Role of Renal Nerves and Salt Intake on Erythropoietin Secretion in Rats following Carbon Monoxide Exposure

Renal Nerve-Mediated Erythropoietin Release Confers Cardioprotection During Remote Ischemic Preconditioning

BACKGROUND

Remote ischemic preconditioning (RIPC) induced by transient limb ischemia is a powerful innate mechanism of cardioprotection against ischemia. Several described mechanisms explain how RIPC may act through neural pathways or humoral factors; however, the mechanistic pathway linking the remote organ to the heart has not yet been fully elucidated. This study aimed to investigate the mechanisms underlying the RIPC-induced production of Janus kinase (JAK)-signal transducer and activator of the transcription (STAT)-activating cytokines and cardioprotection by using mouse and human models of RIPC.

METHODS AND RESULTS

Screened circulating cardioprotective JAK-STAT-activating cytokines in mice unexpectedly revealed increased serum erythropoietin (EPO) levels after RIP induced by transient ischemia. In mice, RIPC rapidly upregulated EPO mRNA and its main transcriptional factor, hypoxia-inducible factor-1α (HIF1α), in the kidney. Laser Doppler blood flowmetry revealed a prompt reduction of renal blood flow (RBF) after RIPC. RIPC activated cardioprotective signaling pathways and the anti-apoptotic Bcl-xL pathway in the heart, and reduced infarct size. In mice, these effects were abolished by administration of an EPO-neutralizing antibody. Renal nerve denervation also abolished RIPC-induced RBF reduction, EPO production, and cardioprotection. In humans, transient limb ischemia of the upper arm reduced RBF and increased serum EPO levels.

CONCLUSIONS

Based on the present data, we propose a novel RIPC mechanism in which inhibition of infarct size by RIPC is produced through the renal nerve-mediated reduction of RBF associated with activation of the HIF1α-EPO pathway.

Lack of effect of extracellular adenosine generation and signaling on renal erythropoietin secretion during hypoxia

Previous studies have yielded conflicting results as to whether extracellular adenosine generation and signaling contributes to hypoxia-induced increases in renal erythropoietin (EPO) secretion. In this study, we combined pharmacological and genetic approaches to elucidate a potential contribution of extracellular adenosine to renal EPO release in mice. To stimulate EPO secretion, we used murine carbon monoxide exposure (400 and 750 parts per million CO, 4 h), ambient hypoxia (8% oxygen, 4 h), or arterial hemodilution. Because the ecto-5-nucleotidase (CD73, conversion of AMP to adenosine) is considered the pacemaker of extracellular adenosine generation, we first tested the effect of blocking extracellular adenosine generation with the specific CD73-inhibitor adenosine 5'-(alpha,beta-methylene) diphosphate (APCP) or by gene-targeted deletion of cd73. These studies showed that neither APCP-treatment nor targeted deletion of cd73 resulted in changes of stimulated EPO mRNA or serum levels, although the increases of adenosine levels in the kidney following CO exposure were attenuated in mice with APCP treatment or in cd73(-/-) mice. Moreover, pharmacological studies using specific inhibitors of individual adenosine receptors (A1 AR, DPCPX; A 2A AR, DMPX; A 2B AR, PSB 1115; A3AR, MRS 1191) showed no effect on stimulated increases of EPO mRNA or serum levels. Finally, stimulated EPO secretion was not attenuated in gene-targeted mice lacking A1A(-/-, A2A AR-/-, A2BAR(-/-), or A3AR-/-. Together, these studies combine genetic and pharmacological in vivo evidence that increases of EPO secretion during limited oxygen availability are not affected by extracellular adenosine generation or signaling.

Expression and localization of S-adenosylhomocysteine-hydrolase in the rat kidney following carbon monoxide induced hypoxia

BACKGROUND/AIMS

Tissue hypoxia induces a variety of functional changes including enhanced transcriptional activity associated with high transmethylation activity (e.g. mRNA cap methylation) in the nucleus. It is well known that the kidney responds to hypoxia with enhanced transcription of erythropoietin (EPO) in the interstitial cells. Since S-adenosylhomocysteine (AdoHcy)-hydrolase regulates most S-adenosylmethionine (AdoMet) dependent transmethylation reactions by hydrolyzing the potent feedback inhibitor AdoHcy to adenosine and homocysteine we studied the effect of hypoxia by carbon monoxide (CO) inhalation (1200 ppm) on AdoHcy-hydrolase gene expression and its localization in rat kidneys.

RESULTS

CO lowered renal AdoHcy-hydrolase mRNA expression by 64% whereas AdoHcy-hydrolase activity was not changed during 4h of CO exposure 0.7+/-0.04 mU/mg (control) vs. 0.75+/-0.06 mU/mg protein. Using two-channel immunofluorescence confocal laser scanning microscope AdoHcy-hydrolase was visualized in different cells of the hypoxic rat kidney. A very bright immunofluorescence of AdoHcy-hydrolase was observed in the nuclei of single interstitial cells of renal cortex and outer medulla which respond to hypoxia with increased EPO secretion indicating translocation of AdoHcy-hydrolase from the cytosol to the nucleus.

CONCLUSIONS

These data suggest that AdoHcy-hydrolase accumulation in the nucleus of adult mammalian cells is involved in maintaining efficient transmethylation reactions in transcriptionally active cells by removing the product inhibitor AdoHcy.