Sodium-pump gene-expression, protein abundance and enzyme activity in isolated nephron segments of the aging rat kidney.

Aging is associated with alteration in renal tubular functions, including sodium handling and concentrating ability. Na-K-ATPase plays a key role in driving tubular transport, and we hypothesized that decreased concentrating ability of the aging kidney is due in part to downregulation of Na-K-ATPase. In this study, we evaluated Na and K balance, aldosterone levels, and Na-K-ATPase gene expression, protein abundance, and activity in aging rat kidney. Na-K-ATPase activity (assayed microfluorometrically), mRNA (RT-PCR), and protein abundance (immunoblotting) were quantitated in the following isolated nephron segments: PCT, PST, MTAL, DCT, and CCD from 2, 8, 15, and 24 month-old-rats. In the course of aging, creatinine clearance decreased from 0.48 ± 0.02 mL/min/100 g BW to 0.28 ± 0.06 (P < 0.001) and aldosterone decreased from 23.6 ± 0.8 ng/dL to 13.2 ± 0.6 (P < 0.001). Serum Na(+) and K(+) increased by 4.0% and 22.5%, respectively. Na-K-ATPase activity, mRNA, and protein abundance of the α1 subunit displayed similar trends in all assayed segments; increasing in PCT and PST; decreasing in MTAL and DCT; increasing in CCD: in PCT they increased by 40%, 75%, and 250%, respectively; while in PST they increased by 80%, 50%, and 100%, respectively (P < 0.001). In MTAL they declined by 36%, 24%, and 34%, respectively, and in DCT by 38%, 59%, and 60%, respectively (P < 0.001). They were higher in CCD by 110%, 115%, and 246%, respectively (P < 0.001). Rats maintained Na/K balance; however with a steady state elevated serum K(+). These results reveal quantitative changes in axial distribution of Na-K-ATPase at the level of gene expression, protein abundance, and activity in the nephrons of aging animals and may explain, in part, the pathophysiology of the senescent kidney.

Psammomys obesus, a particularly important animal model for the study of the human diabetic nephropathy.

The Psammomys obesus lives in natural desert habitat on low energy (LE) diet, however when maintained in laboratory conditions with high energy (HE) diet it exhibits pathological metabolic changes resembling those of type 2 diabetes. We have evaluated and correlated the histopathology, metabolic and functional renal alterations occurring in the diabetic Psammomys. Renal function determined by measuring glomerular filtration rate (GFR), protein excretion, protein/creatinine ratio and morpho-immunocytochemical evaluations were performed on HE diet diabetic animals and compared to LE diet control animals. The diabetic animals present a 54% increase in GFR after one month of hyperglycemic condition and a decrease of 47% from baseline values after 4 months. Protein excretion in diabetic animals was 5 folds increased after 4 months. Light microscopy showed an increase in glomeruli size in the diabetic Psammomys, and electron microscopy and immunocytochemical quantitative evaluations revealed accumulation of basement membrane material as well as frequent splitting of the glomerular basement membrane. In addition, glycogen-filled Armanni-Ebstein clear cells were found in the distal tubules including the thick ascending limbs of the diabetic animals. These renal complications in the Psammomys, including changes in GFR with massive proteinuria sustained by physiological and histopathological changes, are very similar to the diabetic nephropathy in human. The Psamommys obesus represents therefore a reliable animal model of diabetic nephropathy.

Regulation of the renal sodium-dependent phosphate cotransporter NaPi2 (Npt2) in acute renal failure due to ischemia and reperfusion.

BACKGROUND: Acute renal failure (ARF) is associated with hyperphosphatemia and decreased urinary phosphate excretion. The present study was undertaken to characterize the effects of ARF due to ischemia and reperfusion on renal phosphate transport and on gene and protein expression of type IIa NaPi cotransporter (Npt2) the physiologically most relevant renal sodium-dependent phosphate cotransporter. METHODS: The following groups of rats with intact parathyroid glands were studied: (1) sham operated (sham); (2) after 1 h ischemia by bilateral renal artery clamping (I), and after 1 h ischemia and reperfusion of 1 h (I + R 1 h); (3) 24 h (I + R 24 h); (4) 48 h (I + R 48 h), and (5) 72 h (I + R 72 h) duration. The effect of ARF on Npt2 mRNA and protein expression was also examined after parathyroidectomy (PTX) of 2 and 4 days' duration. RESULTS: Ischemia and reperfusion were associated with increases in plasma creatinine, hyperphosphatemia, and with decreased tubular phosphate reabsorption. Npt2 mRNA was significantly downregulated in the cortex, maximal at 24 and 48 h of reperfusion. The degree of Npt2 mRNA downregulation was not affected by PTX of 2-4 days' duration. The abundance of Npt2 protein in proximal tubular apical brush border membrane was markedly decreased after reperfusion. Npt2 protein, however, was more abundant in PTX animals than in those with intact parathyroids and a similar degree of renal insufficiency. The immunohistochemical analysis of proximal tubular apical brush border membrane showed a progressive decrease of Npt2 protein labeling after ischemia and reperfusion, with progressive regeneration after 72 h. CONCLUSION: These results suggest that downregulation of Npt2 protein may contribute to the decreased tubular reabsorption of phosphate in acute ischemic renal failure and hyperphosphatemia.

