Interaction of amylin with calcitonin gene-related peptide receptors in the microvasculature of the hamster cheek pouch in vivo

Amylin peptide levels are raised in infants of diabetic mothers

BACKGROUND

Amylin is a novel 37 amino acid peptide hormone that is co-secreted with insulin from the pancreas in response to food intake. As a potent inhibitor of gastric emptying it plays an important role in the control of carbohydrate absorption. Feed intolerance is common in infants of diabetic mothers (IDM).

AIMS

To establish a normal range of amylin levels in healthy neonates, and to determine whether serum amylin levels are raised in IDM.

METHODS

A serial sample of 221 infants > or =28 weeks gestation was enrolled prior to delivery over a 12 month period. Blood samples collected immediately after birth (umbilical cord), and at the routine Guthrie test were analysed for amylin and insulin levels.

RESULTS

Amylin levels in umbilical cord (n = 181) and Guthrie samples (n = 33) of healthy infants were 5.7 (3.0-9.1) and 6.9 (2.9-9.0) pmol/l respectively. IDM had significantly raised amylin levels in both cord (n = 31; 32.7 pmol/l, 25.9-48.1) and Guthrie samples (n = 8; 18.1 pmol/l, 15.3-23.6). Amylin correlated positively with insulin (n = 42; r = 0.67; 95% CI 0.4 to 0.81), birth weight (r = 0.22; 95% CI 0.08 to 0.36), and gestation (r = 0.18; 95% CI 0.03 to 0.32). Umbilical cord venous amylin levels showed agreement with arterial cord amylin levels (n = 34, mean bias -0.2, 95% CI 3.1 to -3.6).

CONCLUSIONS

Amylin levels are significantly increased in the umbilical cord and Guthrie blood samples in IDM.

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.

Vascular Actions of Calcitonin Gene-Related Peptide and Adrenomedullin

This review summarizes the receptor-mediated vascular activities of calcitonin gene-related peptide (CGRP) and the structurally related peptide adrenomedullin (AM). CGRP is a 37-amino acid neuropeptide, primarily released from sensory nerves, whilst AM is produced by stimulated vascular cells, and amylin is secreted from the pancreas. They share vasodilator activity, albeit to varying extents depending on species and tissue. In particular, CGRP has potent activity in the cerebral circulation, which is possibly relevant to the pathology of migraine, whilst vascular sources of AM contribute to dysfunction in cardiovascular disease. Both peptides exhibit potent activity in microvascular beds. All three peptides can act on a family of CGRP receptors that consist of calcitonin receptor-like receptor (CL) linked to one of three receptor activity-modifying proteins (RAMPs) that are essential for functional activity. The association of CL with RAMP1 produces a CGRP receptor, with RAMP2 an AM receptor and with RAMP3 a CGRP/AM receptor. Evidence for the selective activity of the first nonpeptide CGRP antagonist BIBN4096BS for the CGRP receptor is presented. The cardiovascular activity of these peptides in a range of species and in human clinical conditions is detailed, and potential therapeutic applications based on use of antagonists and gene targeting of agonists are discussed.

Receptor autoradiography as mean to explore the possible functional relevance of neuropeptides: focus on new agonists and antagonists to study natriuretic peptides, neuropeptide Y and calcitonin gene-related peptides

Over the past 20 years, receptor autoradiography has proven most useful to provide clues as to the role of various families of peptides expressed in the brain. Early on, we used this method to investigate the possible roles of various brain peptides. Natriuretic peptide (NP), neuropeptide Y (NPY) and calcitonin (CT) peptide families are widely distributed in the peripheral and central nervous system and induced multiple biological effects by activating plasma membrane receptor proteins. The NP family includes atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP). The NPY family is composed of at least three peptides NPY, peptide YY (PYY) and the pancreatic polypeptides (PPs). The CT family includes CT, calcitonin gene-related peptide (CGRP), amylin (AMY), adrenomedullin (AM) and two newly isolated peptides, intermedin and calcitonin receptor-stimulating peptide (CRSP). Using quantitative receptor autoradiography as well as selective agonists and antagonists for each peptide family, in vivo and in vitro assays revealed complex pharmacological responses and radioligand binding profile. The existence of heterogeneous populations of NP, NPY and CT/CGRP receptors has been confirmed by cloning. Three NP receptors have been cloned. One is a single-transmembrane clearance receptor (NPR-C) while the other two known as CG-A (or NPR-A) and CG-B (or NPR-B) are coupled to guanylate cyclase. Five NPY receptors have been cloned designated as Y(1), Y(2), Y(4), Y(5) and y(6). All NPY receptors belong to the seven-transmembrane G-protein coupled receptors family (GPCRs; subfamily type I). CGRP, AMY and AM receptors are complexes which include a GPCR (the CT receptor or CTR and calcitonin receptor-like receptor or CRLR) and a single-transmembrane domain protein known as receptor-activity-modifying-proteins (RAMPs) as well as an intracellular protein named receptor-component-protein (RCP). We review here tools that are currently available in order to target each NP, NPY and CT/CGRP receptor subtype and establish their respective pathophysiological relevance.

