptophan (Trp), serotonin (5-HT), monoamino oxidase (MAO) and 5-hydroxyindole acetic acid (5-HIAA) in brain and subesophagic ganglions of earthworms. Effects of Parathion

Summary Tryptophan, 5-HT, MAO and 5-HIAA were determined in the first 5 segments of earthworms (Oligochaetae) where the brain and subesophagic ganglions are located. Tranylcypromine (IMAO) decreased MAO activity increasing 5-HT and decreasing 5-HIAA. Motility and survival of worms were disturbed. InAllolobophora species (young worms), parathion fumigation decreased acetylcholinesterase (AChE) activity and increased Trp, 5-HT and 5-HIAA. Motility was diminished after 24 h: it worsened after 72, but returned to normal levels 40/50 days later.Publication of the National Institute of Agrarian Technology (INTA), Argentina.Acknowledgments. We are most grateful to Prof. Zlatko Tomsic (from the Lillo Institute, Tucumán) and to Leticia Alvarado (from INTA) and to Prof. Mirta P. de Matosian for earthworm classification.     

Serotonin metabolism in the CNS in cerebellar ataxic mice

The metabolism of 5-hydroxytryptamine (5-HT) in the CNS was investigated in four kinds of morphologically different ataxic mice; reeler, staggerer, weaver and Purkinje cell degeneration mutants, and in hypocerebellar mice experimentally produced by injection of cytosine arabinoside. 5-HT and 5-hydroxyidoleacetic acid concentrations and tryptophan hydroxylase (TrpOH) activity were measured in the cerebrum, cerebellum and brain stem, respectively. TrpOH activity was significantly reduced only in the reeler mouse. The enhancements of the cerebellar 5-HT metabolism observed in the ataxic mice other than the reeler were supposed to be pseudo-enhancements subsequent to the cerebellar hypoplasia.     

The enteric neural receptor for 5-hydroxytryptamine

An enteric neural receptor for serotonin (5-HT) has been characterized. This receptor was assayed, using 3H-5-HT as a radioligand, by rapid filtration of isolated enteric membranes and by radioautography. In addition, intracellular recordings were made from ganglion cells of the myenteric plexus. High affinity, saturable, reversible, and specific binding of 3H-5-HT was demonstrated both to membranes of the dissected longitudinal muscle with adherent myenteric plexus and the mucosa-submucosa. Radioautographs showed these 3H-5-HT binding sites to be in myenteric ganglia and in a broad unresolved band at the mucosal-submucosal interface. Antagonists active at receptors for other neurotransmitters than 5-HT, at either of the two known types of CNS 5-HT receptor, and at 5-HT uptake sites on serotonergic neurons failed to inhibit binding of 3H-5-HT. The structural requirements of analogues for binding to the enteric 5-HT receptor matched the known pharmacology of M or neural 5-HT receptors. A novel 5-HT antagonist was found. This compound, N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide (5-HTP-DP), antagonized the action of 5-HT on type II/AH cells of the myenteric plexus but did not affect the release or actions of acetylcholine (nicotinic or muscarinic) or substance P. 5-HTP-DP was also an equally potent displacer of 3H-5-HT from its binding sites on enteric membranes. It is concluded that the sites responsible for specific binding of 3H-5-HT are enteric M or neural 5-HT receptors. These receptors differ from those now known to be present in the CNS.     

