Role of prostaglandins and renal nerves in the renal actions of adrenomedullin

Adrenomedullin Inhibits Osmotic Water Permeability in Rat Inner Medullary Collecting Ducts

Adrenomedullin (ADM) is a vasodilator that causes natriuresis and diuresis. However, the direct effect of ADM on osmotic water permeability in the rat inner medullary collecting duct (IMCD) has not been tested. We investigated whether ADM and its ADM receptor components (CRLR, RAMP2, and 3) are expressed in rat inner medulla (IM) and whether ADM regulates osmotic water permeability in isolated perfused rat IMCDs. The mRNAs of ADM, CRLR, and RAMP2 and 3 were detected in rat IM. Abundant protein of CRLR and RAMP3 were also seen but RAMP2 protein level was extremely low. Adding ADM (100 nM) to the bath significantly decreased osmotic water permeability. ADM significantly decreased aquaporin-2 (AQP2) phosphorylation at Serine 256 (pS256) and increased it at Serine 261 (pS261). ADM significantly increased cAMP levels in IM. However, inhibition of cAMP by SQ22536 further decreased ADM-attenuated osmotic water permeability. Stimulation of cAMP by roflumilast increased ADM-attenuated osmotic water permeability. Previous studies show that ADM also stimulates phospholipase C (PLC) pathways including protein kinase C (PKC) and cGMP. We tested whether PLC pathways regulate ADM-attenuated osmotic water permeability. Blockade of either PLC by U73122 or PKC by rottlerin significantly augmented the ADM-attenuated osmotic water permeability and promoted pS256-AQP2 but did change pS261-AQP2. Inhibition of cGMP by L-NAME did not change AQP2 phosphorylation. In conclusion, ADM primarily binds to the CRLR-RAMP3 receptor to initiate signaling pathways in the IM. ADM reduced water reabsorption through a PLC-pathway involving PKC. ADM-attenuated water reabsorption may be related to decreased trafficking of AQP2 to the plasma membrane. cAMP is not involved in ADM-attenuated osmotic water permeability.

Gene expression of adrenomedullin in canine normal tissues and diseased hearts

The purposes of this study were to determine the tissue distribution of canine adrenomedullin (AM) and to determine whether increased canine AM mRNA expression is associated with congestive heart failure (CHF) due to mitral regurgitation (MR). Canine AM mRNA expression was detectable in various normal tissues, including cardiovascular tissues. In addition, the AM mRNA expression in the left atrium of dogs with MR was significantly higher than that in normal subjects. In conclusion, AM is a potential neurohumoral factor in dogs with CHF due to MR.

Increased renal adrenomedullin expression in rats with ureteral obstruction

Ureteral obstruction is characterized by decreased renal blood flow that is associated with hypoxia within the kidney. Adrenomedullin (AM) is a peptide hormone with tissue-protective capacity that is stimulated through hypoxia. We tested the hypothesis that ureteral obstruction stimulates expression of AM and hypoxia-inducible factor-1 (HIF-1α) in kidneys. Rats were exposed to bilateral ureteral obstruction (BUO) for 2, 6, 12, and 24 h or sham operation and compared with unilateral obstruction (UUO). AM mRNA expression was measured by quantitative PCR in cortex and outer medulla (C+OM) and inner medulla (IM). AM and HIF-1α protein abundance and localization were determined in rats subjected to 24-h BUO. AM mRNA expression in C+OM increased significantly after 12-h BUO and further increased after 24 h. In IM, AM mRNA expression increased significantly in response to BUO for 6 h and further increased after 24 h. AM peptide abundance was enhanced in C+OM and IM after 24-h BUO. Immunohistochemical labeling of kidneys showed a wider distribution and more intense AM signal in 24-h BUO compared with Sham. In UUO rats, AM mRNA expression increased significantly in IM of the obstructed kidney compared with nonobstructed and Sham kidney whereas AM peptide increased in IM compared with Sham. HIF-1α protein abundance increased significantly in IM after 24-h BUO compared with Sham and HIF-1α immunoreactive protein colocalized with AM. In summary, AM and HIF-1α expression increases in response to ureteral obstruction in agreement with expected oxygen gradients. Hypoxia acting through HIF-1α accumulation may be an important pathway for the renal response to ureteral obstruction.