Characterization of adenosine receptors mediating the vasodilator effects of adenosine receptor agonists in the microvasculature of the hamster cheek pouch in vivo

1 The aim of this study was to characterize the adenosine receptor mediating vasodilation in the microvasculature of the hamster cheek pouch in vivo. A range of adenosine agonists was used including N6-cyclopentyladenosine (CPA) (A1 agonist), 5'-N-ethylcarboxamidoadenosine (NECA) (non-selective), 2-chloroadenosine (2CADO) (non-selective), 2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680) (A2A agonist), N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (IBMECA) (A3 agonist) and adenosine, as well as the adenosine antagonists 8-sulphophenyltheophylline (8-SPT) (A1/A2 antagonist), 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) (A1 antagonist) and 4-(2-[7-amino-2-(2-furyl)[1,2,4]-triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385) (A2A antagonist). 2 All the adenosine analogues used induced vasodilation at concentrations between 10 nm and 1 microm, and the potency order was NECA > CGS 21680 > 2CADO > CPA=IBMECA >> adenosine, indicating an action at A2A receptors. 8-SPT (50 microm) antagonized vasodilator responses to NECA with an apparent pKB of 5.4, consistent with an action at A1 or A2 receptors and confirming that A3 receptors are not involved in this response. 3 DPCPX (10 nm) had no effect on vasodilation evoked by NECA, suggesting that this response was not mediated via A1 receptors, while ZM 241385 (10 nm) antagonized dilator responses to NECA with an apparent pKB of 8.9 consistent with an action via A2A receptors. 4 Overall these results suggest that adenosine A2A receptors mediate vasodilation in the hamster cheek pouch in vivo.

Production of cAMP by adrenomedullin in human oligodendroglial cell line KG1C: comparison with calcitonin gene-related peptide and amylin

The actions and the presence of adrenomedullin (AM) were investigated in cultured human oligodendroglial cell line KG1C. AM and AM mRNA were detected in KG1C cells by immunohistochemistry and RT-PCR. mRNAs for calcitonin receptor-like receptor (CRLR) and receptor-activity-modifying proteins (RAMPs) 1, 2 and 3 but not for calcitonin receptors were detected in the cells, while mRNAs for CRLR, calcitonin receptors and all RAMPs were detected in the human cerebellum. Application of AM resulted in time- and concentration-dependent increases in the cAMP level of KG1C cells. Calcitonin gene-related peptide (CGRP) and amylin, peptides structurally related to AM, also increased cAMP. The potencies for the cAMP production of the three peptides were CGRP > or =AM >> amylin with EC(50) of 8, 18, 90 nM, respectively. The responses induced by AM were strongly inhibited by the CGRP(1) receptor antagonist human CGRP(8-37), and inhibited also by the AM receptor antagonist human AM(22-52). In contrast, the responses induced by CGRP or amylin were inhibited only by CGRP(8-37) and not by AM(22-52). The responses induced by all three peptides were unaffected by the amylin receptor antagonist human amylin(8-37). The CGRP(2) receptor agonist human [Cys(Acm)(2,7)]CGRP significantly increased the cAMP level but the increase was smaller than that caused by CGRP. This increase in cAMP was unaffected by CGRP(8-37), AM(22-52) or by amylin(8-37). These results suggest that in KG1C cells, AM increases cAMP through AM and CGRP(1) receptors, whereas CGRP does so through CGRP(1) and CGRP(2) receptors, and amylin exerts its effects through CGRP(1) receptors. Collectively, these findings imply that AM released from oligodendroglial cells may play a role in the regulation of oligodendrocytes via autocrine/paracrine through AM receptors and CGRP(1) receptors.

