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

Age and Pre-operative HbA1c levels affect renal function compensation in living kidney donors

OBJECTIVES

About 90% of Japanese kidney transplantations are conducted from living donors, and their safety and the maintenance of their renal function are critical. This study aims to identify factors that affect the compensation of renal function in living kidney donors after donor nephrectomy.

METHOD

In a retrospective cohort study, we reviewed data from 120 patients who underwent nephrectomy as living kidney transplant donors in our department from 2012 to 2021. Univariable and multivariable linear regression analyses were performed for donor factors affecting renal function after donor nephrectomy.

RESULT

The multivariable linear regression model revealed that the donor's age (p = 0.025), preoperative estimated Glomerular Filtration Rate (eGFR) (p < 0.001), and hemoglobin A1c (HbA1c) (p = 0.043) were independent risk factors for eGFR at six months after nephrectomy. The eGFR deterioration was more strongly associated with age in females than in males, whereas higher HbA1c values were more strongly associated with eGFR deterioration in males. Higher donor age and higher HbA1c each enhance the deterioration of eGFR six months after living donor nephrectomy. The data suggest that old age in especially female donors and preoperative higher HbA1c in male donors have a harmful impact on their renal function compensation.

Divergent roles of angiotensin II upon the immediate and sustained increases of renal blood flow following unilateral nephrectomy

Although the molecular and functional responses related to renal compensatory hypertrophy after unilateral nephrectomy (UNX) has been well described, many aspects of these events remain unclear. One question is how the remaining kidney senses the absence of the contralateral organ, and another is what the role of the renin-angiotensin system is in these responses. Both acute anesthetized and chronic unanesthetized experiments were performed using the angiotensin II type 1 receptor blocker losartan and the renin inhibitor aliskiren to determine the contribution of the renin-angiotensin system to immediate changes and losartan for chronic changes of renal blood flow (RBF) and the associated hypertrophic events in male Sprague-Dawley rats. Chronic experiments used implanted RBF probes and arterial catheters for continuous data collection, and the glomerular filtration rate was determined by noninvasive transcutaneous FITC-sinistrin measurements. The results of the acute experiments found that RBF increased nearly 25% (4.6 ± 0.5 to 5.6 ± 0.6 mL/min/g kidney wt) during the first 15 min following UNX and that this response was abolished by losartan (6.7 ± 0.7 to 7.0 ± 0.7 mL/min/g kidney wt) or aliskiren (5.8 ± 0.4 to 6.0 ± 0.4 mL/min/g kidney wt) treatment. Thereafter, RBF increased progressively over 7 days, and kidney weight increased by 19% of pre-UNX values. When normalized to kidney weight determined at day 7 after UNX, RBF was not significantly different from pre-UNX levels. Semiquantification of CD31-positive capillaries revealed increases of the glomeruli and peritubular capillaries that paralleled the kidney hypertrophy. None of these chronic changes was inhibited by losartan treatment, indicating that neither the compensatory structural nor the RBF changes were angiotensin II type 1 receptor dependent.NEW & NOTEWORTHY This study found that the immediate increases of renal blood flow (RBF) following unilateral nephrectomy (UNX) are a consequence of reduced angiotensin II type 1 (AT1) receptor stimulation. The continuous monitoring of RBF and intermittent measurement of glomerular filtration rate (GFR) in conscious rats during the 1-wk period of rapid hypertrophy following UNX provided unique insights into the regulation of RBF and GFR when faced with increased metabolic loads. It was found that neither kidney hypertrophy nor the associated increase of capillaries was an AT1-dependent phenomenon.

Diverse Roles of Mitochondria in Renal Injury from Environmental Toxicants and Therapeutic Drugs

Mitochondria are well-known to function as the primary sites of ATP synthesis in most mammalian cells, including the renal proximal tubule. Other functions have also been associated with different mitochondrial activities, including the regulation of redox status and the initiation of mitophagy and apoptosis. Mechanisms for the membrane transport of glutathione (GSH) and various GSH-derived metabolites across the mitochondrial inner membrane of renal proximal tubular cells are critical determinants of these functions and may serve as pharmacological targets for potential therapeutic approaches. Specific interactions of reactive intermediates, derived from drug metabolism, with molecular components in mitochondria have been identified as early steps in diverse forms of chemically-induced nephrotoxicity. Applying this key observation, we developed a novel hypothesis regarding the identification of early, sensitive, and specific biomarkers of exposure to nephrotoxicants. The underlying concept is that upon exposure to a diverse array of environmental contaminants, as well as therapeutic drugs whose efficacy is limited by nephrotoxicity, renal mitochondria will release both high- and low-molecular-weight components into the urine or the extracellular medium in an in vitro model. The detection of these components may then serve as indicators of exposure before irreversible renal injury has occurred.

