Early enhancement of fluid transport in rabbit proximal straight tubules after loss of contralateral renal excretory function

Role of mitochondria in reno-cardiac diseases: A study of bioenergetics, biogenesis, and GSH signaling in disease transition

Acute kidney injury (AKI) and chronic kidney disease (CKD) are global health burdens with rising prevalence. Their bidirectional relationship with cardiovascular dysfunction, manifesting as cardio-renal syndromes (CRS) types 3 and 4, underscores the interconnectedness and interdependence of these vital organ systems. Both the kidney and the heart are critically reliant on mitochondrial function. This organelle is currently recognized as a hub in signaling pathways, with emphasis on the redox regulation mediated by glutathione (GSH). Mitochondrial dysfunction, including impaired bioenergetics, redox, and biogenesis pathways, are central to the progression of AKI to CKD and the development of CRS type 3 and 4. This review delves into the metabolic reprogramming and mitochondrial redox signaling and biogenesis alterations in AKI, CKD, and CRS. We examine the pathophysiological mechanisms involving GSH redox signaling and the AMP-activated protein kinase (AMPK)-sirtuin (SIRT)1/3-peroxisome proliferator-activated receptor-gamma coactivator (PGC-1α) axis in these conditions. Additionally, we explore the therapeutic potential of GSH synthesis inducers in mitigating these mitochondrial dysfunctions, as well as their effects on inflammation and the progression of CKD and CRS types 3 and 4.

Extracellular Vesicles in Redox Signaling and Metabolic Regulation in Chronic Kidney Disease

Chronic kidney disease (CKD) is a world health problem increasing dramatically. The onset of CKD is driven by several mechanisms; among them, metabolic reprogramming and changes in redox signaling play critical roles in the advancement of inflammation and the subsequent fibrosis, common pathologies observed in all forms of CKD. Extracellular vesicles (EVs) are cell-derived membrane packages strongly associated with cell-cell communication since they transfer several biomolecules that serve as mediators in redox signaling and metabolic reprogramming in the recipient cells. Recent studies suggest that EVs, especially exosomes, the smallest subtype of EVs, play a fundamental role in spreading renal injury in CKD. Therefore, this review summarizes the current information about EVs and their cargos’ participation in metabolic reprogramming and mitochondrial impairment in CKD and their role in redox signaling changes. Finally, we analyze the effects of these EV-induced changes in the amplification of inflammatory and fibrotic processes in the progression of CKD. Furthermore, the data suggest that the identification of the signaling pathways involved in the release of EVs and their cargo under pathological renal conditions can allow the identification of new possible targets of injury spread, with the goal of preventing CKD progression.

Progressive Reduction in Mitochondrial Mass Is Triggered by Alterations in Mitochondrial Biogenesis and Dynamics in Chronic Kidney Disease Induced by 5/6 Nephrectomy

The five-sixth nephrectomy (5/6Nx) model is widely used to study the mechanisms involved in chronic kidney disease (CKD) progression. Mitochondrial impairment is a critical mechanism that favors CKD progression. However, until now, there are no temporal studies of the change in mitochondrial biogenesis and dynamics that allow determining the role of these processes in mitochondrial impairment and renal damage progression in the 5/6Nx model. In this work, we determined the changes in mitochondrial biogenesis and dynamics markers in remnant renal mass from days 2 to 28 after 5/6Nx. Our results show a progressive reduction in mitochondrial biogenesis triggered by reducing two principal regulators of mitochondrial protein expression, the peroxisome proliferator-activated receptor-gamma coactivator 1-alpha and the peroxisome proliferator-activated receptor alpha. Furthermore, the reduction in mitochondrial biogenesis proteins strongly correlates with the increase in renal damage markers. Additionally, we found a slow and gradual change in mitochondrial dynamics from fusion to fission, favoring mitochondrial fragmentation at later stages after 5/6Nx. Together, our results suggest that 5/6Nx induces the progressive reduction in mitochondrial mass over time via the decrease in mitochondrial biogenesis factors and a slow shift from mitochondrial fission to fusion; both mechanisms favor CKD progression in the remnant renal mass.

Cre/loxP approach-mediated downregulation of Pik3c3 inhibits the hypertrophic growth of renal proximal tubule cells

