Gene regulation of atrial natriuretic peptide A, B, and C receptors in rat glomeruli

Regional heterogeneity of expression of renal NPRs, TonEBP, and AQP-2 mRNAs in rats with acute kidney injury

To understand the pathophysiology of ischemia/reperfusion (I/R) - induced acute kidney injury (AKI), the present study defined changes in renal function, plasma renotropic hormones and its receptors in the kidney 2, 5, or 7 days after 45 min-renal ischemia in rats. Blood urea nitrogen, plasma creatinine, and osmolarity increased 2 days after I/R injury and tended to return to control level 7 days after I/R injury. Decreased renal function tended to return to control level 5 days after I/R injury. However, plasma concentrations of atrial natriuretic peptide and renin did not change. In control kidney, natriuretic peptide receptor (NPR)-A, -B and -C mRNAs were highly expressed in medulla (ME), inner cortex (IC), and outer cortex (OC), respectively, and tonicity-responsive enhancer binding protein (TonEBP), auqaporin-2 (AQP-2) and eNOS mRNAs were highly expressed in ME. NPR-A and -B mRNA expressions were markedly decreased 2 days after I/R injury. On 5 days after I/R injury, NPR-A mRNA expression increased in OC and recovered to control level in IC but not in ME. NPR-B mRNA expression was increased in OC, and recovered to control level in IC and ME. NPR-C mRNA expression was markedly decreased in OC 2 and 5 days after I/R injury. TonEBP, APQ-2 and eNOS mRNA expressions were markedly decreased 2 days after I/R injury and did not recover in ME 7 days after I/R injury. Therefore, we suggest that there is a regional heterogeneity of regulation of renal NPRs, TonEBP, and APQ-2 mRNA in AKI.

Osmoregulation of natriuretic peptide receptors in bromoethylamine-treated rat kidney

Extracellular osmolarity is known as an important factor for the regulation of natriuretic peptide receptors (NPRs). We investigated the intra-renal osmoregulation of NPRs using renal medullectomized rats with bromoethylamine hydrobromide (BEA, 200mg/kg). The administration of BEA caused the decreased food intake and body weight. Water intake was decreased on the first day and then increased from the second day. Urine volume was persistently increased from the first day and free water clearance was also increased from the second day. Urinary excretions of sodium and potassium were decreased on the second day and then recovered to control level. Plasma levels of atrial natriuretic peptide (ANP) and Dendroaspis natriuretic peptide (DNP) in BEA-treated rats were not different from control rats. The inactive renin was increased. The maximum binding capacities of (125)I-ANP as well as (125)I-DNP decreased in glomeruli and medulla of BEA-treated rat kidneys but the binding affinity was not changed. In renal cortex, the gene expressions of ANP, NPR-A, and NPR-B were not changed but that of NPR-C decreased. In renal medulla, the gene expressions of NPR-A, -B, and -C decreased without change in ANP mRNA. Both renal medullary osmolarity and sodium concentration by BEA treatment were lower than those in control kidney. The cGMP concentrations in renal medulla and urine in BEA-treated rats were higher than those in control rats. These results suggest that the increased cGMP production may be partly involved in the decrease in NPRs mRNA expression and their binding capacities by BEA-induced medullectomy.

Effect of exercise on gene expression of atrial natriuretic peptide receptor of kidney

To study the effect of exercise on gene expression of natriuretic peptide receptors (NPRs) in the kidney, with in situ hybridization and the computerized image analysis, we investigated the alterations of gene expression of NPRs on the animal model of exercise training of different intensity. We found that after exercise training of different intensity, renal NPR-A mRNA and NPR-C mRNA expression showed different changes, the expression of NPR-A mRNA upregulated and NPR-C mRNA downregulated in the kidney. With the increase exercise intensity, change in NPR-A mRNA expression was insignificant, but downregulation in NPR-C mRNA expression was more significant. The result suggested that the effect of exercise on renal NPRs mRNA expression was mainly on the modulation level of NPR-C mRNA, it could reduce the clearance rate of ANP, increase the level of ANP, and enhance the biological effect of ANP on the kidney and regulative action of kidney in exercise.

RENAL RESPONSES TO ATRIAL NATRIURETIC PEPTIDE ARE PRESERVED IN BILATERAL URETERAL OBSTRUCTION AND AUGMENTED BY NEUTRAL ENDOPEPTIDASE INHIBITION

PURPOSE

Atrial natriuretic peptide (ANP) contributes to post-obstructive diuresis in bilateral ureteral obstruction (BUO). In this study we examined the activity of neutral endopeptidase (NEP), an enzyme responsible for degradation of ANP, in the kidney in rats subjected to BUO for 24 hours.

MATERIALS AND METHODS

Renal function was examined by the clearance method in sham operated rats and BUO rats after obstruction release. Renal responses to an intravenous bolus injection of ANP (5 microg/kg) were studied in sham operated and BUO rats with or without pretreatment with intravenous phosphoramidon (100 microg/kg per minute), a NEP inhibitor.

RESULTS

In BUO rats natriuresis and diuresis occurred despite a marked decrease in the glomerular filtration rate (GFR). ANP administration increased GFR and induced marked natriuresis and diuresis in sham operated and BUO rats. Inhibition of ANP degradation by phosphoramidon induced natriuresis and diuresis, and accentuated these renal responses to ANP.

