Constitutive nitric oxide synthesis in the kidney--functions at the juxtaglomerular apparatus

Concerted regulation of renal plasma flow and glomerular filtration rate by renal dopamine and NOS I in rats on high salt intake

Under high sodium intake renal dopamine (DA) increases while NOS I expression in macula densa cells (MD) decreases. To explore whether renal DA and NOS I, linked to natriuresis and to the stability of the tubuloglomerular feedback, respectively, act in concert to regulate renal plasma flow (RPF) and glomerular filtration rate (GFR). Male Wistar rats were studied under a normal sodium intake (NS, NaCl 0.24%) or a high sodium intake (HS, NaCl 1% in drinking water) during the 5 days of the study. For the last two days, the specific D1-like receptor antagonist SCH 23390 (1 mg kg bwt-1 day-1, sc) or a vehicle was administered. HS intake increased natriuresis, diuresis, and urinary DA while it decreased cortical NOS I expression (P < 0.05 vs. NS), Nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) activity in MD (P < 0.001 vs. NS) and cortical nitrates+nitrites (NOx) production (NS 2.04 ± 0.22 vs. HS 1.28 ± 0.10 nmol mg protein-1, P < 0.01). Treatment with SCH 23390 to rats on HS sharply decreased hydroelectrolyte excretion (P < 0.001 vs. HS) while NOS I expression, NADPH-d activity and NOx production increased (P < 0.05 vs. HS for NOS I and P < 0.001 vs. HS for NADPH-d and NOx). SCH 23390 increased RPF and GFR in HS rats (P < 0.01 HS+SCH vs. HS). It did not cause variations in NS rats. Results indicate that when NS intake is shifted to a prolonged high sodium intake, renal DA through the D1R, and NOS I in MD cells act in concert to regulate RPF and GFR to stabilize the delivery of NaCl to the distal nephron.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

Prion protein expression in bovine podocytes and extraglomerular mesangial cells

The cellular form of the prion protein (PrP(c)) is thought to be a substrate for an abnormal isoform of the prion protein (PrP(sc)). One emerging hypothesis is that the proposed conversion phenomenon takes place at the site at which the infectious agent meets PrP(c). PrP(c) is abundant in the central nervous system, but little is known about the cell-type-specific distribution of PrP(c) in non-neuronal tissues of cattle. We have studied whether PrP(c), a protein found predominantly in neurons, also exists in bovine podocytes, since neurons and podocytes share a large number of similarities. We have therefore examined the expression of PrP(c) by immunohistochemistry, reverse transcription/polymerase chain reaction and enzyme-linked immunosorbent analysis. Immunostained serial sections and specific antibodies against PrP(c) have revealed that PrP(c) is selectively localized in podocytes and is particularly strongly expressed in extraglomerular mesangial cells but not in endothelial or intraglomerular mesangial cells. The selective expression of PrP(c) in podocytes is of special importance, as it suggests that these cells represent possible targets for peripheral infection with prions and demonstrates that PrP(c) can be added to the list of neuronal factors expressed in mammalian podocytes.

Role of renal nitric oxide synthase in diabetic kidney disease during the chronic phase of diabetes

BACKGROUND

Several studies have suggested that an early increase in renal nitric oxide (NO) production or activity mediates pathophysiologic and morphologic changes in diabetic nephropathy. To evaluate the role of NO in developing diabetic kidney disease, we studied the NO system in streptozotocin (STZ)-induced diabetic rats for a period of 8 weeks.

METHODS

Control rats, STZ-induced diabetic rats, and STZ-induced diabetic rats treated with insulin were monitored and sacrificed at 1, 2, and 8 weeks. Urinary cGMP was measured, and the levels and activity of NO synthase (NOS) isoforms in the kidney cortex were determined at specific times by immunoblotting and diaphorase staining.

RESULTS

Diabetic rats had increased kidney weight, urinary volume, glucose, sodium and potassium excretion, which was precluded by insulin treatment. Creatinine clearance was increased in the diabetic group and reversed by insulin treatment. Urinary cGMP decreased by 71, 93, and 92% at 1, 2, and 8 weeks of diabetes, respectively, compared with the control animals. Insulin treatment curtailed the urinary cGMP reduction in diabetic animals. Total NOS activity in the renal cortex was reduced by 65, 52, and 44% after 1, 2, and 8 weeks of diabetes, respectively, and returned to normal levels upon insulin treatment. NADPH diaphorase staining of renal cortical slices showed a 77, 63, and 70% decrease in neuronal NOS isoform activity in the macula densa after 1, 2, and 8 weeks of diabetes, respectively, compared with control non-diabetic animals. This reduction was normalized by insulin treatment. Endothelial NOS protein expression in the kidney cortex tended to increase after 1 week of diabetes and its level was elevated significantly after 2 and 8 weeks of diabetes. However, neuronal NOS protein expression in the kidney cortex was reduced by 52% in 2-week diabetic animals, but this reduction was normalized by insulin treatment.

CONCLUSIONS

The decreased renal NOS activity during the late phase of diabetes is partially associated with a decrease in neuronal NOS activity and protein expression in kidney macula densa.