Nephron loss stimulates residual functioning nephrons to undergo compensatory growth. Excessive nephron growth may be a maladaptive response that sets the stage for progressive nephron damage, leading to kidney failure. To date, however, the mechanism of nephron growth remains incompletely understood. Our previous study revealed that class III phosphatidylinositol-3-kinase (Pik3c3) is activated in the remaining kidney after unilateral nephrectomy (UNX)-induced nephron loss, but previous studies failed to generate a Pik3c3 gene knockout animal model. Global Pik3c3 deletion results in embryonic lethality. Given that renal proximal tubule cells make up the bulk of the kidney and undergo the most prominent hypertrophic growth after UNX, in this study we used Cre-loxP-based approaches to demonstrate for the first time that tamoxifen-inducible SLC34a1 promoter-driven CreER^T2 recombinase-mediated downregulation of Pik3c3 expression in renal proximal tubule cells alone is sufficient to inhibit UNX- or amino acid-induced hypertrophic nephron growth. Furthermore, our mechanistic studies unveiled that the SLC34a1-CreER^T2 recombinase-mediated Pik3c3 downregulation inhibited UNX- or amino acid-stimulated lysosomal localization and signaling activation of mechanistic target of rapamycin complex 1 (mTORC1) in the renal proximal tubules. Moreover, our additional cell culture experiments using RNAi confirmed that knocking down Pik3c3 expression inhibited amino acid-stimulated mTORC1 signaling and blunted cellular growth in primary cultures of renal proximal tubule cells. Together, both our in vivo and in vitro experimental results indicate that Pik3c3 is a major mechanistic mediator responsible for sensing amino acid availability and initiating hypertrophic growth of renal proximal tubule cells by activation of the mTORC1-S6K1-rpS6 signaling pathway.

Mitochondrial bioenergetics, redox state, dynamics and turnover alterations in renal mass reduction models of chronic kidney diseases and their possible implications in the progression of this illness

Nowadays, chronic kidney disease (CKD) is considered a worldwide public health problem. CKD is a term used to describe a set of pathologies that structurally and functionally affect the kidney, it is mostly characterized by the progressive loss of kidney function. Current therapeutic approaches are insufficient to avoid the development of this disease, which highlights the necessity of developing new strategies to reverse or at least delay CKD progression. Kidney is highly dependent on mitochondrial homeostasis and function, consequently, the idea that mitochondrial pathologies could play a pivotal role in the genesis and development of kidney diseases has risen. Although many research groups have recently published studies of mitochondrial function in acute kidney disease models, the existing information about CKD is still limited, especially in renal mass reduction (RMR) models. This paper focuses on reviewing current experimental information about the bioenergetics, dynamics (fission and fusion processes), turnover (mitophagy and biogenesis) and redox mitochondrial alterations in RMR, to discuss and integrate the mitochondrial changes triggered by nephron loss, as well as its relationship with loss of kidney function in CKD, in these models. Understanding these mechanisms would allow us to design new therapies that target these mitochondrial alterations.

Compensatory Structural and Functional Adaptation After Nephrectomy in Obese Patients According to Waist Circumference

OBJECTIVE

To investigate whether the postnephrectomy renal adaptation mechanism, focused on functional hyperfiltration as well as structural hypertrophy, was affected by abdominal obesity.

MATERIALS AND METHODS

We retrospectively evaluated 358 patients who underwent simple or radical nephrectomy and nephroureterectomy between 2009 and 2013. Patients were classified according to waist circumference (WC), with values >102 cm in men and >88 cm in women considered high (obesity). Functional renal volume (FRV) was measured using computed tomography performed preoperatively and 6 months postoperatively to evaluate the degree of remnant kidney hypertrophy. The degree of hyperfiltration was calculated from the difference between the preoperative and postoperative glomerular filtration rate (GFR)/FRV.

RESULTS

The mean preoperative GFR, FRV, and GFR/FRV were 72.1 mL/min/1.73 m², 282.8 cm³, and 0.25 mL/min/1.73 m²/cm³, respectively. The percent GFR reduction was significantly greater in the high WC group (high, 25.9% vs normal, 16.0%, P = .036), although the degree of hypertrophic volume in the remnant kidney showed no difference. The change in GFR/FRV was statistically lower in the high WC group (high, 25.7% vs normal, 40.2%, P = .009). The factors associated with postoperative increased GFR/FRV were low preoperative GFR, proteinuria, high predictive preserved functional parenchymal volume ratio, absence of hypertension, increased levels of high-density lipoprotein cholesterol, and normal WC (all P < .05).

CONCLUSION

Patients with high WC might have a large reduction in postoperative renal function, owing to a lower degree of functional hyperfiltration.

The effect of cis-Diamminedichloroplatinum II on Na+ and K+ transport in the rabbit cortical collecting duct

