Renal amylin binding in normotensive and hypertensive rats: effects of angiotensin converting enzyme inhibition with perindopril

The association between renal accumulation of pancreatic amyloid-forming amylin and renal hypoxia

Chronic kidney disease (CKD) is increasing worldwide and is associated with diabetic states (obesity, prediabetes and type-2 diabetes mellitus). The kidney is intrinsically susceptible to low oxygen (hypoxia) and renal hypoxia plays a vital role in the progression of CKD. Recent studies suggest an association between CKD and renal deposition of amyloid-forming amylin secreted from the pancreas. Renal accumulation of amyloid-forming amylin is associated with hypertension, mitochondrial dysfunction, increased production of reactive oxygen species (ROS) and activation of hypoxia signaling in the kidney. In this review we will discuss potential associations between renal amylin amyloid accumulation, hypertension, and mechanism of hypoxia-induced kidney dysfunction, including activation of hypoxia-inducible factors (HIFs) and mitochondrial dysfunction.

Renal Effects

Amylin bound to kidney cortex in a distinctive pattern. Binding appeared specific in that it was displaceable with amylin antagonists. It was associated with activation of cyclic AMP (cAMP), and was thereby likely to represent receptor binding and activation. Amylin's principal effects at the kidney included a stimulation of plasma renin activity, reflected in aldosterone increases at quasi-physiological amylin concentrations. It was unclear whether this was a local or a systemic effect. Other renal effects in rats included a diuretic effect and a natriuretic effect. The latter was mainly driven by the diuresis, since urinary sodium concentration did not change. Amylin had a transient effect to lower plasma potassium concentration. This effect was likely to be a consequence of activation of Na+/K+-ATPase, an action shared with insulin and catecholamines. Amylin lowered plasma calcium, particularly ionized calcium, likely due to an antiresorptive effect at osteoclasts. Immunoreactive amylin was detected in the developing kidney. It appeared to have a trophic effect in kidney, and its absence resulted in renal dysgenesis. Neurons in the subfornical organ (SFO), which has a role in fluid/electrolyte homeostasis, were potently activated by amylin. The dipsogenic and renal effects of amylin may be related to effects at the SFO.

Receptor pharmacology

Despite clear evidence for a distinct amylin pharmacology and localization of such pharmacology to sites such as the nucleus accumbens,efforts to clone an amylin receptor were fruitless for over a decade. This enigma led many to doubt the status of amylin as a bona fide hormone. Yet it became apparent during those cloning efforts that, whatever the amylin receptor was, it was somehow similar to a calcitonin receptor. The enigma of the amylin receptor was solved following the identification of receptor activity modifying proteins (RAMPs). These single transmembrane spanning molecules, when associated with a calcitonin receptor, altered its pharmacology from calcitonin-preferring to amylin-preferring. With at least two forms of the calcitonin receptor and three forms of RAMP, there is the potential for six subtypes of amylin receptors. Of these, two appear to predominate. The CTa (shorter form) calcitonin receptor, dimerized with RAMP1 [amylin 1 (a) receptor], appears to represent binding sites at the nucleus accumbens and the subfornical organ. Binding sites at area postrema appear to be composed of CTa + RAMP3 [amylin3 (a) receptors]. Thus far, RAMP proteins have been associated in vivo only with the CT/CLR receptor system. It is presently unknown whether RAMPs are more general modulators of receptor function, dynamically modifying responsivity with time or across other receptor classes. The largest and first identified amylin-binding field was in the nucleus accumbens. The function of these receptors is yet undetermined, but because the nucleus accumbens is within the blood-brain barrier, the cognate ligand is unlikely to be circulating amylin. Dense amylin binding is present at the circumventricular organs, including the subfornical organ, the organum vasculosum lateralis terminalis (OVLT), and the area postrema. There is no diffusional (blood-brain) barrier at these structures, so they most likely respond to circulating (beta-cell-derived) amylin. Despite pharmacological evidence of amylin sensitivity in several peripheral tissues, selective amylin binding outside of the brain is observed only in the renal cortex. The newly designated amylinomimetic drug class was defined on the basis of its unique pharmacology prior to the molecular characterization of amylin receptors. Currently, the class includes any agent that acts as antagonist at characterized amylin receptors. Several peptides, typically analogs of truncated salmon calcitonin, have been developed as potent and selective amylin antagonists and have been useful in identifying amylinergic responses. Of these, AC187 (30Asn32Tyr[8-32]sCT; Amylin Pharmaceuticals Inc.) is particularly selective and potent, and has been most often cited in studies using amylin antagonists. Antagonism of a response with an order of potency of AC187> AC66 > CGRP[8-37] is suggestive that it is mediated via amylin receptors. Activation of a response with salmon calcitonin (sCT) > amylin >calcitonin gene-related peptide (CGRP) > mammalian CT suggests activation via the amylinl (a) receptor, while sCT = amylin >> CGRP >mammalian CT suggests activation via amylin3 (a) receptors. Absence of response to other ligands (e.g., adrenomedullin) is useful for excluding certain pharmacologies.