Vasorelaxant effect of glucagon-like peptide-(7-36)amide and amylin on the pulmonary circulation of the rat

The gastrointestinal peptides glucagon-like peptide-1(7-36)amide (GLP-1) and amylin are currently being tested in clinical trials for the treatment of diabetes mellitus due to their effects in lowering blood glucose. Receptors for these polypeptides also exist in the lung and since polypeptides are known to modulate airway and pulmonary vascular tone, we investigated whether GLP-1 and amylin act similarly in the lung. We compared their effects with the well-known actions of calcitonin gene-related peptide (CGRP) and vasoactive intestinal peptide (VIP). Both GLP-1 and amylin induced a dose-dependent and time-reversible endothelial-dependent relaxation of preconstricted pulmonary artery rings. Amylin was approximately as strong as VIP and CGRP, GLP-1 however, was 2.3-fold less potent. GLP-1 as well as amylin also reduced the vascular tone in the isolated, perfused and ventilated rat lung. In contrast to their action on the pulmonary vasculature, neither GLP-1 nor amylin showed any effect on the tone of isolated preconstricted trachea rings. In conclusion, GLP-1 and amylin represent two additional peptides which may modulate pulmonary vascular tone.

Characterization of CGRP(1) receptors in the guinea pig basilar artery

The purpose of the present study was to characterise receptors mediating calcitonin gene-related peptide (CGRP)-induced relaxation of guinea pig basilar artery. This was done by investigating vasomotor responses in vitro and performing autoradiographic binding studies. We also intended to study the importance of an intact endothelium. Agonist studies showed that peptides of the CGRP family induced relaxation of the guinea pig basilar artery with the following order of potency: human beta-CGRP=human alpha-CGRP>>adrenomedullin=[acetamidomethyl-Cys(2,7)]alpha-human CGRP ([Cys(ACM)(2,7)]CGRP)=amylin. These data are in concord with those of the autoradiographic binding studies that showed displacement of [125I] human alpha-CGRP binding with the following order of potency: human alpha-CGRP=human beta-CGRP>>adrenomedullin=human alpha-CGRP-(8-37)>>Cys(ACM)(2,7)]CGRP. In blockade experiments, the relaxant responses to human alpha- and human beta-CGRP were competitively blocked by the CGRP(1) receptor antagonist human alpha-CGRP-(8-37), while those of adrenomedullin and amylin were blocked non-competitively. In order to examine whether amylin induced relaxation via amylin or CGRP receptors, we studied the antagonistic effect of amylin-(8-37) on the weak relaxant response to amylin and found that it was not blocked by amylin-(8-37). These findings, together with the finding that the CGRP(2) receptor agonist [Cys(ACM)(2,7)]CGRP only induced a weak relaxation in the highest concentrations examined, suggest that the CGRP family of peptides mediate relaxation by CGRP(1)-type receptors. Removal of the endothelium, the addition of N(G)-nitro-L-arginine methyl ester (L-NAME), methylene blue or indomethacin did not affect the concentration-response curves of the CGRP analogues, neither in the presence nor in the absence of human CGRP-(8-37). The study shows the presence of a relaxant CGRP(1) receptor on the smooth muscle cells of guinea pig basilar artery. Various endothelial factors did not influence relaxant responses.

The molecular pharmacology of CGRP and related peptide receptor subtypes

Calcitonin gene-related peptides (alpha and beta isoforms), better known as CGRPalpha and CGRPbeta, were isolated twenty years ago. In fact, these were the first peptides to be characterized using a molecular cloning strategy, which is not the traditional approach of biochemical extraction and purification. Paradoxically, progress in the characterization of CGRP receptor subtypes has been extremely slow as a result of difficulties in their cloning and the lack of selective receptor subtype agonists and antagonists. However, exciting progress has been made overthe pasttwo years and is briefly reviewed here.