cis-Diamminedichloroplatinum II (CDDP) is an antineoplastic drug against solid malignant tumors. However, its clinical use is limited by nephrotoxicity. CDDP also causes hypokalemia and in vivo microperfusion method have demonstrated that luminal CDDP increases K+ secretion by hyperpolarization of the transepithelial voltage difference through stimulating Na+ transport in the distal segments. However, there is no direct evidence for this mechanism. We therefore examined the effect of luminal CDDP on Na+ and K+ transport in the rabbit cortical collecting duct (CCD) using in vitro isolated tubular microperfusion. Luminal CDDP hyperpolarized the transepithelial voltage difference (V(T)) in a dose-dependent manner at concentrations from 10(-5) M to 10(-3) M and at 10(-3) M CDDP, V(T) was hyperpolarized from -11.6+/-2.3 mV to -16.6+/-3.3 mV (P<0.001). A concentration of 10(-5) M ouabain, 10(-4) M amiloride and 2 mM BaCl2 all completely abolished CDDP-induced hyperpolarization. To confirm the mechanism, Na+ and K+ flux were measured in the presence of 10(-3) M CDDP. CDDP decreased net K+ secretion from -22.2+/-5.7 to -15.2+/-2.9 pmol mm(-1) min(-1) (P<0.01) without any effect on the lumen-to-bath isotope flux of Na+ (52.6+/-10.6 to 52.1+/-10.7 pmol mm(-1) min(-1)). These data suggest that luminal CDDP hyperpolarizes V(T) primarily by inhibiting K+ conductance but did not influence Na+ transport of the luminal membrane. We conclude that the CCD does not play a role in CDDP-induced hypokalemia when CDDP is applied from the luminal side.

Acute ureteral obstruction and glomerulotubular function in rats

Urinary tract obstruction is a common cause of acute renal failure (ARF). During unilateral ureteral obstruction (UUO) arteriolar vasoconstriction, increase in tubular pressure, and ultrafiltrate retrodiffusion occur. We studied renal function of rats with surgical UUO for 24 hr. After this period of UUO, the contralateral kidney was removed and the right ureter was deobstructed. The control uninephrectomized group consisted of normal rats submitted to left uninephrectomy (UNx). Functional studies were performed 12 and 24 hr, and 7 days after deobstruction and UNx. We measured creatinine clearance, and fractional excretion of sodium and lithium. Using conventional formulas we calculated fractional proximal and distal sodium reabsorption. Initially we observed a reduction in glomerular filtration rate (GFR) after deobstruction (12 and 24 hr). However, after 7 days, the GFR was significantly higher in deobstructed rats than in controls (340.3 +/- 18.3 vs. 286.4 +/- 9.3 microL/min/100 g, p < 0.01). The dry kidney weight was also increased in these rats. The fractional sodium excretion was increased in deobstructed rats, mainly in early studies (12 and 24 hr). Whereas fractional proximal reabsorption was reduced in both groups, the fractional distal reabsorption was significantly decreased in the deobstructed group compared to UNX controls (93.9 +/- 0.9 vs. 98.9 +/- 0.1% after 24 hr, p < 0.01). Our data showed that UUO influenced both glomerular and tubular functions. A salient finding was the overcorrection of GFR 7 days after deobstruction. The renal release of hormones and growth factors could mediate these alterations in renal function through their vascular, tubular, and proliferative actions.

Electrical properties of the rabbit cortical collecting duct from obstructed and contralateral kidneys after unilateral ureteral obstruction

Electrophysiological techniques were used to determine the electrical properties of the collecting duct (CD) cell in the isolated cortical collecting duct from obstructed (UUOOK) and contralateral (UUOCK) kidneys in rabbits 24 h after unilateral ureteral obstruction (UUO); results were compared with those from sham-operated kidneys. The lumen-negative transepithelial voltage and the basolateral membrane voltage (VB) were decreased in the UUOOK, and increased in the UUOCK. The transepithelial conductance (GT) was decreased in parallel with an increase in the fractional apical membrane resistance (fRA) and a decrease in apical membrane conductance in the UUOOK. By contrast, the GT was increased in parallel with increases in apical and basolateral membrane conductances in the UUOCK. The amiloride-sensitive changes in apical membrane voltage (VA), GT and fRA were lower in the UUOOK, but greater in the UUOCK. The changes in VA and GT upon raising the perfusate K+ concentration and upon addition of luminal Ba2+ were decreased in the UUOOK, and increased in the UUOCK. Addition of ouabain to the bath resulted in a smaller depolarization of VB in the UUOOK, but in a greater depolarization in the UUOCK. Upon lowering bath Cl-, the change in basolateral membrane electromotive force (delta EMF) was increased in the UUOOK, and decreased in the UUOCK. Reversely, upon raising bath K+, the delta EMF was decreased in the UUOOK, and increased in the UUOCK. We conclude: (a) the conductances of Na+ and K+ in the apical membrane, and active Na(+)-K+ pump activity and relative K+ conductance in the basolateral membrane are decreased in the UUOOK, and increased in the UUOCK; (b) the relative basolateral membrane Cl- conductance was increased in the UUOOK, and decreased in the UUOCK.