Interaction of the renal amylin and renin-angiotensin systems in animal models of diabetes and hypertension

The range of known actions of amylin are reviewed together with the proposal that an important role for amylin may be the hormonal integration of diverse physiological systems activated with feeding. Major targets for the action of amylin are found within the kidney. Components of the amylin system (AS) have been shown to influence the activity of components of the renin-angiotensin system (RAS), and vice versa, in normal, hypertensive and diabetic models. For instance, amylin injected into humans and rats elicits a rapid rise in plasma renin activity. Furthermore, in two models of hypertension (the spontaneously hypertensive rat (SHR) and the model with subtotal nephrectomy (STNx)), the density of amylin-binding sites in the renal cortex associated with the proximal tubules, was associated with elevation of blood pressure. In normotensive controls and in the STNx model, but not in the SHR model, treatment with angiotensin-converting enzyme (ACE) inhibitors reduced blood pressure and the density of amylin binding in the renal cortex. In Sprague-Dawley rats, angiotensin II (Ang II) infusion was associated with increased density of amylin-binding sites as well as elevated blood pressure. Thus, there appears to be a direct relationship between the activity of Ang II and the binding sites for amylin in the renal cortex. From these studies it has been postulated that the activation of the AS in the kidney may play a role in the genesis and/or development of hypertension in certain contexts. The transient expression of amylin mRNA has been detected perinatally, using in situ hybridization, in the subnephrogenic zone of the metanephros and is associated with proximal tubules of the developing nephron. These cells situated close to the glomeruli, represent a subset of brush border epithelial cells. Amylin immunoreactivity (IR) is also found in these cells and colocalizes with angiotensinogen IR. Thus a second important role for amylin is described in which it plays a role as a growth factor in the developing kidney and in renal regrowth in the adult kidney. In a model of IDDM (streptozotocin diabetes), amylin and angiotensinogen IR are both restricted to a subset of brush border epithelial cells close to glomeruli which, in the developing kidney, expressed amylin mRNA. Thus in this IDDM model, we hypothesize that amylin mRNA transcription which is normally downregulated in the adult, is upregulated in this subset of these brush border epithelial cells, and that it stimulates the activity of a local RAS by an intracellular mechanism, leading to the biosynthesis of Ang II. It remains to be determined that if amylin is playing a role in stimulating local Ang II production at these sites, this provides a mechanism for activation of TGF-beta, ultimately leading to interstitial fibrosis.

Amylin: physiological roles in the kidney and a hypothesis for its role in hypertension

1. There are high-affinity binding sites for amylin in the renal cortex associated with proximal tubules. These appear to represent seven transmembrane (heptatopic) receptors that are known to form ternary complexes with G-proteins and activate second messenger systems. 2. Amylin stimulates sodium/water reabsorption from the basolateral side of the proximal tubules and plays a role in sodium homeostasis. 3. The transient expression of amylin-like mRNA has been detected perinatally, using in situ hybridization, in the subnephrogenic zone of the metanephros and is associated with proximal tubules of the developing nephron. There it is thought to play a role as a growth factor for brush border epithelial cells in the developing kidney and in renal regrowth in the adult kidney. 4. In two models of hypertension, the spontaneously hypertensive rat (SHR) and one created surgically by subtotal nephrectomy, renal amylin receptors are activated. In the SHR, activation precedes the rise in blood pressure and suggests that activation of the amylin system may be an important event in the development of hypertension.