Association between kinin B(1) receptor expression and leukocyte trafficking across mouse mesenteric postcapillary venules

Using intravital microscopy, we examined the role played by B(1) receptors in leukocyte trafficking across mouse mesenteric postcapillary venules in vivo. B(1) receptor blockade attenuated interleukin (IL)-1beta-induced (5 ng intraperitoneally, 2 h) leukocyte-endothelial cell interactions and leukocyte emigration ( approximately 50% reduction). The B(1) receptor agonist des-Arg(9)bradykinin (DABK), although inactive in saline- or IL-8-treated mice, caused marked neutrophil rolling, adhesion, and emigration 24 h after challenge with IL-1beta (when the cellular response to IL-1beta had subsided). Reverse transcriptase polymerase chain reaction and Western blot revealed a temporal association between the DABK-induced response and upregulation of mesenteric B(1) receptor mRNA and de novo protein expression after IL-1beta treatment. DABK-induced leukocyte trafficking was antagonized by the B(1) receptor antagonist des-arg(10)HOE 140 but not by the B(2) receptor antagonist HOE 140. Similarly, DABK effects were maintained in B(2) receptor knockout mice. The DABK-induced responses involved the release of neuropeptides from C fibers, as capsaicin treatment inhibited the responses. Treatment with the neurokinin (NK)(1) and NK(3) receptor antagonists attenuated the responses, whereas NK(2), calcitonin gene-related peptide, or platelet-activating factor receptor antagonists had no effect. Substance P caused leukocyte recruitment that, similar to DABK, was inhibited by NK(1) and NK(3) receptor blockade. Mast cell depletion using compound 48/80 reduced DABK-induced leukocyte trafficking, and DABK treatment was shown histologically to induce mast cell degranulation. DABK-induced trafficking was inhibited by histamine H(1) receptor blockade. Our findings provide clear evidence that B(1) receptors play an important role in the mediation of leukocyte-endothelial cell interactions in postcapillary venules, leading to leukocyte recruitment during an inflammatory response. This involves activation of C fibers and mast cells, release of substance P and histamine, and stimulation of NK(1), NK(3), and H(1) receptors.

Equipotent in vitro actions of alpha- and beta-CGRP on guinea pig basilar artery are likely to be mediated via CRLR derived CGRP receptors

The aim of the present study was to investigate and compare specific in vitro pharmacological actions of human alpha- and beta-CGRP applied as single concentrations to prostaglandin F2alpha precontracted segments of guinea pig basilar artery. To support the suggestion of a possible link between the pharmacological actions of alpha- and beta-CGRP and a specific receptor, we wished to determine whether mRNAs required for the expression of calcitonin receptor-like receptor (CRLR) derived CGRP receptors were present in the guinea pig basilar artery. In the pharmacological experiments we demonstrated an increase in the cAMP content by 2.5-fold and a concomitant significant vasorelaxation of the precontracted basilar artery segments following 1 min of stimulation by 10(-7) M alpha- or beta-CGRP. In another set of experiments, the time course of alpha- and beta-CGRP induced vasodilatation was investigated and concentration dependent responses of the two peptides were demonstrated. No significant differences between the actions of alpha- and beta-CGRP regarding induction of cAMP formation, amount of vasodilatation, time course of vasodilatation and mode of inhibition by the CGRP receptor antagonist, human alpha-CGRP(8-37), could be detected. The presence of mRNA encoding the guinea pig CRLR and the guinea pig CGRP receptor component protein (RCP) in the guinea pig basilar artery was demonstrated by RT-PCR methods. Furthermore, a partial sequence of mRNA encoding the guinea pig CRLR was determined. The expression in this tissue of a CRLR derived CGRP receptor and a functional RCP is therefore likely, and the equipotent pharmacological actions of alpha- and beta-CGRP might be mediated via CRLR derived CGRP receptors.

Amylin evokes phosphorylation of P20 in rat skeletal muscle

To investigate the signal transduction events underlying amylin's actions, the amylin-evoked protein phosphorylation cascade was analysed using two-dimensional gel electrophoresis. We found that phosphorylation of three isoelectric variants of P20 (termed ARPP1, ARPP2 and ARPP3) was associated with amylin's actions in rat skeletal muscle. Amylin decreased phosphorylation of ARPP1 and increased phosphorylation of ARPP2 and ARPP3 in a dose-dependent manner. Insulin inhibited amylin-evoked phosphorylation of ARPP2 and ARPP3. The amylin-selective antagonist rat amylin-(8-37) completely reversed amylin's action on ARPP3 and partially decreased phosphorylation of ARPP2. By contrast, the CGRP-selective antagonist, human CGRP-(8-37) blocked phosphorylation of ARPP2 but had little effect on ARPP3. These results suggest that amylin modifies phosphorylation of P20 via two independent mechanisms, and that P20 might be a molecule mediating amylin's biological functions.