Amylin and Syndrome-X

Hypertension and Type 2 Diabetes-The Novel Treatment Possibilities

Elevated blood pressure and hyperglycaemia frequently coexist and are both components of metabolic syndrome. Enhanced cardiovascular risk is strongly associated with diabetes and the occurrence of hypertension. Both hypertension and type 2 diabetes, if treated inappropriately, lead to serious complications, increasing the mortality of patients and generating much higher costs of health systems. This is why it is of great importance to find the missing link between hypertension and diabetes development and to simultaneously search for drugs influencing these two disorders or even drugs aimed at their pathological bases. Standard antihypertensive therapy mainly focuses on blood pressure reduction, while novel drugs also possess a wide range of pleiotropic modes of actions, such as cardio- and nephroprotective properties or body weight reduction. These properties are especially desirable in a situation when type 2 diabetes coexists with hypertension. This review describes the connections between diabetes and hypertension development and briefly summarises the current knowledge regarding attempts to define targets for the treatment of high blood pressure in diabetic patients. It also describes the standard hypotensive drugs preferred in patients with type 2 diabetes, as well as novel drugs, such as finerenone, esaxerenone, sodium-glucose co-transporter-2 inhibitors, glucagon-like peptide-1 analogues and sacubitril/valsartan.

Amylin and the integrated control of nutrient influx

The most potent actions of amylin that occur at physiological plasma concentrations include inhibition of food intake, gastric emptying, acid and digestive enzyme secretion, and glucagon secretion. These actions share a common outcome; they each help regulate the rate at which nutrients (including glucose) appear in the blood (Ra). Amylin physiologically orchestrates, via several parallel processes, the rate of entry of nutrient into the circulation, as shown schematically in Fig. 1. In this way, amylin's function may be viewed as complementary to that of insulin (secreted from the same pancreatic beta-cells), which orchestrates the exit of nutrient from blood and its storage in peripheral tissues. The following discussion addresses the emerging picture that, although amylin is co-secreted with an endocrine hormone from endocrine tissue (the pancreatic islets), the target for its most potent and physiologically relevant effects appears to be the central nervous system. Amylin thus may be primarily regarded as a neuroendocrine hormone (Young et al., 2000).

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.

Calcitonin and calcitonin receptors: bone and beyond

Calcitonin (CT), a 32 amino acid peptide hormone produced primarily by the thyroid, and its receptor (CTR) are well known for their ability to regulate osteoclast mediated bone resorption and enhance Ca2+ excretion by the kidney. However, recent studies now suggest that CT and CTRs may play an important role in a variety of processes as wide ranging as embryonic/foetal development and sperm function/physiology. In this review article, CT and CTR gene transcription, signal transduction and function are addressed. The effects of CT on the physiology of a variety of organ systems are discussed and the relationship between polymorphisms in the CTR gene and bone mineral density (BMD)/osteoporosis is examined. Recent studies demonstrating the ability of receptor activity modifying proteins (RAMPs) to post-translationally modify the calcitonin receptor-like receptor (CRLR) are detailed and studies employing transgenic mouse technology to determine the temporal and tissue specific transcriptional activity of the CTR gene in vivo are discussed.

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.

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

OBJECTIVES

To investigate the effect of angiotensin converting enzyme inhibition with perindopril on the binding density of [125I]-rat amylin in the renal cortex in normotensive Sprague-Dawley rats, renally ablated hypertensive rats and spontaneously hypertensive rats.

DESIGN

Sprague-Dawley rats, renally ablated hypertensive rats and spontaneously hypertensive rats were administered either the angiotensin converting enzyme inhibitor perindopril or no treatment.

METHODS

The density of [125I]-rat amylin binding was measured in the renal cortex using autoradiography in vitro. The systolic blood pressure was measured by indirect tail-cuff plethysmography. The plasma renin activity was measured by radioimmunoassay.

RESULTS

The density of [125I]-amylin binding was reduced by approximately 50% in Sprague-Dawley and subtotally nephrectomized Sprague-Dawley rats after treatment with perindopril. These changes were associated with a reduction in systolic blood pressure and an increase in plasma renin activity. In contrast, amylin binding in the perindopril-treated spontaneously hypertensive rats was not reduced, despite the prevention of a rise in systolic blood pressure and an increase in plasma renin activity.