Tubular sodium handling and tubuloglomerular feedback in compensatory renal hypertrophy

Tubular sodium handling and tubuloglomerular feedback (TGF) activity were assessed in established compensatory renal hypertrophy in Sprague Dawley rats. Hyperfiltration at the level of the single nephron was confirmed 4-6 weeks following a reduction in renal mass. TGF activity, determined as the difference between late proximal and early distal measurements of single-nephron glomerular filtration rate (SNGFR), was significantly increased in compensatory renal hypertrophy, being 7.8 +/- 1.0 vs 23.3 +/- 1.9 vs 25.5 +/- 2.6 nl/min (P for analysis of variance less than 0.05) following sham operation, unilateral nephrectomy, and 1 1/3 nephrectomy, respectively. Enhanced net tubular Na transport was also observed, with total Na reabsorption up to the late proximal site being 1.8 +/- 0.2 vs 2.7 +/- 0.1 vs 3.1 +/- 0.3 nmol/min (P less than 0.05), and to the early distal site being 3.4 +/- 0.5 vs 5.8 +/- 0.6 vs 7.9 +/- 0.8 nmol/min (P less than 0.05) in the three animal groups respectively. Comparison of proximal tubular length demonstrated a 71.9 +/- 8.1% increase in uninephrectomised vs sham-operated animals. This increase was proportionately greater than the increase in proximal Na reabsorption (50.0 +/- 4.0%) observed in the corresponding animal groups. Concurrent electron microprobe experiments in uninephrectomised and sham-operated animals demonstrated that the proximal tubular intracellular Na concentration was significantly lower following uninephrectomy (16.8 +/- 0.6 vs 18.9 +/- 0.5 mmol/kg wet weight, P less than 0.01), in association with evidence of reduced basolateral Na/K-ATPase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal metallothionein metabolism after a reduction of renal mass. I. Effect of unilateral nephrectomy and compensatory renal growth on basal and metal-induced renal metallothionein metabolism

The effects of unilateral nephrectomy and compensatory renal growth on renal metallothionein metabolism were evaluated in the present study. In rats, the renal content of metallothionein increased in proportion to the increase in renal mass after unilateral nephrectomy and compensatory renal growth. However, when zinc was used to induce the synthesis of renal metallothionein, the remnant kidney in uninephrectomized (NPX) rats produced significantly greater amounts of metallothionein on a per gram kidney basis than a normal kidney in sham-operated (SO) rats. In both NPX and SO rats, zinc pretreatment caused metallothionein synthesis to increase primarily in the renal cortex and renal outer stripe of the outer medulla. Zinc pretreatment also changed the pattern for the intrarenal accumulation of inorganic mercury in NPX rats. After pretreatment with zinc, the accumulation of inorganic mercury predominated in the renal cortex rather than in the outer stripe of the outer medulla in the NPX rats. In addition, both NPX and SO rats were afforded complete protection against the nephrotoxic effects of a low, toxic dose of inorganic mercury when they were pretreated with inorganic zinc. The protection is postulated to be related to the alteration in the pattern of renal accumulation of inorganic mercury. In conclusion, the capacity to synthesize metallothionein increases significantly in rats after they have undergone unilateral nephrectomy and compensatory renal growth. The increased capacity of the remnant kidney to synthesize metallothionein may involve adaptive changes both in transcriptional and/or translational controls of metallothionein synthesis.

Renal accumulation and intrarenal distribution of inorganic mercury in the rabbit: effect of unilateral nephrectomy and dose of mercuric chloride

The effects of unilateral nephrectomy and dose of mercuric chloride on the short-term renal accumulation and intrarenal distribution of inorganic mercury were studied in the rabbit. The renal accumulation of inorganic mercury, on a per gram basis, was increased in uninephrectomized (NPX) rabbits compared with that in sham-operated (SO) rabbits 24 h after the animals received either a nontoxic 2.0 mumol/kg or nephrotoxic 4.0 mumol/kg dose of mercuric chloride. In the NPX rabbits given the 2.0 mumol/kg dose of mercuric chloride, the increased accumulation of inorganic mercury was due to increased accumulation of mercury in the outer stripe of the outer medulla. In the NPX rabbits given the 4.0 mumol/kg dose of mercuric chloride, the increased renal accumulation of mercury appeared to be due to increased accumulation of mercury in both the renal cortex and outer stripe of the outer medulla. Interestingly, no differences in the renal accumulation of inorganic mercury were found between NPX and SO rabbits given a low nontoxic 0.5 mumol/kg dose of mercuric chloride. As the dose of mercuric chloride was increased from 0.5 to 4.0 mumol/kg, the percent of the administered dose of mercury that accumulated in each gram of renal tissue decreased substantially. The findings in the present study indicate that the renal accumulation of inorganic mercury increases after unilateral nephrectomy when certain nontoxic and nephrotoxic doses of mercuric chloride are administered. In addition, they indicate that the percent of the administered dose of mercury that accumulates in the renal tissue of both NPX and SO rabbits decreases as the dose of mercuric chloride is increased.