Recovery of renal tubule phosphate reabsorption despite reduced levels of sodium-phosphate transporter.

BACKGROUND: The acute effect of parathyroid hormone (PTH) on phosphate transport has been reported to be mediated by rapid downregulation of sodium-phosphate transporter (NaPi-IIa) protein, but the association was observed with pharmacological doses of PTH. OBJECTIVE: To explore the effects of physiological doses of PTH on NaPi-IIa protein and its relationship to phosphate transport. METHODS: Acute clearance studies were performed in parathyroidectomized rats given a bolus i.v. physiological dose (1 microg) of bovine PTH(1-34) and NaPi-IIa protein concentrations were examined at different time intervals. RESULTS: Fractional excretion of phosphate increased from 0.031+/-0.006 (mean+/-S.E.) to 0.238+/-0.059 (P<0.01 compared with baseline and compared with controls) at 40 min and returned to control values by 120 min. Urinary cAMP concentrations were increased at 20 min only. Superficial cortex brush-border membrane (BBM) NaPi-IIa protein was decreased from baseline at both 40 and 120 min (P<0.01) and did not recover at 240 min (P<0.01 compared with baseline and compared with controls). CONCLUSION: These results confirm that PTH, even in physiological dosage, causes a rapid decrease in BBM NaPi-IIa, but subsequent recovery of phosphate reabsorption is poorly correlated with BBM concentrations of NaPi-IIa protein. This suggests that transport mechanisms other than NaPi-IIa are important in renal phosphate reabsorption.

Regulation of NaPi-IIa mRNA and transporter protein in chronic renal failure: role of parathyroid hormone (PTH) and dietary phosphate (Pi).

Chronic renal failure (CRF) is associated with a high fractional phosphate excretion (FEPi), secondary hyperparathyroidism, and resistance to parathyroid hormone (PTH). This study was undertaken to characterize the role of PTH and dietary Pi in the regulation of PTH/PTH-related peptide receptor (PTHrP-R) mRNA and NaPi-IIa mRNA and protein in CRF. The following groups of rats were studied: (1) sham-operated (control); (2) CRF: 6 weeks after 5/6 nephrectomy (NPX); (3) NPX and parathyroidectomy (NPX + PTX); (4) NPX rats fed a low-Pi diet (NPX + LP); (5) sham-operated rats fed a low-Pi diet (control + LP); (6) sham-operated after PTX (control + PTX). Expression of NaPi-IIa mRNA and PTH/PTHrP-R mRNA was determined in the renal cortex by Northern hybridization. NaPi-IIa protein abundance was determined in cortical brush border membranes by immunoblotting. In NPX rats creatinine clearance decreased to 40 +/- 4%, PTH/PTHrP-R mRNA to 52.1 +/- 2% and NaPi-IIa mRNA to 41.2 +/- 5.5% of control. The PTH/PTHrP-R and NaPi-IIa mRNA in the NPX + PTX and NPX + LP group was similar to that in NPX. NaPi-IIa protein abundance was reduced in NPX compared with control, but was increased dramatically in NPX + PTX and NPX + LP compared to NPX, paralleled by a decrease in FEPi. These findings suggest that the elevated FEPi in CRF is maintained by decreased NaPi-IIa mRNA and NaPi-IIa protein abundance. In contrast, the observed decrease in FEPi with PTX or LP diet in CRF is mediated, at least partly, by increased NaPi-IIa protein abundance with no change in NaPi-IIa mRNA, suggesting post-transcriptional regulation of the NaPi-IIa transporter.

Hyperkalemia with concomitant watery diarrhea: an unusual association.

