Jialong Zhuo

Angiotensin receptors in the nervous system

In addition to its traditional role as a circulating hormone, angiotensin is also involved in local functions through the activity of tissue renin-angiotensin systems that occur in many organs, including the brain. In the brain, both systemic and presumptive neurally derived angiotensin and angiotensin metabolites act through specific receptors to modulate many functions. This review examines the distribution of these specific angiotensin receptors and discusses evidence regarding the function of angiotensin peptides in various brain regions. Angiotensin AT1 and AT2 receptors occur in characteristic distributions that are highly correlated with the distribution of angiotensin-like immunoreactivity in nerve terminals. Acting through the AT1 receptor in the brain, angiotensin has effects on fluid and electrolyte homeostasis, neuroendocrine systems, autonomic pathways regulating cardiovascular function and behavior. Angiotensin AT1 receptors are also found in many afferent and efferent components of the peripheral autonomic nervous system. The role of the AT2 receptor in the brain is less well understood, although recent knockout studies point to an involvement with behavioral and cardiovascular functions. In addition to the AT1 and AT2 receptors, receptors for other fragments of angiotensin have been proposed. The AT4 binding site, which binds angiotensin, has a widespread distribution in the brain quite distinct from that of the AT1 and AT2 receptors. It is associated with many cholinergic neuronal groups and also several sensory nuclei, but its function remains to be determined. Our discovery that another brain-derived peptide binds to the AT4 binding site in the brain and may represent the native ligand is discussed. Overall, the distribution of angiotensin receptors in the brain indicate that they play diverse and important physiological roles in the nervous system.

Localization and function of angiotensin AT1 receptors

The distributions of angiotensin AT1 and AT2 receptors have been mapped by in vitro autoradiography throughout most tissues of many mammals, including humans. In addition to confirming that AT1 receptors occur in sites known to be targets for the physiologic actions of angiotensin, such as the adrenal cortex and medulla, renal glomeruli and proximal tubules, vascular and cardiac muscle and brain circumventricular organs, many new sites of action have been demonstrated. In the kidney, AT1 receptors occur in high density in renal medullary interstitial cells. The function of these cells, which span the interstitial space between the tubules and the vasa rectae, remains to be determined. Renal medullary interstitial cells possess receptors for a number of vasoactive hormones in addition to AT1 receptors and this, in concert with their anatomic location, suggests they may be important for the regulation of fluid reabsorption or renal medullary blood flow. In the heart, the highest densities of AT1 receptors occur in association with the conduction system and vagal ganglia. In the central nervous system, high AT1 receptor densities occur in many regions behind the blood-brain barrier, supporting a role for neurally derived angiotensin as a neuromodulator. The physiologic role of angiotensin in many of these brain sites remains to be determined. The AT2 receptor also has a characteristic distribution in several tissues including the adrenal gland, heart, and brain. The role of this receptor in physiology is being elucidated, but it appears to inhibit proliferation and to participate in development. Thus, receptor-binding studies, localizing the distribution of AT1 and AT2 receptors, provide many insights into novel physiologic roles of angiotensin.

Mapping tissue angiotensin-converting enzyme and angiotensin AT1, AT2 and AT4 receptors.

Background The renin–angiotensin system (RAS) functions as both a circulating endocrine system and a tissue paracrine/autocrine system. As a circulating peptide, angiotensin II (Ang II) plays a prominent role in blood-pressure control and body fluid and electrolyte balance by acting on the AT1 receptor in the brain and peripheral tissues. As a paracrine/autocrine peptide, locally formed Ang II also plays additional roles in tissues involving the regulation of regional haemodynamics, cell growth and remodelling, and neurotransmitter release. Evidence is emerging that Ang II is not the only active peptide of the RAS, and other Ang II fragments may also have important biological activities. Objectives To provide a morphological basis for understanding novel actions of angiotensin-converting enzyme (ACE), Ang II and related peptides in tissues, this article will review the localization of ACE and AT1, AT2 and AT4 receptors in the central nervous system, blood vessels and kidney. Results and conclusion Autoradiographic mapping of the major components of the RAS has proved a valuable strategy to reveal, or suggest, cellular sites of novel actions for Ang II and related peptides in tissues. First, colocalization of ACE and AT1 receptors in the substantia nigra, the caudate nucleus and putamen of human and rat brain, which contain the dopamine-synthesizing neurons, suggests that the central RAS may be important in modulating central dopamine release. Secondly, the distribution of AT4 receptors with a striking association with cholinergic neurons, motor and sensory nuclei in the brain reveals that Ang IV may modulate central motor and sensory activities and memory. Thirdly, the occurrence of high levels of ACE and AT1 and/or AT2 receptors in the adventitia of blood vessels suggests important paracrine roles of the vascular RAS. Finally, the identification of abundant AT1 receptor and elucidation of its roles in the renomedullary interstitial cells of the kidney may provide a new impetus to study further the role of Ang II in the regulation of renal medullary function and blood pressure. Overall, circulating and locally produced Ang II and related peptides may exert a remarkable range of actions in the brain, kidney and cardiovascular system through multiple angiotensin receptors.