The enteric neural receptor for 5-hydroxytryptamine

Summary An enteric neural receptor for serotonin (5-HT) has been characterized. This receptor was assayed, using³H-5-HT as a radiologand, by rapid filtration of isolated enteric membranes and by radioautography. In addition, intracellular recordings were made from ganglion cells of the myenteric plexus. High affinity, saturable, reversible, and specific binding of³H-5-HT was demonstrated both to membranes of the dissected longitudinal muscle with adherent myenteric plexus and the mucosa-submucosa. Radioautographs showed these³H-5-HT binding sites to be in myenteric ganglia and in a broad unresolved band at the mucosal-submucosal interface. Antagonists active at receptors for other neurotransmitters than 5-HT, at either of the two known types of CNS 5-HT receptor, and at 5-HT uptake sites on serotonergic neurons failed to inhibit binding of³H-5-HT. The structural requirements of analogues for binding to the enteric 5-HT receptor matched the known pharmacology of M or neural 5-HT receptors. A novel 5-HT antagonist was found. This compound, N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide (5-HTP-DP), antagonized the action of 5-HT on type II/AH cells of the myenteric plexus but did not affect the release or actions of acetylcholine (nicotinic or muscarinic) or substance P. 5-HTP-DP was also an equally potent displacer of³H-5-HT from its binding sites on enteric membranes. It is concluded that the sites responsible for specific binding of³H-5-HT are enteric M or neural 5-HT receptors. These receptors differ from those now known to be present in the CNS.     

Taste perception and coding in the periphery

Recent identification of taste receptors and their downstream signaling molecules, expressed in taste receptor cells, led to the understanding of taste coding in the periphery. Ion channels appear to mediate detection of salty and sour taste. The sensations of sweet, umami and bitter taste are initiated by the interaction of sapid molecules with the G-protein-coupled receptors T1Rs and T2Rs. Mice lacking either PLCβ2 or TRPM5 diminish behavioral and nerve responses to sweet, umami and bitter taste stimuli, suggesting that both receptor families converge on a common signaling pathway in the taste receptor cells. Nevertheless, separate populations of taste cells appear to be uniquely tuned to sweet, umami and bitter taste. Since PLCβ2-deficient mice still respond to sour and salty stimuli, sour and salty taste are perceived independent of bitter, umami and sweet taste. In this review, the recent characterization of the cellular mechanisms underlying taste reception and perception, and of taste coding in the periphery will be discussed. Received 5 March 2006; received after revision 2 May 2006; accepted 10 June 2006     

Daily variation in the eye's 5-HT stores

Summary Eyes from mice have been assayed for 5-HT content at various times during the day. 5-HT levels are highest midway in the light period and lowest during the dark period. In general this daily variation conforms with other published reports for variation of 5-HT stores in brain and pineal.     

Platelet monoamine oxidase B: use and misuse

The human platelet in addition to having serotonin (5-HT) receptors, uptake carriers (receptor) and transmitter storage vesicles, primarily possesses mitochondrial monoamine oxidase (MAO) type B. Similar to the major form of MAO in the human brain, this enzyme actively oxidizes A-B and B substrates (tyramine, dopamine, phenylethylamine) as well as the novel secondary amine anticonvulsant, milacemide and dopaminergic neurotoxin, MPTP. 5-HT oxidation is hardly affected by the platelet enzyme and MAO inhibitors have no net effect on its accumulation. MAO-B is selectively inhibited by 1-deprenyl and thus the platelet enzyme may be useful to monitor the anti-Parkinson activity of such drugs, as related to their ability to inhibit brain MAO-B. The oxidation of the anticonvulsant, milacemide, to glycine in vitro and in vivo by MAO-B, may herald new prospects for the development of inert prodrugs capable of being metabolized to neuroactive substances by MAO-B. The plasma levels of their metabolites may be an index of MAO-B activity found in the platelet and brain.     

Platelet monoamine oxidase B: Use and misuse

The human platelet in addition to having serotonin (5-HT) receptors, uptake carriers (receptor) and transmitter storage vesicles, primarily possesses mitochondrial monoamine oxidase (MAO) type B. Similar to the major form of MAO in the human brain, this enzyme actively oxidizes A-B and B substrates (tyramine, dopamine, phenylethylamine) as well as the novel secondary amine anticonvulsant, milacemide and dopaminergic neurotoxin, MPTP. 5-HT oxidation is hardly affected by the platelet enzyme and MAO inhibitors have no net effect on its accumulation. MAO-B is selectively inhibited by 1-deprenyl and thus the platelet enzyme may be useful to monitor the anti-Parkinson activity of such drugs, as related to their ability to inhibit brain MAO-B. The oxidation of the anticonvulsant, milacemide, to glycine in vitro and in vivo by MAO-B, may herald new prospects for the development of inert prodrugs capable of being metabolized to neuroactive substances by MAO-B. The plasma levels of their metabolites may be an index of MAO-B activity found in the platelet and brain.     