Resistance to oxidative stress by chronic infusion of angiotensin II in mouse kidney is not mediated by the AT2 receptor

Wild-type mice are resistant to ANG II-induced renal injury and hence form an attractive model to study renal defense against ANG II. The present study tested whether ANG II induces expression of antioxidative genes via the AT2 receptor in renal cortex and thereby counteracts prooxidative forces. ANG II was infused in female C57BL/6J mice for 28 days and a subgroup received AT2 receptor antagonist (PD-123,319) for the last 3 days. ANG II induced hypertension and aortic hypertrophy; proteinuria and renal injury were absent. Urinary nitric oxide metabolites (NOx) were decreased, and lipid peroxide (TBARS) excretion remained unchanged. Expression of NADPH oxidase components was decreased in renal cortex but induced in aorta. Heme oxygenase-1 (HO-1) was induced in both renal cortex and aorta. In contrast, ANG II suggestively increased AT2 receptor expression in kidney but not in aorta. AT2 receptor blockade enhanced hypertension in ANG II-infused mice, reversed ANG II effects on NOx excretion, but did not affect TBARS. Despite its prohypertensive effect, expression of prooxidative genes in the renal cortex decreased rather than increased after short-term AT2 receptor blockade and renal HO-1 induction after ANG II was normalized. Thus chronic ANG II infusion in mice induces hypertension but not oxidative stress. In contrast to the response in aorta, gene expression of components of NADPH-oxidase was not enhanced in renal cortex. Although ANG II administration induced renal cortical AT2 receptor expression, blockade of that receptor did not unveil the AT2 receptor as intrarenal dampening factor of prooxidative forces.

Stimulation of AQP2 membrane insertion in renal epithelial cells in vitro and in vivo by the cGMP phosphodiesterase inhibitor sildenafil citrate (Viagra)

Vasopressin-stimulated insertion of the aquaporin 2 (AQP2) water channel into the plasma membrane of kidney collecting duct principal cells is a key event in the urinary concentrating mechanism. The paradigm for vasopressin-receptor signaling involves cAMP-mediated protein kinase A activation, which results in the functionally critical phosphorylation of AQP2 on amino acid serine 256. We previously showed that a parallel cGMP-mediated signaling pathway also leads to AQP2 membrane insertion in AQP2-transfected LLC-PK1 (LLC-AQP2) cells and in outer medullary collecting duct principal cells in situ (Bouley R, Breton S, Sun T, McLaughlin M, Nsumu NN, Lin HY, Ausiello DA, and Brown D. J Clin Invest 106: 1115-1126, 2000). In the present report, we show by immunofluorescence microscopy, and Western blotting of plasma membrane fractions, that 45-min exposure of LLC-AQP2 cells to the cGMP phosphodiesterase type 5 (PDE5) inhibitors sildenafil citrate (Viagra) or 4-{[3',4'-methylene-dioxybenzyl]amino}-6-methoxyquinazoline elevates intracellular cGMP levels and results in the plasma membrane accumulation of AQP2; i.e., they mimic the vasopressin effect. Importantly, our data also show that acute exposure to PDE5 inhibitors for 60 min induces apical accumulation of AQP2 in kidney medullary collecting duct principal cells both in tissue slices incubated in vitro as well as in vivo after intravenous injection of Viagra into rats. These data suggest that AQP2 membrane insertion can be induced independently of vasopressin-receptor activation by activating a parallel cGMP-mediated signal transduction pathway with cGMP PDE inhibitors. These results provide proof-of-principle that pharmacological activation of vasopressin-independent, cGMP signaling pathways could aid in the treatment of those forms of nephrogenic diabetes insipidus that are due to vasopressin-2 receptor dysfunction.

Neuronal nitric oxide synthase and renin stimulation by sodium deprivation are dependent on the renal nerves

OBJECTIVE

The aim of the present study was to evaluate the role of the renal nerves in the regulation of neuronal nitric oxide synthase (nNOS) gene expression in normotensive rats on different sodium balance.

METHODS

Thirty-six male Sprague-Dawley rats were divided into six experimental groups combining three diets with different NaCl content (normal, 0.4%; low, 0.04%; or high, 4.0%), and bilateral renal denervation or sham denervation. After 7 days of dietary treatment, all rats were sacrificed and plasma renin activity (PRA) measured. The nNOS and renin messenger RNA (mRNA) levels in the renal cortex were determined by semiquantitative polymerase chain reaction.

RESULTS

PRA was higher in animals with low sodium diet compared with those with standard diet, while it was lower in animals with high sodium diet. Renal denervation decreased PRA in normal and low sodium groups, while it did not alter the PRA values in the high sodium group. The nNOS gene expression significantly increased in rats fed with the low sodium diet compared with the standard diet group, and it significantly decreased in rats with the high sodium diet. Renal denervation significantly reduced nNOS mRNA levels in rats receiving the low sodium diet, but did not significantly influence nNOS mRNA in normal and high sodium groups. Renin mRNA was influenced by diets and denervation in a parallel way to nNOS mRNA.