Access of peripherally administered DuP 753 to rat brain angiotensin II receptors.

The in vivo access of the nonpeptide angiotensin II (Ang II) antagonist, DuP 753 (10 mg kg−1, i.v.), to Ang II receptors of rat brain was investigated by in vitro autoradiography with [125I]‐[Sar1, Ile8] Ang II as a ligand. DuP 753 markedly inhibited the binding to sites which contain exclusively AT1 receptors both outside and within the blood brain barrier, such as the circumventricular organs, paraventricular hypothalamic nucleus, median preoptic nucleus and nucleus of the solitary tract. However, binding to other nuclei containing AT2 receptors was not significantly inhibited. These results demonstrate that DuP 753 and/or its active metabolite readily cross the blood brain barrier in vivo and selectively inhibit binding to AT1 receptors in specific brain nuclei.

Haemodynamic and renal tubular effects of low doses of endothelin in anaesthetized rats.

1. Renal haemodynamic and tubular transport responses to low‐dose infusions (1 and 10 ng kg‐1 min‐1) of endothelin were investigated in anaesthetized rats. 2. Both doses caused transient increases in mean arterial blood pressure (17 +/‐ 5 mmHg, P < 0.05 at 1 ng kg‐1 min‐1) followed by sustained hypotension (‐14 +/‐ 5 mmHg, P < 0.05), reduced renal vascular resistance (‐42%, P < 0.05) and increased renal plasma flow (46%, P < 0.05). Glomerular filtration rate was unchanged. 3. Each dose caused profound diuresis and natriuresis. At 1 ng kg‐1 min‐1 urine flow rate and fractional water excretion increased 5‐fold and fractional sodium excretion 10‐fold. Fractional potassium excretion and solute‐free water clearance were unaltered. 4. End‐proximal fluid delivery estimated by lithium clearance doubled (P < 0.05) and fractional proximal and distal sodium reabsorption decreased 10‐20% (P < 0.05). Absolute proximal reabsorption also fell with the higher dose. 5. Hypotension and natriuresis persisted for 30 min after terminating infusions. Time‐control animals showed no changes in haemodynamics or renal tubular transport. 6. It is concluded that endothelin, at low concentrations, causes renal vasodilatation with concomitant natriuresis due to reduced sodium transport in proximal and distal nephron segments.

BLOCKADE BY INTRAVENOUS LOSARTAN OF AT1 ANGIOTENSIN II RECEPTORS IN RAT BRAIN, KIDNEY AND ADRENALS DEMONSTRATED BY IN VITRO AUTORADIOGRAPHY

1. The in vivo inhibition of angiotensin II (AII) receptor binding in the rat brain, kidney and adrenal was investigated after intravenous administration of the AT1‐selective AII receptor antagonist losartan.

Distribution of angiotensin II receptor subtypes in the rabbit brain

We have determined the distribution of angiotensin II receptor subtypes in rabbit brain using in vitro autoradiography. AT1 receptors were found in very high concentrations in the forebrain circumventricular organs--the subfornical organ, organum vasculosum of the lamina terminalis, and the median eminence as observed in other mammals. However, there was very little labeling in the area postrema. In the paraventricular nucleus, median preoptic nucleus, supraoptic nucleus there were high levels of predominantly AT1 receptors. High densities of AT1 receptors were also found in the nucleus of the solitary tract and the rostral and caudal ventrolateral medulla. All of these regions have putative roles in the regulation of blood pressure and fluid and electrolyte balance. In the rabbit brain there is less AT2 receptor binding than the rat, with most AT2 binding found in the molecular layer of the cerebellum and in the septohypothalamic nucleus. In the subthalamic nucleus, the mediodorsal and ventroposterior nuclei of the thalamus, locus coeruleus and inferior olivary nuclei, areas containing mostly AT2 receptors in the rat, no binding was detected in the rabbit except in the locus coeruleus which contains moderate levels of AT1 receptors. Taken in conjunction with our previous results in the rat and human brains, these results reveal that AT1 receptors predominate in rostral forebrain, hypothalamus and autonomic control centers of the medulla oblongata in all three species. However, the distribution and density of AT2 bearing sites in regions such as the septum, thalamus subthalamic nuclei, locus coeruleus, cerebellum and inferior olivary nuclei show marked species differences.

In vitro Autoradiography Reveals Predominantly AT1 Angiotensin II Receptors in Rat Kidney

Angiotensin II (Ang II) receptor subtypes in the rat kidney were investigated by using type 1 (AT1) and type 2 (AT2) Ang II receptor antagonists to discriminate specific 125I-[Sar1,Ile8] Ang II binding sites with in vitro autoradiography. DuP 753, a nonpeptide Ang II antagonist specific for the AT1 sites, potently displaced binding in glomeruli (Ki = 23.9 +/- 3.3 nM) and proximal tubules (Ki = 43.4 +/- 17 nM). By contrast, the AT2 antagonists, PD 123177 and CGP 42112A, were very weak in competing for specific 125I-[Sar1,Ile8] Ang II binding sites. AT1 receptors, as determined in the presence of an excess concentration (10 microM) of the AT2 antagonist, PD 123177, account for 95% of total renal Ang II receptors, whereas AT2 receptors, as determined in the presence of an excess concentration (10 microM) of the AT1 antagonist, DuP 753, represent approximately 5% of total renal Ang II receptors. In addition, the reducing agent, dithiothreitol, produces a dose-dependent inhibition of Ang II receptor binding with an IC50 of 2 mM, a characteristic of the AT1 receptors. These findings indicate that the AT1 receptor is the predominant subtype at multiple anatomical sites in the rat kidney.