What’s new in the renin-angiotensin system?

Angiotensin-converting enzyme-2 (ACE2) is the first human homologue of ACE to be described. ACE2 is a type I integral membrane protein which functions as a carboxypeptidase, cleaving a single hydrophobic/basic residue from the C-terminus of its substrates. ACE2 efficiently hydrolyses the potent vasoconstrictor angiotensin II to angiotensin (1-7). It is a consequence of this action that ACE2 participates in the renin-angiotensin system. However, ACE2 also hydrolyses dynorphin A (1-13), apelin-13 and des-Arg(9) bradykinin. The role of ACE2 in these peptide systems has yet to be revealed. A physiological role for ACE2 has been implicated in hypertension, cardiac function, heart function and diabetes, and as a receptor of the severe acute respiratory syndrome coronavirus. This paper reviews the biochemistry of ACE2 and discusses key findings such as the elucidation of crystal structures for ACE2 and testicular ACE and the development of ACE2 inhibitors that have now provided a basis for future research on this enzyme.     

The involvement of the renin-angiotensin system in the regulation of cell proliferation in the rat endometrium

Oestrogens are known to enhance angiotensin biosynthesis by increasing the elaboration of its precursor, angiotensinogen. On the other hand, we found that inhibition of angiotensin-converting enzyme (ACE) suppressed the proliferative response of the rat anterior pituitary gland to oestrogens. To answer the question whether the angiotensin system is involved in the control of the cell proliferation of the uterine epithelium, the effects of an ACE inhibitor, enalapril maleate, and of angiotensins II and IV, alone or together with losartan, an antagonist of angiotensin receptor type 1 (AT1), on endometrial epithelial cell proliferation have been studied. The experiments were performed on ovariectomized female Wistar rats. In the first experiment the animals were injected with a single dose of oestradiol benzoate or received an injection of solvent only. Half of the oestrogen-treated rats were injected additionally with enalapril maleate (EN, twice daily). The incorporation of bromodeoxyuridine (BrDU) into endometrial cell nuclei was used as an index of cell proliferation. It was found that oestradiol alone dramatically increased the BrDU labelling index (LI) of endometrial cell nuclei, and this effect was partially blocked by the simultaneous treatment with EN. In the second experiment, the animals were injected intraperitoneally with angiotensin II (AII), angiotensin IV (AIV) or saline, alone or together with losartan. It was found that AIV induced an increase in the LI in uterine epithelium, and this effect was not blocked by the simultaneous treatment with losartan. The increase in LI in uterine epithelium was also observed in the rats treated with AII and with losartan. These findings suggest an involvement of angiotensin IV in the control of uterine epithelium cell proliferation. Received 12 October 1998; received after revision 6 January 1999; accepted 2 February 1999     

What’s new in the renin-angiotensin system?

The angiotensin AT(4) receptor was originally defined as the specific, high-affinity binding site for the hexapeptide angiotensin IV (Ang IV). Subsequently, the peptide LVV-hemorphin 7 was also demonstrated to be a bioactive ligand of the AT(4) receptor. Central administration of Ang IV, its analogues or LVV-hemorphin 7 markedly enhance learning and memory in normal rodents and reverse memory deficits observed in animal models of amnesia. The AT(4) receptor has a broad distribution and is found in a range of tissues, including the adrenal gland, kidney, lung and heart. In the kidney Ang IV increases renal cortical blood flow and decreases Na(+) transport in isolated renal proximal tubules. The AT(4) receptor has recently been identified as the transmembrane enzyme, insulin-regulated membrane aminopeptidase (IRAP). IRAP is a type II integral membrane spanning protein belonging to the M1 family of aminopeptidases and is predominantly found in GLUT4 vesicles in insulin-responsive cells. Three hypotheses for the memory-potentiating effects of the AT(4) receptor/IRAP ligands, Ang IV and LVV-hemorphin 7, are proposed: (i) acting as potent inhibitors of IRAP, they may prolong the action of endogenous promnestic peptides; (ii) they may modulate glucose uptake by modulating trafficking of GLUT4; (iii) IRAP may act as a receptor, transducing the signal initiated by ligand binding to its C-terminal domain to the intracellular domain that interacts with several cytoplasmic proteins.     