Compensatory renal hypertrophy following nephrectomy: When and how?

Following surgical removal of one kidney, the other enlarges and increases its function. The mechanism for the sensing of this change and the growth is incompletely understood but begins within days and compensatory renal hypertrophy (CRH) is the dominant contributor to the growth. In many individuals undergoing nephrectomy for cancer or kidney donation this produces a substantial and helpful increase in renal function. Two main mechanisms have been proposed, one in which increased activity by the remaining kidney leads to hypertrophy, the second in which there is release of a kidney specific factor in response to a unilateral nephrectomy that initiates CRH. Whilst multiple growth factors and pathways such as the mTORC pathway have been implicated in experimental studies, their roles and the precise mechanism of CRH are not defined. Unrestrained hypoxia inducible factor activation in renal cancer promotes growth and may play an important role in driving CRH.

Acute and Chronic Regulation of the Renal Na+/H+ Exchanger NHE3 in Rats with STZ-Induced Diabetes mellitus

BACKGROUND

Early stages of diabetic nephropathy are characterized by alterations of glomerular filtration, increased tubular sodium and water reabsorption, and systemic volume expansion, which may be a major cause for the development of hypertension. As a significant fraction of renal salt and water transport is mediated by the proximal tubular Na+/H+ exchanger NHE3, we investigated its regulation in rats with STZ-induced diabetes mellitus.

METHODS

Male Sprague-Dawley rats were injected +/- streptozotocin (STZ, 60 mg/kg), and sacrificed after 2, 7 or 14 days. Renal cortical BBM vesicles were prepared to measure Na+/H+ exchange (NHE) activity and NHE3 protein abundance. Cortical NHE3 mRNA was extracted to perform Northern blot analysis. Pharmacological inhibitors were used in vivo and in vitro in order to identify isoform specificity conferring changes in NHE activity mediated by the diabetic milieu.

RESULTS

Compared to control rats, STZ rats were clearly hyperglycemic at all time points studied. NHE activity was significantly increased by 40 and 37% in diabetic rats after 7 and 14 days, respectively, but not after 2 days. The increase in Na+/H+ exchange activity was not inhibited by HOE-642 (3 microM). Administration of exogenous insulin to diabetic rats resulted in lower blood sugars, but not NHE activity. Moreover, serum glucose concentration did not correlate with NHE activity in any subgroup nor in all animals analyzed together. However, in STZ rats supplemented with exogenous insulin NHE activity was positively correlated with serum insulin concentrations (r = 0.86, p < 0.01). In vivo, the increase in NHE activity induced by STZ could be completely inhibited when rats were fed 6 ppm of HOE-642 with the diet over 14 days. The changes in Na+/H+ exchange activity were not paralleled by changes in NHE3 protein or mRNA abundance in diabetic rats at any of the time points investigated.

CONCLUSIONS

These results suggest that proximal tubular Na/H exchange activity is modified in the early stage of diabetes mellitus.

Insulin activates Na(+)/H(+) exchanger 3: biphasic response and glucocorticoid dependence

Insulin is an important regulator of renal salt and water excretion, and hyperinsulinemia has been implicated to play a role in hypertension. One of the target proteins of insulin action in the kidney is Na(+)/H(+) exchanger 3 (NHE3), a principal Na(+) transporter responsible for salt absorption in the mammalian proximal tubule. The molecular mechanisms involved in activation of NHE3 by insulin have not been studied so far. In opossum kidney (OK) cells, insulin increased Na(+)/H(+) exchange activity in a time- and concentration-dependent manner. This effect is due to activation of NHE3 as it persisted after pharmacological inhibition of NHE1 and NHE2. In the early phase of stimulation (2-12 h), NHE3 activity was increased without changes in NHE3 protein and mRNA. At 24 h, enhanced NHE3 activity was accompanied by an increase in total and cell surface NHE3 protein and NHE3 mRNA abundance. All the effects of insulin on NHE3 activity, protein, and mRNA were amplified in the presence of hydrocortisone. These results suggest that insulin stimulates renal tubular NHE3 activity via a biphasic mechanism involving posttranslational factors and an increase in NHE3 gene expression and the effects are dependent on the permissive action of hydrocortisone.