Increased Na/H antiporter and Na/3HCO3 symporter activities in chronic hyperfiltration. A model of cell hypertrophy

The effect of chronic hyperfiltration, a model of cell hypertrophy, on H/HCO3 transporters was examined in the in vivo microperfused rat proximal tubule. Hyperfiltration was induced by uninephrectomy with subsequent increased dietary protein. After 2 wk the hyperfiltration group had a higher glomerular filtration rate (2.21 +/- 0.13 vs. 1.48 +/- 0.12 ml/min), associated with increased kidney weight (1.71 +/- 0.05 vs. 1.23 +/- 0.04 g). HCO3 absorptive rate measured in tubules perfused with an ultrafiltrate-like solution (25 mM HCO3) was higher in the hyperfiltration group (183 +/- 17 vs. 109 +/- 16 pmol/mm per min). The activities of the apical membrane Na/H antiporter and basolateral membrane Na/3HCO3 symporter were assayed using the measurement of cell pH [(2'7')-bis(carboxyethyl)-(5,6)-carboxyfluorescein] in the doubly microperfused tubule in the absence of contact with native fluids. After 2 wk of hyperfiltration Na/H antiporter activity, assayed as the effect of luminal Na removal on cell pH, was increased 114%. Basolateral membrane Na/3HCO3 symporter activity, assayed as the effect of a decrease in peritubular [HCO3] (25 to 5 mM) or in peritubular [Na] (147 to 25 mM) in the absence of luminal and peritubular chloride, was increased 77 and 113%, respectively, in the hyperfiltration group. Steady-state cell pH, measured with physiologic, ultrafiltrate-like luminal and peritubular perfusates, was significantly higher in the hyperfiltration group (7.27 +/- 0.02 vs. 7.14 +/- 0.03). In similar studies, performed 24 h after uninephrectomy and protein feeding, kidney weight was increased 10%, Na/H antiporter activity 39%, and Na/3HCO3 symporter activity 46%. At this time cell pH was not different between the two groups. The results demonstrate that chronic hyperfiltration is associated with parallel increases in Na/H antiporter and Na/3HCO3 symporter activities. If a decrease in cell pH is the signal that triggers these adaptations, it occurs early, and the adaptations can be maintained in the absence of sustained cell acidification.

Glomerular and tubular adaptive responses to acute nephron loss in the rat. Effect of prostaglandin synthesis inhibition

These studies, using in vivo micropuncture techniques in the Munich-Wistar rat, document the magnitude of changes in glomerular and tubular function and structure 24 h after approximately 75% nephron loss (Nx) and compared these results with those obtained in sham-operated rats. The contribution of either nephron hypertrophy or renal prostaglandin to these adjustments in nephron function was also explored. After acute Nx, single nephron GFR (SNGFR) was increased, on average by approximately 30%, due primarily to glomerular hyperperfusion and hypertension. The approximately 45% reduction in preglomerular and the constancy in postglomerular vascular resistances was entirely responsible for these adaptations. Although increases in fluid reabsorption in proximal convoluted tubules correlated closely with increase in SNGFR, the fractional fluid reabsorption between late proximal and early distal tubular segments was depressed. Nephron hypertrophy could not be substantiated based on either measurements of protein content in renal tissue homogenates or morphometric analysis of proximal convoluted tubules. However, acute Nx was associated with increased urinary excretory rates per functional nephron for 6-keto-PGF1 alpha and TXB2. Prostaglandin synthesis inhibition did not affect function in control nephrons, but this maneuver was associated with normalization of glomerular and tubular function in remnant nephrons. The results suggest that enhanced synthesis of cyclooxygenase-dependent products is one of the earliest responses to Nx, and even before hypertrophy the pathophysiologic effects of prostaglandin may be important contributors to the adaptations in remnant nephron function.

Glucagon stimulates chloride transport independently of cyclic AMP in the rat medullary TAL

The effect of glucagon on chloride transport was studied in the rat medullary thick ascending limb (MTAL) perfused in vitro. In the bath, 10(-6) M glucagon increased the efflux coefficient of Cl (KeCl) from 6.88 +/- 0.21 x 10(5) to 9.65 +/- 0.38 x 10(-5) cm.sec -1 (P less than 0.01) without changing the influx coefficient (KiCl; 2.87 +/- 0.54 x 10(-5) in control vs. 2.83 +/- 0.57 x 10(-5) cm.sec-1 with glucagon) or transepithelial potential difference (4.8 +/- 0.76 in control vs. 5.0 +/- 0.71 mV with glucagon). A physiological concentration of glucagon (10(-8), (10(-10) M) also increased chloride efflux significantly. Pretreatment of tubules with luminal furosemide (10(-5) M) and/or basolateral ouabain (10(-4) M) completely abolished the effect of glucagon. In isolated MTALs incubated in the same medium as that used in the microperfusion study, 10(-6) M glucagon stimulated cAMP production by 255.2 +/- 33.7% (P less than 0.01). However, neither dibutyryl cAMP (10(-3), 10(-4) M) nor forskolin (10(-4), 10(-6) M) increased the chloride efflux. It is concluded that: 1) Glucagon stimulates net Cl reabsorption by increasing Cl efflux in the rat MTAL; and 2) cyclic AMP is not responsible for this effect of glucagon.