Serotoninergic modulation of mesolimbic and frontal cortical dopamine neurons

Potentiation of the effect of haloperidol on dopamine metabolism by the 5-HT uptake inhibitor CGP 6085 A, and antagonism of this effect by the 5-HT antagonist mianserin were observed in the mesolimbic area and the frontal cortex of the rat brain. A similar effect was reported earlier in the corpus striatum. This suggests that serotoninergic modulation of dopamine neurons is a generally-occurring phenomenon in the brain.     

Serotoninergic modulation of mesolimbic and frontal cortical dopamine neurons

Summary Potentiation of the effect of haloperidol on dopamine metabolism by the 5-HT uptake inhibitor CGP 6085 A, and antagonism of this effect by the 5-HT antagonist mianserin were observed in the mesolimbic area and the frontal cortex of the rat brain. A similar effect was reported earlier in the corpus striatum. This suggests that serotoninergic modulation of dopamine neurons is a generally-occurring phenomenon in the brain.     

A discoidin domain receptor 1 knock-out mouse as a novel model for osteoarthritis of the temporomandibular joint

Discoidin domain receptor 1 (DDR-1)-deficient mice exhibited a high incidence of osteoarthritis (OA) in the temporomandibular joint (TMJ) as early as 9 weeks of age. They showed typical histological signs of OA, including surface fissures, loss of proteoglycans, chondrocyte cluster formation, collagen type I upregulation, and atypical collagen fibril arrangements. Chondrocytes isolated from the TMJs of DDR-1-deficient mice maintained their osteoarthritic characteristics when placed in culture. They expressed high levels of runx-2 and collagen type I, as well as low levels of sox-9 and aggrecan. The expression of DDR-2, a key factor in OA, was increased. DDR-1-deficient chondrocytes from the TMJ were positively influenced towards chondrogenesis by a three-dimensional matrix combined with a runx-2 knockdown or stimulation with extracellular matrix components, such as nidogen-2. Therefore, the DDR-1 knock-out mouse can serve as a novel model for temporomandibular disorders, such as OA of the TMJ, and will help to develop new treatment options, particularly those involving tissue regeneration.     

Not just angiotensinases: new roles for the angiotensin-converting enzymes

The renin-angiotensin system (RAS) is a critical regulator of blood pressure and fluid homeostasis. Angiotensin II, the primary bioactive peptide of the RAS, is generated from angiotensin I by angiotensin-converting enzyme (ACE). A homologue of ACE, ACE2, is able to convert angiotensin II to a peptide with opposing effects, angiotensin-(1-7). It is proposed that disturbance of the balance of ACE and ACE2 expression and/or function is important in pathologies in which angiotensin II plays a role. These include cardiovascular and renal disease, lung injury and liver fibrosis. The critical roles of ACE and ACE2 in regulating angiotensin II levels have traditionally focussed attention on their activities as angiotensinases. Recent discoveries, however, have illuminated the roles of these enzymes and of the ACE2 homologue, collectrin, in intracellular trafficking and signalling. This paper reviews the key literature regarding both the catalytic and non-catalytic roles of the angiotensin-converting enzyme gene family.     

Pre- and postnatal lead exposure affects the serotonergic system in the immature rat brain.

The effect of pre- and postnatal lead exposure on the development of the serotonergic system in striatum and brain stem was investigated. Serotonin and its metabolite 5-HIAA where determined by HPLC-EC. A significant decrease of 5-HT was detected in the brain stem at postnatal day 28. At both days 6 and 28 postnatal, 5-HIAA was reduced in striatum and brain stem. The results provide support to the hypothesis that developing 5-HT neurons are sensitive to relatively low levels of lead exposure.