Intensive diabetes management decreases Na-Li countertransport in young subjects with type 1 diabetes and enlarged kidneys

In type 1 diabetes, increases in sodium-lithium countertransport (Na-Li CT), kidney volume (KV), and albumin excretion rate (AER) may precede the development of persistent microalbuminuria. Limited data exist on reversibility of these factors early in the evolution of diabetic nephropathy. A crossover design was used to study the separate effects of enalapril and intensive diabetes management (IDM) on Na-Li CT, KV and AER in 17 children and adolescents with type 1 diabetes (5-10 years duration) with large kidneys (>275 ml/1. 73 m(2)) and predominantly normoalbuminuria. Subjects were randomized to receive 3 months of either enalapril (0.25 mg/kg/day) or IDM, a 3-month washout, followed by the alternate treatment for 3 months. During IDM, HbA1c decreased 2.5% (pre 9.5+/-0.3% (mean+/-SE), post 7.0+/-0.1%, p<0.0001), but was unchanged while on enalapril (pre 8.8+/-0.3%, post 8.5+/-0.3%, p=0.1). A significant decrease in Na-Li CT was seen with IDM (pre 0.43+/-0.05, post 0.36+/-0.04 mmol/l RBC/h, p=0.006) but not angiotensin converting enzyme inhibition (ACE-i) (pre 0.39+/-0.04, post 0.38+/-0.04 mmol/RBC/h, p=0.4). Neither ACE-i nor IDM affected KV or AER. It is concerning that kidney enlargement does not appear reversible at this early stage in the pathogenesis of diabetic nephropathy, although our conclusions are limited by the short duration of intervention and small sample size. The reduction in Na-Li CT with IDM suggests this may be a potentially modifiable risk factor for diabetic nephropathy.

A cell cycle-dependent mechanism of renal tubule epithelial cell hypertrophy

The response of renal epithelial cells to injury can include hyperplasia (increase in cell number), apoptosis (cell death), antiproliferation (growth arrest), or hypertrophy (cells physically enlarge). Examining cell size and the protein:DNA ratio can differentiate between the growth response patterns, but it is proposed that the degree of activation of cyclin D kinase in the late G1 phase of the cell cycle differentiates between hyperplasia and hypertrophy.

TGF-beta 1 dissociates human proximal tubule cell growth and Na(+)-H+ exchange activity

Stimulation of proximal tubule cell (PTC) growth in a variety of physiological and pathological renal conditions is preceded by increased renal production of transforming growth factor-beta 1 (TGF-beta 1) and by augmented tubular sodium transport via activated sodium hydrogen exchange (NHE). Since TGF-beta 1 has been shown to be an important paracrine and autocrine regulator of PTC growth, the hypothesis that TGF-beta 1 modulates basal and mitogen-stimulated PTC growth via an effect on NHE activity was examined. Confluent, quiescent, human PTC were incubated for 24 hours in serum-free media containing vehicle (control) or 1 ng/ml TGF-beta 1, in the presence or absence of 100 ng/ml insulin-like growth factor-1 (IGF-I). Under basal conditions, TGF-beta 1 inhibited thymidine incorporation (73.5 +/- 7.3% of control, P < 0.05), but exerted no effect on cellular protein content (97.4 +/- 10.7% of control), an index of hypertrophy. There was no significant alteration of NHE activity, measured as ethylisopropylamiloride (EIPA)-sensitive H+ efflux (2.72 +/- 0.50 vs. control 3.26 +/- 0.68 mmol/liter/min) or 22Na+ influx (2.20 +/- 0.23 vs. control 2.19 +/- 0.19 nmol/mg protein/min). When co-incubated with IGF-I. TGF-beta 1 induced significant PTC hypertrophy (116.9 +/- 8.2% of control, P < 0.05), which was not seen with either agent alone. TGF-beta 1 counteracted the stimulatory effect of IGF-I on DNA synthesis (TGF-beta 1 + IGF-I 103.0 +/- 7.3% vs. IGF-I alone 181.2 +/- 30.3% of control, P < 0.05), but did not affect IGF-I-stimulated EIPA-sensitive 22Na+ influx (3.63 +/- 0.63 vs. IGF-I alone 3.67 +/- 0.50 nmol/mg protein/min, P = NS, both vs. control 2.19 +/- 0.19 nmol/mg protein/min, P < 0.05). Similar results were obtained when NHE activity was measured as EIPA-sensitive H+ efflux. Moreover, the kinetics of NHE activation by the combination of TGF-beta 1 and IGF-I (involving an increase in Vmax) were identical to that previously found for PTC exposed to IGF-I alone. The study demonstrates that TGF-beta 1 elicits distinct PTC growth responses in the presence and absence of IGF-I, without modification of NHE activity. The combination of predominant PTC hypertrophy and enhanced proximal tubule Na+ reabsorption found in many conditions that are associated with renal growth is likely to require the integrated actions of both TGF-beta 1 and IGF-I.