Four patients presented to the emergency room with life-threatening hyperkalemia and concomitant watery diarrhea. Hypovolemia, acidosis, and renal insufficiency were present in all 4 cases. In 2 patients, hyperkalemia followed initiation of angiotensin-converting enzyme (ACE) inhibitor therapy, whereas 1 patient experienced hyperkalemia after a dose increase of an ACE inhibitor, and the fourth patient was on continuous ACE-inhibitor therapy at the time of the hyperkalemia episode. Two of the 3 patients with functioning kidneys required hemodialysis to correct the hyperkalemia, whereas the other patient was on long-term hemodialysis therapy. In the 2 patients in whom transtubular potassium (K+) gradients were available, their values ranged far below normal, indicating tubular failure to secrete K+. This abnormality was attributed to decreased distal delivery of sodium and water and to renin/angiotensin II/aldosterone blockade. It has been proposed that aldosterone blockade impairs the capacity of the colonic epithelial cells to secrete K+. In all 4 patients the watery diarrhea ceased in parallel with the correction of serum K+ to normal values. It is suggested that hyperkalemia, most likely by stimulating intestinal motility, induced the watery diarrhea in all 4 patients. The watery diarrhea, however, failed to compensate for the renal tubular failure to secrete K+.

Does type 2 diabetes mellitus delay renal failure in autosomal dominant polycystic kidney disease?

Autosomal dominant polycystic kidney disease (ADPKD) is a common renal disease without an effective therapeutic intervention to delay renal failure. Within kindreds, renal dysfunction often develops at a similar age in affected individuals, although there are known modifying factors. Two kindreds with ADPKD have shown a striking pattern of delayed onset of renal insufficiency in those individuals also suffering from type 2 diabetes mellitus. Eight nondiabetic patients with ADPKD had onset of dialysis or renal death at ages 38-52 years, (mean +/- SEM 46 +/- 1.9, n = 7) as compared with four diabetics who started dialysis or are still off dialysis at the age of 61 +/- 2.8 years (p < 0.01). Two of the four diabetics still have reasonable renal function at age 61 and 66. The diabetes was diagnosed at age 32 +/- 2 years and was treated with oral hypoglycemics for 19 +/- 2 years before institution of insulin. Cardiovascular disease dominated the clinical picture in the diabetics. In conclusion, onset of renal failure in ADPKD was delayed for over 15 years in individuals who also suffered from type 2 diabetes mellitus, in two ADPKD kindreds. Possible mechanisms are discussed, including glibenclamide inhibition of the cystic fibrosis transmembrane conductance regulator. The striking delay associated with type 2 diabetes mellitus in ADPKD induced renal failure should be evaluated further.

Parathyroid hormone-independent osteoclastic resorptive bone disease: a new variant of adynamic bone disease in haemodialysis patients.

BACKGROUND: Osteitis fibrosa cystica (OFC) caused by secondary hyperparathyroidism is the pre-eminent form of uraemic osteodystrophy. In recent years, however, new bone abnormalities have been described. Among them adynamic bone disease (ABD) has become a focus of growing interest. Marked suppression of dynamic bone measurements with normal or near-normal static bone-forming parameters are the hallmarks of this disorder. Depressed parathyroid hormone (PTH) levels, frequently evident in this entity, have been linked causally with low bone turnover. METHODS: We reviewed bone biopsy specimens from 96 patients with end-stage renal disease undergoing chronic haemodialysis. RESULTS: We found OFC in 50% of our patients, 20% had mixed bone disease, 24% showed bone morphology of ABD and a minority (6%) had osteomalacia, mostly due to aluminium accumulation. In the patients that were affected by ABD there was a distinct subgroup with bone morphology featuring a striking increase in osteoclast number and osteoclast surface, whereas the osteoid volume, osteoid thickness, osteoblast surface, tetracycline uptake and bone formation rates were diminished as in ordinary ABD. Similarly the PTH levels in this subgroup were low or undetectable. CONCLUSION: We describe patients undergoing chronic haemodialysis with static and dynamic bone forming parameters, indistinguishable from that of ABD, but differing from the classic ABD by the presence of increased osteoclastic bone resorption. The suppressed PTH levels in this subgroup suggests that factors other than PTH activate osteoclasts in some patients on chronic haemodialysis. Uraemic cytokines and/or toxic metabolites, including beta-microglobulin, may be involved in this disorder. The precise nature of this bone abnormality remains to be defined by further studies.