CONCLUSIONS

Renal responses to ANP and renal NEP activity were preserved in 24-hour BUO. NEP inhibition to attenuate ANP degradation augmented responses to ANP in increasing GFR, natriuresis and diuresis. These findings provide the theoretical potential for facilitating the recovery of GFR after BUO release by inhibiting ANP degradation by pharmacological means.

C-type natriuretic peptide as a podocyte hormone and modulation of its cGMP production by glucose and mechanical stress

BACKGROUND

High glucose and mechanical strain resulting from capillary hypertension are relevant risk factors affecting glomerular cells in diabetes. Altered activity of the natriuretic peptide (NP) system acting via vasorelaxing cyclic guanosine 5' monophosphate (cGMP) has been proposed to be one of the reasons for diabetes-dependent impairment of kidney function. Podocytes possess the NP receptors (NPRs) coupled to particular guanylyl cyclase. We investigated whether mechanical stress and high ambient glucose influence cGMP generation in podocytes stimulated with NPs. Additionally, the C-type natriuretic peptide (CNP) system has been characterized in these cells.

METHODS

Conditionally immortalized mouse podocytes were stimulated with NP for 15 minutes and cGMP was determined by enzymatic immunoassay. The mRNA expression for CNP and CNP-specific NPR-B was confirmed by reverse transcriptase-polymerase chain reaction (RT-PCR). In addition, cGMP synthesis was measured in cells exposed to mechanical stress and to 30 mmol/L glucose for 3 days.

RESULTS

Production of cGMP upon stimulation of the NPR-B receptor with CNP and inhibition by an antagonist HS-142-1 was dose-dependent. RT-PCR showed that podocytes express mRNA not only for NPR-B but also for CNP. Mechanical stress reduced the cGMP response by 50%, both to atrial natriuretic peptide (ANP) and to CNP. Conversely, upon high glucose, the CNP-induced production of cGMP was elevated twofold in stretched and in control cells. Furthermore, as compared to ANP, stimulation with CNP caused a larger increase in cGMP levels in stretched as well as in nonstretched cells.

CONCLUSION

Expression of CNP together with potent NPR-B receptors suggests that in podocytes, CNP may act in an autocrine and/or paracrine manner. Furthermore, in a diabetic kidney, high glucose and mechanical stress may modulate the CNP-dependent cGMP production in podocytes in an opposite manner.

Renal C-type natriuretic peptide and natriuretic peptide receptor B mRNA expression are affected by water deprivation in the Spinifex Hopping mouse

This study investigated the effect of water deprivation on the expression of C-type natriuretic peptide (CNP) and natriuretic peptide receptor B (NPR-B) mRNA, and the ability of NPR-B to generate cGMP in the Spinifex Hopping mouse, Notomys alexis. This rodent is a native of central and western Australia that is well adapted to survive in arid environments. Initially, CNP and NPR-B cDNAs (partial for NPR-B) were cloned and sequenced, and were shown to have high homology with those of rat and mouse. RT-PCR analysis showed CNP mRNA expression in the kidney, proximal and distal colon and small intestine, whilst NPR-B mRNA expression was found in the kidney, proximal and distal colon and the atria. Using a semi-quantitative multiplex PCR technique, the expression of renal CNP and NPR-B mRNA was determined in 7- and 14-day water-deprived hopping mice, in parallel with control hopping mice (access to water). Water deprivation significantly decreased the relative levels of CNP and NPR-B mRNA expression in both the 7- and 14-day water-deprived hopping mice, when compared to control hopping mice. In contrast, the ability of CNP to stimulate cGMP production was significantly increased after 14 days of water deprivation. This study shows that alterations in the renal CNP/NPR-B system may be an important physiological adjustment when water is scarce.

The effect of water deprivation on the expression of atrial natriuretic peptide and its receptors in the spinifex hopping mouse, Notomys alexis

This study investigated the mRNA expression of the atrial natriuretic peptide (ANP) system (peptide and receptors) during water deprivation in the spinifex hopping mouse, Notomys alexis, a native of central and western Australia that is well adapted to survive in arid environments. Initially, ANP, NPR-A and NPR-C cDNAs (partial for receptors) were cloned and sequenced, and were shown to have high homology with those of rat and mouse. Using a semi-quantitative multiplex PCR technique, the expression of cardiac ANP mRNA and renal ANP, NPR-A, and NPR-C mRNA was determined in 7- and 14-day water-deprived hopping mice, in parallel with control mice (access to water). The levels of ANP mRNA expression in the heart remained unchanged, but in the kidney ANP mRNA levels were increased in the 7-day water-deprived mice, and were significantly decreased in the 14-day water-deprived mice. NPR-A mRNA levels were significantly higher in 7-day water-deprived mice while no change for NPR-A mRNA expression was observed in 14-day water-deprived mice. No variation in NPR-C mRNA levels was observed. This study shows that water deprivation differentially affects the expression of the ANP system, and that renal ANP expression is more important than cardiac ANP in the physiological adjustment to water deprivation.