Human renal fibroblasts modulate proximal tubule cell growth and transport via the IGF-I axis

To determine the paracrine interactions involved in the tubulointerstitial response to progressive renal disease, the role of insulin-like growth factor-I (IGF-I) and its binding proteins (IGFBPs) in in vitro interactions between human proximal tubule cells (PTC) and renal cortical fibroblasts (CF) were studied in primary cell culture. PTC growth and transport were increased in the presence of CF-conditioned media (CF-CM), as shown by increased thymidine incorporation, cellular protein content and sodium-hydrogen exchange (NHE) activity, to 185 +/- 31% (P < 0.01), 150 +/- 18% (P < 0.05) and 195 +/- 27% (P < 0.01) of the control values, respectively. IGF-I was produced by cultured CF at a rate of 64.6 +/- 7.5 ng/mg protein/day. Exogenous IGF-I applied to PTC provoked similar enhancement of growth and NHE activity as CF-CM and the stimulatory effect of CF-CM was blocked by specific immunoneutralization of IGF-I receptors. These receptors were threefold more abundant on PTC basolateral versus apical membranes. IGF binding proteins (IGFBP)-2 and IGFBP-3 were secreted by CF at rates of 694 +/- 88 and 1769 +/- 45 ng/mg/day, with the release of IGFBP-3 being enhanced in the presence of PTC-CM (120.0 +/- 9.7% of control, P < 0.01). Moreover, the addition of CF-CM to PTC increased cell-associated IGFBP-3 on PTC surfaces, without changes in IGF-I receptor numbers or affinity and without changes in PTC mRNA for IGFBP-3. Des(1-3)IGF-I, an analog that binds to the IGF-I receptor but not to IGFBPs, provided a less potent stimulus for PTC growth compared with IGF-I, indicating that cell-associated IGFBP-3 facilitates the action of IGF-I on PTC. The results support important paracrine roles for both IGF-I and IGFBPs in the interstitial regulation of proximal tubule growth and transport.

Effects of insulin and lipid emulsion on renal haemodynamics and renal sodium handling in IDDM patients

To evaluate the role of insulin and hypertriglyceridaemia in the regulation of renal haemodynamics and sodium handling in insulin-dependent diabetes mellitus (IDDM), 11 IDDM patients without microalbuminuria and 13 weight-, age-, protein intake- and sex-matched healthy control subjects were studied. Clearances of inulin (Cin), para-amino-hippuric acid (CPAH), sodium (CNa), and lithium (CLi) were measured in four 60-min clearance periods (periods I, II, III and IV) during isoinsulinaemia with lipid emulsion infusion (study 1), a hyperinsulinaemic isoglycaemic clamp with Intralipid infusion (study 2), and during time-controlled isoinsulinaemia (study 3). We found that Cin, CPAH and filtration fraction were comparable in IDDM and control subjects, whereas CNa was decreased in diabetic subjects (2.01 +/- 1.11 vs 3.03 +/- 1.32 ml/min; p < 0.05) due to elevations of proximal tubular fractional and absolute reabsorptions of sodium (p < 0.05). Insulin infusion did not affect Cin, increased CPAH (p < 0.05) and, consequently, lowered the filtration fraction (p < 0.01) in both groups. While acute hyperinsulinaemia resulted in increases in distal tubular fractional and absolute reabsorptions of sodium (p < 0.01) contributing to a fall in CNa (p < 0.01) in control subjects, in diabetic subjects the sodium-retaining effect of insulin was not significant. The lipid emulsion did not alter any of the estimated parameters. We conclude that IDDM without microalbuminuria is associated with a tendency to sodium retention which is not aggravated by insulin when compared to control subjects. Acutely induced hypertriglyceridaemia does not alter renal haemodynamics or renal sodium handling.