Increased sodium transport by cortical collecting tubules from remnant kidneys

To determine whether intrinsic changes in cortical collecting tubule (CCT) transport contribute to the maintenance of sodium and acid-base balance after loss of renal mass, we studied transport functions in isolated perfused CCT from rabbit remnant kidneys. The rabbits were sacrificed three weeks after surgical reduction of renal mass (by 3/4 to 7/8) at which time they were mildly azotemic but had no systemic electrolyte or acid-base disturbances. When perfused by standard methods in vitro, CCT from remnant kidneys exhibited sodium transport rates (lumen-to-bath 22Na-flux) approximately twice as high as those in CCT from control animals (111 +/- 19 vs. 54 +/- 7 pmol/min mm, P less than 0.02). A similar difference was present in the ouabain-sensitive sodium fluxes (81 +/- 16 vs. 39 +/- 8 pmol/min mm, P less than 0.05). In contrast, there were no significant differences in net bicarbonate transport. Significant hypertrophy of the remnant kidney CCT was reflected by 30 to 45% increases in tubule diameters. To examine the possible role of differences in food intake, we studied a separate group of weight-matched, pair-fed sham-operated and remnant kidney rabbits. Similar differences in total and ouabain-sensitive 22Na-flux, and in tubule size persisted in the pair-fed animals. A dissociation between active sodium transport and tubule hypertrophy was documented in the outer medullary collecting tubule: despite the lack of active sodium transport, hypertrophy was present. Our studies show that loss of renal mass results in a selective augmentation of certain transport processes in the CCT, implying selective or specific signals and mechanisms.

The solitary kidney: a model of chronic hyperfiltration in humans

Reduction of renal mass (11/12) in rats leads to progressive azotemia, proteinuria, and hypertension. Less extensive renal ablation resulting from uninephrectomy also accelerates the progression of focal glomerulosclerosis (FGS) induced by experimental diabetes, renal irradiation, aminonucleoside nephrosis, or aging. The consequence of the absence of one kidney in man are examined in three different clinical situations. Unilateral renal agenesis seems to predispose to the development of FGS, but most reports include isolated cases and the true incidence of FGS is not known. The solitary kidney following uninephrectomy for acquired unilateral disease undergoes a compensatory rise in glomerular filtration rate (GFR) that remains stable for several decades. Finally, kidney donors followed for over 2 decades show unimpaired GFR, elevated at 70% to 80% of the normal (two-kidney) GFR. Some donors develop mild, nonprogressive proteinuria. Their incidence of hypertension matches that in the control population. Thus, hyperfiltration secondary to 50% reduction of renal mass in humans does not lead to loss of function of the remaining parenchyma.

Short-term and long-term stimulation of Na+-H+ exchange in cortical brush-border membranes during compensatory growth of the rat kidney

The effect of unilateral nephrectomy on Na+-H+ exchange in rat renal cortical brush-border membrane vesicles (BBMV) was studied by the method of acridine orange fluorescence quenching. The exchanger activity in BBMV from remnant kidney increased rapidly by 70-75% within first 30 min following uninephrectomy. Only a slight further increase was found in later stages of renal growth, i.e. 30 min to 7 days following uninephrectomy. The changes in antiporter activity were restricted to Vmax, whereas the Km for Na+ was similar in control and compensatory growing kidney. The increase of Na+-H+ exchange at 15 min was not affected by actinomycin D in vivo, whereas the increase at 48 h was completely abolished indicating that protein synthesis could be involved in the late, but not in the initial stimulation of renal Na+-H+ exchange. The late, but not the initial stimulations of Na+-H+ exchange were associated with elevated activities of cortical (Na++K+)-ATPase indicating that changes in antiporter activity precede those in the (Na++K+)-pump. The early stimulation of Na+-H+ exchange in BBMV in one kidney was induced also by the occlusion of blood flow through the contralateral kidney for 15 min, without removing it. Thirty min after the occlusion was removed and the reflow established, the Na+-H+ exchange in BBMV from the intact kidney decreased to the control values. The observed modulations in renal Na+-H+ exchanger may be regulated by phosphorylation-dephosphorylation events.(ABSTRACT TRUNCATED AT 250 WORDS)

Mercuric chloride-induced nephrotoxicity in the rat following unilateral nephrectomy and compensatory renal growth