Plasma and urinary adrenomedullin levels in children with renal parenchymal scar and vesicoureteral reflux

Adrenomedullin (AM) is a strong vasodilator peptide with proven antimitogenic and antiproliferative effects in renal mesangial cells, as well as diuretic and natriuretic actions. Its gene expression is stimulated by endotoxins (lipopolysacharides) and cytokines. Consequently, its plasma and urinary levels are known to deviate from normal levels in many renal diseases. The purpose of this study is to determine plasma and urinary AM levels in children with renal parenchymal scar (RPS) and vesicoureteral reflux (VUR). The study was carried out on 74 children with recurrent urinary tract infections, arranged in groups: 25 patients with RPS with VUR (group I), 16 patients with RPS without VUR (group II), 12 patients with VUR without RPS (group III) and 21 healthy children as the control group. Plasma and urinary AM concentrations were both determined by high performance liquid chromotography (HPLC). Plasma AM was measured as picomoles per milliliter (pM/ml) and urinary AM as pM/mg urinary creatinine. In addition, serum creatinine, creatinine clearance and fractional sodium excretion (FE(Na)) were measured. All cases with RPS and VUR had normal blood pressure levels. The plasma AM levels were higher, although not significantly, in the control group (56.2+/-14.0 pM/ml) than in group I (50.6+/-4.2 pM/ml), group II (49.6+/-3.7 pM/ml) and group III (50.6+/-3.6 pM/ml) ( P =0.162). The urinary AM levels were higher in the control group (80.1+/-33.9 pM/mg) than in the three study groups (52+/-7.6 pM/mg, 58.6+/-7.5 pM/mg and 44.2+/-6.4 pM/mg; P =0.003, P =0.002 and P =0.002, respectively). There were no differences among the 4 groups (group I, group II, group III and the control group) in terms of FE(Na) and creatinine clearance ( P >0.05 and P >0.05, respectively). The finding that diminished urinary AM levels in patients with RPS and VUR implies that AM can be a prognostic factor in the long-term follow-up of cases with these diseases.

Role of increased circulating and renal adrenomedullin in rats with malignant hypertension

Although it has been reported that the circulating adrenomedullin (AM) level is elevated in hypertension and renal failure, the pathophysiological significance of circulating and intrarenal AM in malignant hypertension remains unknown. We investigated the circulating and intrarenal AM system in rats with malignant hypertension by measuring the plasma level, renal tissue level, and mRNA abundance of AM and the mRNA abundance of AM receptor. We also investigated the effects of intravenously infused calcitonin gene-related peptide (CGRP)-(8-37), an antagonist of AM, on the hemodynamics and renal tubular function. We studied the following four groups: control Wistar-Kyoto rats (WKY), control spontaneously hypertensive rats (C-SHR), salt-loaded SHR (S-SHR), and DOCA-salt SHR (D-SHR). After 3 wk of DOCA treatment, D-SHR developed malignant hypertension. D-SHR were characterized by higher blood pressure, kidney weight, urinary protein excretion and blood urea nitrogen, and lower creatinine clearance compared with the other three groups. The plasma AM level and urinary excretion of AM were markedly higher in D-SHR than in the other three groups. In the kidney, the tissue AM level and the expression of AM mRNA in the renal medulla were significantly increased in D-SHR compared with the other three groups, whereas there were no significant differences in these levels in the renal cortex among the four groups. In the renal AM receptor system, the expression of the gene for receptor activity modifying protein 3 was significantly increased in the renal medulla in D-SHR compared with the other three groups. An immunohistochemical study revealed that AM immunostaining in renal collecting duct cells and distal tubules was more intense in D-SHR than in the other three groups. After CGRP-(8-37) infusion, blood pressure increased significantly and urinary sodium excretion and urine flow decreased significantly only in D-SHR. These results suggest that the increased circulating AM and renal AM and the increased expression of the mRNA for AM and its receptor may at least partly compensate for the malignant hypertensive state in certain forms of malignant hypertension via the hypotensive, natriuretic, and diuretic actions of AM.