CONCLUSION

The renal nerves mediate the increase of renin and nNOS mRNA during sodium restriction, while the suppression of nNOS and renin gene expression during a sodium load is independent of the presence of the renal nerves. The parallel changes in renin and nNOS mRNA during different sodium intakes suggest that nNOS can be part of the complex, and still largely unclarified, macula densa mechanism of renin regulation.

Renal injury in apolipoprotein E-deficient mice

Hyperlipidemia is thought to accelerate the progression of renal diseases, but the mechanisms by which hyperlipidemia exerts its deleterious effect is still poorly understood. The aim of this study was to describe the renal pathology in a hyperlipidemic mouse strain, the apolipoprotein E-deficient mice (apoE-/-). Renal specimens from a total of 34 mice were studied, including 19 apoE-/- females at the age of 36 weeks, 9 apoE-/- females at the age of 24 weeks, and 6 wild-type females (C57BL/6) as controls. Kidneys were evaluated by histologic examination, immunohistochemistry, and electron microscopy. Immunohistochemistry was used to detect MAC-2-expressing monocyte/macrophages, and the proliferation marker PCNA. Glomerular cell number, glomerular matrix area, and glomerular area were quantified by morphometry. Glomerular lesions in apoE-/- mice were characterized by macrophage accumulation, commonly with foam cell appearance, deposition of extracellular matrix, glomerular hyperplasia, and at times prominent mesangiolysis associated with capillary microaneurysms. Some cases demonstrated lipid deposits filling glomerular capillaries. Arterioles of the vascular pole demonstrated a "foamy" degeneration of smooth muscle cells. These lesions related to hyperlipidemia in this well-established mouse strain have not been previously described. Because this mouse strain is among the most widely studied for interventions aimed at altering hyperlipidemia and the progression of atherosclerosis, we believe that our observations may be of major importance for the accurate interpretation of interventional studies in this strain and offer a new opportunity to study mechanisms of hyperlipidemic renal injury.

Cellular distribution and function of soluble guanylyl cyclase in rat kidney and liver

Soluble guanylyl cyclase (sGC) catalyzes the biosynthesis of cGMP in response to binding of L-arginine-derived nitric oxide (NO). Functionally, the NO-sGC-cGMP signaling pathway in kidney and liver has been associated with regional hemodynamics and the regulation of glomerular parameters. The distribution of the ubiquitous sGC isoform alpha 1 beta 1 sGC was studied with a novel, highly specific antibody against the beta 1 subunit. In parallel, the presence of mRNA encoding both subunits was investigated by using in situ hybridization and reverse transcription-PCR assays. The NO-induced, sGC-dependent accumulation of cGMP in cytosolic extracts of tissues and cells was measured in vitro. Renal glomerular arterioles, including the renin-producing granular cells, mesangium, and descending vasa recta, as well as cortical and medullary interstitial fibroblasts, expressed sGC. Stimulation of isolated mesangial cells, renal fibroblasts, and hepatic Ito cells with a NO donor resulted in markedly increased cytosolic cGMP levels. This assessment of sGC expression and activity in vascular and interstitial cells of kidney and liver may have implications for understanding the role of local cGMP signaling cascades.

Feedback control of glomerular vascular tone in neuronal nitric oxide synthase knockout mice

For further elucidation of the role of neuronal nitric oxide synthase (nNOS) in macula densa (MD) cells, experiments were performed in anesthetized nNOS knockout mice (nNOS -/-). At comparable levels of arterial BP, renal blood flow was not significantly different between nNOS +/+ and nNOS -/- (1.7 +/- 0.2 versus 1.4 +/- 0.1 ml/min), and autoregulation of renal blood flow was maintained to a pressure level of approximately 85 mmHg in both groups of mice (n = 6 in each group). The fall in proximal tubular stop-flow pressure in response to an increase in loop of Henle perfusion rate from 0 to 30 nl/min was comparable in nNOS +/+ and -/- mice (40.7 +/- 1.6 to 32 +/- 2 mmHg versus 40.6 +/- 1.6 to 31.6 +/- 2 mmHg; not significant; n = 13 versus 18 nephrons). Luminal application of the nonselective NOS inhibitor nitro-L-arginine (10(-3) and 10(-2) M) enhanced the perfusion-dependent fall in stop-flow pressure in nNOS +/+ (7 +/- 1 to 13 +/- 2 mmHg; P < 0.05) but not in nNOS -/- (7 +/- 1 to 8 +/- 1 mmHg; not significant) mice. nNOS -/- mice exhibited a lower nephron filtration rate, compared with nNOS +/+, during free-flow collections from early distal tubules (influence of MD intact, 7 +/- 0.7 versus 10.9 +/- 1 nl/min; P = 0.002) but not from late proximal tubule (influence of MD minimized, 10.1 +/- 1 versus 11.7 +/- 1 nl/min; not significant; n = 16 nephrons). Distal Cl concentration and fractional absorption of fluid or chloride up to the early distal tubule was not different between nNOS -/- and +/+ mice. The data indicate that nNOS in MD tonically attenuates the GFR-lowering influence of ambient luminal NaCl, which may serve to increase the fluid and electrolyte load to the distal tubule, consistent with a role of MD nNOS in tubuloglomerular feedback resetting.