The nephropathy induced by mercuric chloride was assessed in unilaterally nephrectomized (NPX) and sham-operated (SO) rats using histological and urinalysis techniques. This assessment was carried out in order to test whether or not rats are more susceptible to the nephrotoxic effects of mercuric chloride after unilateral nephrectomy and a period allowing for compensatory renal growth. Twelve days after surgery both NPX and SO rats were given a single 1.5, 2.0 or 2.5 mumol/kg dose of mercuric chloride (i.v.). Twenty-four hours after the 1.5 or 2.0 mumol/kg dose of mercuric chloride was administered, cellular and tubular necrosis in the pars recta segments of proximal tubules in the outer medulla was more severe in NPX rats than in SO rats. Moreover, the urinary excretion of a number of cellular enzymes (e.g. lactate dehydrogenase) and plasma solutes (e.g. albumin) was greater in NPX rats than in SO rats. At the 2.5 mumol/kg dose of mercuric chloride, renal tubular damage was quite extensive in both groups of rats; to such an extent that possible differences in renal tubular damage between the NPX and SO rats could not be determined histologically. However, the urinary excretion of alanine aminopeptidase was greater in the NPX rats than in the SO rats. Therefore, based on the aforementioned findings, rats that have undergone and adapted to a reduction in renal mass (i.e. unilateral nephrectomy) appear to be more vulnerable to the nephrotoxic effects of mercuric chloride than rats with two normal kidneys.

Acute adaptative changes to unilateral nephrectomy in humans

Renal function was monitored in 20, living-related kidney donors before and after uninephrectomy. Urinary protein excretion and retinoid metabolism respectively were studied in 10 and 6 of these donors. The functional adaptation was characterized by an increase in glomerular filtration rate and tubular function, which began in the first two days after uninephrectomy. Changes in tubular function were also demonstrated by significant increases in the urinary excretion of beta 2 microglobulin (beta 2M), retinol binding protein (RBP), kappa and lambda light chains of immunoglobulins. In addition, a protein identical to or homologous to cellular retinoic acid binding protein (CRABP), appeared in the urine after nephrectomy. We did not find CRABP in serum samples either before or after nephrectomy, suggesting that urinary CRABP was synthesized by the remaining kidney. Increases in serum levels of Vitamin A and RBP were also observed in the post-nephrectomy period. These modifications in retinol metabolism suggest that these substances could have a role as renotropic growth factors in compensatory hypertrophy.

Phorbol myristate acetate, dioctanoylglycerol, and phosphatidic acid inhibit the hydroosmotic effect of vasopressin on rabbit cortical collecting tubule

We explored the role for protein kinase C (PKC) in modulating vasopressin (AVP)-stimulated hydraulic conductivity (Lp) in rabbit cortical collecting tubule (CCT) perfused in vitro at 37 degrees C. In control studies, 10 microU/ml AVP increased Lp (mean +/- SE, X 10(-7) centimeters/atmosphere per second) from 4.4 +/- 0.9 to 166.0 +/- 10.4. Pretreatment with dioctanoylglycerol (DiC8) suppressed AVP stimulated peak Lp (peak Lp, 21.9 +/- 3.1). Pretreatment with 10(-9) and 10(-7) M 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) also blocked the increase in Lp in a dose-dependent fashion (peak Lp, 59.3 +/- 7.5 and 18.6 +/- 4.8, respectively). Inactive phorbol ester, 4 alpha-phorbol 12 beta,13 alpha-didecanoate (10(-7) M), had no effect. PMA also suppressed the increase in Lp induced by 10(-4) M 8-p-chlorophenylthio-cyclic AMP (CcAMP): peak Lp was 169.4 +/- 14.9 in control, 79.2 +/- 5.5 with 10(-9) M PMA, and 25.7 +/- 2.9 with 10(-7) M PMA. Furthermore, when 10(-7) M PMA was added to the bath 10 min after exposure to AVP, the Lp response to AVP was blocked. Peak Lp was 52.4 +/- 9.6 with PMA vs. 165.1 +/- 10.0 in control. Phosphatidic acid (PA), which is thought to stimulate phosphatidylinositol (PI) turnover, produced similar inhibitory effects on AVP as well as CcAMP-stimulated Lp: PA suppressed 10-microU/ml AVP-induced peak Lp from a control value of 159.6 +/- 7.9 to 88.9 +/- 15.8, and 10(-4) M CcAMP induced peak Lp from 169.4 +/- 14.9 to 95.5 +/- 7.7. We conclude that PMA, at concentrations known to specifically activate PKC, suppresses the hydroosmotic effect of AVP on CCT; This suppression is primarily a post-cAMP event; Inhibition of AVP-stimulated Lp by DiC8 and PA also suggests an inhibitory role for the PKC system; The ability of pre- and post-AVP administration of PMA to blunt the AVP response suggests that agents that act through modulation of PI turnover in CCT may regulate the hydroosmotic effect of AVP.