Roles of AT1 and AT2 Receptors in the Hypertensive Ren-2 Gene Transgenic Rat Kidney

Adult Ren-2 gene transgenic rats, TGR(mRen-2)27, exhibit elevated circulating and kidney angiotensin II (Ang II) levels in the presence of severe hypertension. The aim of this study was to examine whether AT1 and AT2 receptors in the kidney and renal hemodynamic and tubular responses to blockade of these receptors were altered in the Ren-2 gene transgenic rats during the maintenance phase of hypertension. Renal AT1 and AT2 receptors were mapped by in vitro autoradiography (n=8), and the effects of blockade of these receptors on mean arterial pressure (MAP), heart rate (HR), and renal cortical (CBF) and medullary blood flows (MBF) were studied in anaesthetized, adult age-matched male homozygous TGR rats (n=12) and Sprague-Dawley (SD) rats (n=7). TGR rats showed higher basal MAP (P<0.001), heart and kidney weight (P<0.001), plasma renin activity (P<0.05) and plasma Ang II level (P<0.05), and CBF (P<0.05) and MBF (P<0.05) than SD rats. AT1 receptor binding was significantly increased in the glomeruli, proximal tubules, and the inner stripe of the outer medulla of TGR rats (P<0.01), while the AT2 receptor binding was low at all renal sites of TGR and SD rats. Immunohistochemistry revealed that this increased AT1 receptor labeling occurred mainly in vascular smooth muscle layer of intrarenal blood vessels including afferent and efferent arterioles, juxtaglomerular apparatus, glomerular mesangial cells, proximal tubular cells, and renomedullary interstitial cells (RMICs) in the transgenic rats. Blockade of AT1 receptors with losartan in TGR rats markedly reduced MAP to the normotensive level (P<0.001) without altering HR. Both CBF (P<0.005) and MBF (P<0.05) were significantly increased by losartan in the transgenic rats. By contrast, losartan only caused a smaller decrease in MAP and an increase in renal CBF in SD rats (P<0.05). PD 123319 was without any renal effect in both SD and TGR rats. These findings suggest that markedly increased AT1 receptors in renal vasculature, glomerular mesangial cells, and RMICs in the presence of fulminant hypertension and elevated circulating and tissue Ang II levels may play an important role in the maintenance of hypertension in the Ren-2 gene transgenic rats.

Angiotensin II Receptor Subtypes in Rat Brain and Peripheral Tissues

Angiotensin II (Ang II) receptor binding was localized in rat adrenal gland, kidney, and brain by in vitro autoradiography using the antagonist analogue 125I-[Sar1, Ile8]Ang II and differentiated into type I (AT-1) and type II (AT-2) subtypes using unlabelled non-peptide antagonists specific for Ang II subtypes. AT-1 binding was determined as that remaining in the presence of an excess of the AT-2 antagonist, PD 123177 (10 microM), and AT-2 binding as that remaining in the presence of an excess of the AT-1 antagonist, DUP753 (10 microM). The reducing agent dithiothreitol decreased the binding to AT-1 receptors and enhanced the binding to AT-2 receptors. The rat adrenal gland contains both AT-1 and AT-2 receptors in the ratio of approximately 3:2 in the cortex and 1:9 in the medulla. By contrast, in the kidney only AT-1 receptors were evident in glomeruli, proximal tubule, and inner stripe of the outer medulla. In the brain, the pattern of Ang II receptor subtypes varies greatly from region to region. Many brain structures known to be involved in blood pressure regulation and fluid and electrolyte balance, such as circumventricular organs (including vascular organ of the lamina terminalis, subfornical organ, median eminence, and area postrema), median preoptic nucleus, hypothalamic paraventricular nucleus, and regions in the medulla oblongata involved in autonomic control (nucleus of the solitary tract, dorsal motor nucleus of the vagus, and intermediate reticular nucleus), contain exclusively AT-1 receptors. By contrast, locus coeruleus, lateral septal nuclei, superior colliculus, subthalamic nucleus, many nuclei of the thalamus, and nuclei of the inferior olive contain predominantly AT-2 receptors. The detailed binding characteristics of each subtype were determined by competition studies with a series of antagonists. The pharmacological specificity obtained in kidney, adrenal cortex and adrenal medulla, superior colliculus, and nucleus of the solitary tract produces specificity patterns which confirm the assignments of AT-1 and AT-2 receptors described above. The present study reveals important pharmacological heterogeneity of Ang II receptors in key target organs. The subtype-specific receptor mapping described here is relevant to the understanding of the role of angiotensin peptides in peripheral organs and in the central nervous system and is relevant to the actions of non-peptide Ang II receptor antagonists.