CONCLUSIONS

These findings provide further evidence for the hypothesis that there is an association among renal amylin binding, the renin-angiotensin system and blood pressure for rats of the Sprague-Dawley strain. In contrast, the lack of an effect of angiotensin converting enzyme inhibition on renal amylin binding for rats of the spontaneously hypertensive rat strain is consistent with previous findings that the changes in amylin binding in rats of this strain are not linked directly to the prevailing systemic blood pressure but may be associated with a developmental abnormality in the kidney of these rats.

Increased density of renal amylin binding sites in experimental hypertension

High-affinity binding sites for the pancreatic beta-cell hormone amylin have been reported in the kidney, and it has been postulated that these sites may be involved in the genesis of hypertension. In the present study, we have used in vivo injection of 125I-amylin and in vitro autoradiographic techniques to assess renal amylin binding in both a genetic and a surgically induced model of hypertension. In the spontaneously hypertensive rat (SHR) at 6 weeks of age, before the rise in systolic blood pressure, there was a 36% increase in density of amylin binding compared with their normotensive counterpart, the Wistar-Kyoto rat (WKY). In SHR, there was a further increase in the density of amylin binding (to 53% greater) as the systolic blood pressure rose between 6 and 12 weeks of age. Histological examination of kidneys from SHR at 12 weeks of age revealed staining for a brush border glycoprotein, normally restricted to the proximal tubules, extending from the urinary pole into half of the epithelial lining of the glomerular capsule. In contrast to WKY, these cells also bound 125I-amylin with high density in SHR. In a rat model of renal ablation and hypertension, systolic blood pressure correlated with the density of 125I-amylin binding in the renal cortex (r=.54, P=.003, n=28). The changes in amylin binding reported here suggest a possible role for this peptide and/or activation of its receptor in the genesis as well as the maintenance of hypertension.

Insulin receptor gene in hypertension

The first molecular genetic association with human essential hypertension (HT) involved the insulin receptor gene (INSR). This highly significant result in Caucasians was for an insertion/deletion polymorphism in intron 9. A polymorphism in exon 8 showed a weak association, but a microsatellite in intron 2 proved negative for HT, although has shown an association with plasma insulin in Japanese. A similar spectrum of genetic associations for variants spanning INSR has been noted for insulin-dependent diabetic patients with rapidly-progressing renal disease, a subgroup having a strong family history of essential HT. Association with HT has also been found for an INSR variant in CHinese. Insulin resistance secondary to an INSR 'defect', or other causes, would increase insulin, which has cardiovascular effects, and insulin can raise angiotensinogen. Also, insulin is co-secreted with amylin, which can increase renin secretion. In the spontaneously HT rat there is evidence for reduced down-regulation of INSR expression in response to NaCl-loading, consistent with a promoter effect. When combined with observations of insulin resistance in essential HT patients and their pre-HT offspring, the possibility of dys-regulation of INSR merits attention in disease etiology in a proportion of essential HT patients.

Role of adrenomedullin and related peptides in the regulation of the hypothalamo-pituitary-adrenal axis

Adrenomedullin (ADM) is a hypotensive peptide, originally isolated from human pheochromocytomas, and then found to be widely distributed in the various body systems. ADM derives from preproadrenomedullin, a 185-amino acid residue prohormone, containing at its N-terminal a 20-amino acid sequence, named proadrenomedullin N-terminal 20 peptide (PAMP). ADM and PAMP immunoreactivities have been detected in the hypothalamo-pituitary-adrenal (HPA) axis of humans, rats, and pigs. Adrenal glands possess binding sites for both ADM and PAMP, the former being mainly of the subtype 1 of calcitonin gene-related peptide (CGRP) receptors. ADM exerts a direct inhibitory action on angiotensin II- or potassium-stimulated aldosterone secretion of zona glomerulosa cells. This effect is mediated by the CGRP1 receptor and its mechanism probably involves the blockade of Ca2+ influx. In contrast, ADM enhances aldosterone production by in situ perfused rat adrenals and human adrenal slices (containing medullary chromaffin cells), again through the activation of CGRP1 receptors. This aldosterone secretagogue effect of ADM is blocked by the beta-adrenoceptor antagonist l-alprenolol, thereby suggesting that it is indirectly mediated by the release of catecholamines by chromaffin cells. The effects of ADM on adrenal glucocorticoid release are doubtful and probably mediated by the increase in adrenal blood flow rate and the inhibition of ACTH release by pituitary corticotropes. The concentrations reached by ADM and PAMP in the blood rule out the possibility that they act on the HPA axis as circulating hormones. Conversely, their content in both adrenal and hypothalamo-pituitary complex is consistent with a paracrine mechanism of action, which may play a potentially important role in the regulation of fluid and electrolyte homeostasis.