Enhanced accumulation of injected inorganic mercury in renal outer medulla after unilateral nephrectomy

The effects of unilateral nephrectomy on renal accumulation and intrarenal distribution of mercury following a single injection of mercuric chloride (HgCl2, 0.5 mumol Hg/kg body wt, ip) were evaluated in the rat. In rats injected with HgCl2 immediately after nephrectomy or 10, 28, or 43 days after nephrectomy, the accumulation of mercury in the renal outer medulla was significantly greater than in respective sham-operated control rats. The increased accumulation of mercury in renal outer medulla was evident at 24 hr after injection of HgCl2 and persisted for at least 72 hr. The effect appeared to be a phenomenon associated specifically with the renal outer medulla in that elevated concentrations of mercury in the outer medulla were observed in the absence of similar elevations in the renal cortex, inner medulla, liver, and blood or in the whole body mercury content. Urinary excretion of mercury was unaffected by nephrectomy. Thus, associated with unilateral nephrectomy are changes in the renal accumulation and intrarenal distribution of systemically administered inorganic mercury that persist long after the rapid phase of compensatory renal growth (0-7 days) is completed.

Load dependence of proximal tubular fluid and bicarbonate reabsorption in the remnant kidney of the Munich-Wistar rat

Studies were undertaken to characterize the pattern of proximal tubular fluid (APRH2O) and bicarbonate reabsorption (APRHCO3) in the remnant kidney of euvolemic Munich-Wistar rats. The remnant kidney rats were placed on a diet containing either low or normal protein. Collections were obtained in the early, mid-, and late proximal convoluted tubule. Single nephron glomerular filtration rate (SNGFR) increased from 40.2 nl/min in controls to 58.8 nl/min in low protein remnant kidney and 78.1 nl/min in normal protein remnant kidney rats. The filtered load of bicarbonate was 1,272, 1,641, and 2,013 pmol/min, in the three groups, respectively. APRH2O and APRHCO3 increased nearly in parallel. Most of the increase in reabsorption occurred in the early proximal tubule. Tubular hypertrophy could account for at least 20-40% of the increase in reabsorption, but the majority of the increase appeared to be a delivery-dependent response similar to that observed in normal rats after an acute increase in SNGFR.

Stimulation of Na+/H+ antiport is an early event in hypertrophy of renal proximal tubular cells

Renal hypertrophy in vivo is achieved by an increase in protein content per cell and an increase in cell size with minimal hyperplasia. Hypertrophied renal tubular cells remain quiescent and demonstrate an increase in transcellular transport rates. This situation was simulated in vitro by exposing a confluent, quiescent primary culture of rabbit renal proximal tubular cells to either insulin, prostaglandin E1, or hypertonic NaCl for 24 or 48 hr. Protein per cell increased by 20-30% with little or no increase in [3H]thymidine incorporation into DNA. Mean cell volume was also increased in insulin- and hypertonic NaCl-treated but not in prostaglandin E1-treated cells. The lag period required to initiate DNA synthesis by a combination of insulin and hydrocortisone was the same in control and hypertrophied cells, indicating a quiescent state of the latter. Two hours of exposure to the growth stimuli increased amiloride-sensitive Na+ uptake, Na-dependent H+ efflux, and ouabain-sensitive Rb+ uptake, indicating that stimulation of Na+/H+ antiport (exchange) occurs as an early event in their action. Hypertrophied cells continued to demonstrate enhanced Na+/H+ antiport after the growth stimuli were removed for 3 hr, by which time their acute effects are reversed.

Adaptation of Na+-H+ exchange in renal microvillus membrane vesicles. Role of dietary protein and uninephrectomy

The ablation of renal mass and institution of a high protein diet both lead to renal cortical hypertrophy and increased glomerular filtration rate (GFR). We studied Na+ transport in rat microvillus membrane vesicles isolated from uninephrectomized or sham operated rats fed 6% (low), 24% (standard), or 40% (high) protein diets. The feeding of high protein, as compared with low protein, was associated with a 50% increase in rates of pH-stimulated 22Na+ transport in isolated vesicles from sham and uninephrectomized animals. Values for the standard protein diet were intermediate to values for high and low protein. At each level of dietary protein intake, vesicular Na+ transport was greater in the uninephrectomized than in sham rats. The high protein diet was also associated with increased vesicular 22Na+ flux inhibitable by 1 mM amiloride. Increases in total and amiloride sensitive flux were also noted in the absence of a pH gradient. Conductive Na+ and H+ transport were not altered, nor were sodium-glucose and sodium-alanine cotransport. Kinetic studies revealed evidence for an increased Vmax of Na+-H+ exchange in uninephrectomized animals fed a 40 vs. a 6% protein diet whereas Km was unchanged. Supplements of NaHCO3 in the 40% protein diet, to adjust for an increased rate of net acid excretion, did not prevent the increased rates of Na+-H+ exchange. However, treatment with actinomycin D (0.12 mg/kg) prevented the increased Na+-H+ activity as well as the increased renal mass and GFR noted 24 h after unilateral nephrectomy. Na+-H+ exchange rate was closely correlated with GFR (r = 0.961; P less than 0.005) and renal mass (r = .986; P less than 0.001). These observations provide evidence for modification of the luminal membrane Na+-H+ exchanger in response to changes in dietary protein content and nephron number.