Effect of race and hypertension on plasma amylin concentrations

Amylin is a recently discovered peptide hormone composed of 37 amino acids that is cosecreted with insulin by pancreatic beta cells. Amylin has been reported to be present in increased amounts in insulin-resistant subjects who are hyper-insulinemic. Because blacks and whites differ in the prevalence of both hypertension and diabetes, we examined amylin levels in 77 individuals; 42 were black (11 hypertensive and 31 normotensive) and 35 were white (10 hypertensive and 25 normotensive) individuals who were either healthy control subjects or hypertensive subjects not receiving antihypertensive medication. Plasma amylin concentrations were measured in two separate monoclonal antibody-based immunofluorescent sandwich-type assays. The F002-2 capture antibody binds amylin plus at least two additional amylin-like peptides, and the F024-4 capture antibody detectably binds only the amylin peptide. There was a significant race-by-diagnosis interaction for levels of amylin immunoreactivity during a 2-hour glucose tolerance test (P<.005 for F002-2 antibody and P<.05 for F024-4 antibody). Highest levels were found in black hypertensive subjects. The results appear to fit with previously observed differences in metabolic status between blacks and whites and with the association between hypertension and alterations in metabolic status.

Amylin mobilizes [Ca2+]i and stimulates the release of pancreatic digestive enzymes from rat acinar AR42J cells: evidence for an exclusive receptor system of amylin

Amylin dose-dependently stimulated the secretion of amylase and cholesterol esterase from rat pancreatic acinar AR42J cells. The biochemical basis of this action was investigated using fura-2-loaded AR42J cells. Amylin increased intracellular free calcium. [Ca2+]i, in a dose-dependent manner. The Ca2+ signal persisted even in Ca(2+)-free medium, suggesting mobilization from intracellular stores rather than influx. Consistently, thapsigargin abolished amylin-induced responses, suggesting that Ca2+ is released from an inositol 1,4,5-triphosphate (IP3)-sensitive pool. This was confirmed by the finding that amylin elevated IP3 levels. AR42J cells pretreated with amylin did not respond to amylin, suggesting that the receptors mediating this response undergo homologous desensitization. However, pretreatment with related peptides, calcitonin gene-related peptide (CGRP) and salmon calcitonin, did not diminish [Ca2+]i mobilization by amylin. CGRP and calcitonin also failed to mobilize [Ca2+]i even at 10 microM. These results suggests that the stimulatory effects of amylin on pancreatic digestive enzyme secretion from AR42J cells are mediated by a G-protein-linked membrane receptor coupled to IP3-dependent calcium pools.

Amylin stimulates plasma renin concentration in humans

Although insulin resistance and hypertension are commonly associated, the underlying cause for this association remains unknown. Plasma concentrations of the recently described hormone amylin, which is cosecreted with insulin by the pancreatic beta cell, are reported to be elevated in various states of insulin resistance, including hypertension and obesity. Preliminary studies by our group have suggested that there are amylin binding sites in the kidney. In nine healthy humans an infusion of human amylin that resulted in steady state plasma amylin levels in the subnanomolar range led to significant increases in plasma renin and aldosterone concentrations. These changes occurred in the absence of significant changes in plasma electrolytes, catecholamines, vasopressin, total renin, or osmolality. Diastolic pressure at 30 minutes and plasma glucose at 60 minutes rose modestly. Since amylin has both metabolic and renal actions, this peptide may be an important link between hypertension, insulin resistance, and the renin-angiotensin system.