Enhanced tumor uptake of carboplatin and survival in glioma-bearing rats by intracarotid infusion of bradykinin analog, RMP-7

OBJECTIVE

Intracarotid infusion of the bradykinin analog, RMP-7, can increase permeability in brain tumor capillaries. This study sought to determine the following: 1) the unidirectional transport, Ki, of radiolabeled [14C]carboplatin into brain tumors with either intravenous or intracarotid RMP-7 infusions; 2) the duration and extent of increased permeability in tumor capillaries during continuous RMP-7 infusions; and 3) the effect on survival of carboplatin combined with RMP-7 treatment in rats with gliomas.

METHODS

Wistar rats with RG2 gliomas were used, and a unidirectional transfer constant, Ki, was determined using quantitative autoradiography. In the survival study, the rats were treated with intra-arterial carboplatin and RMP-7 at Days 5 and 7 after tumor implantation.

RESULTS

Intracarotid infusion of RMP-7 for 15 minutes increased the transport of [14C]carboplatin to tumors by 2.7-fold, as compared with saline infusion alone (P < 0.001). The transports of [14C]dextran and [14C]carboplatin into tumors were significantly higher with 15 minutes of intracarotid infusion of RMP-7 (0.1 microgram/kg/min), compared to those with 10-, 30-, or 60-minute infusions (P < 0.01). Rats treated at Days 5 and 7 after tumor implantation with carboplatin alone (10 mg/kg) exhibited a modest increase in survival at 31 days (37%, compared to < 10% of controls), while those given the combination of carboplatin with RMP-7 exhibited a significantly higher survival rate (74%).

CONCLUSION

Intracarotid infusion of RMP-7 can selectively increase transport of carboplatin into brain tumors and results in higher survival in rats with gliomas. These findings support the use of intracarotid infusion of RMP-7 to enhance the delivery of carboplatin to patients with malignant brain tumors.

Transferable circulating factors and epithelial sodium transport after unilateral nephrectomy in the rat

1. In order to investigate the role of circulating serum factors in the altered renal haemodynamics and enhanced renal tubular transport observed in renal growth, micropuncture experiments were performed on normal animals infused with 20% plasma derived from animals in whom unilateral nephrectomy had been performed 3 days previously. 2. When animals infused with plasma from uninephrectomized animals (NxP) were compared with those infused with control plasma, the former had a higher tubular fluid flow rate measured at both the late proximal (LP; 26.7 +/- 1.6 vs. 18.4 +/- 1.4 nl min-1; P < 0.001) and early distal (ED; 14.9 +/- 1.0 vs. 7.8 +/- 1.0 nl min-1; P < 0.0001) sites, which was reflected in the final urine flow rate (16.1 +/- 3.4 vs. 5.1 +/- 0.8 microliter min-1; P < 0.005). 3. The single nephron glomerular filtration rate (SNGFR) was higher in animals infused with NxP as determined from samples taken at the LP (45.8 +/- 2.8 vs. 35.7 +/- 2.3 nl min-1; P < 0.01) and at the ED (34.5 +/- 2.5 vs. 28.1 +/- 1.8 nl min-1; P = 0.05) sites. However, this increase was not reflected in the whole kidney GFR (1.04 +/- 0.06 vs. 0.89 +/- 0.06; P = 0.07), suggestive of a preferential increase in filtration in the outer cortical nephrons. 4. Tubular Na+ transport was higher in the animals infused with NxP as evidenced by a decrease in the fractional delivery of Na+ at the ED site (4.5 +/- 0.4 vs. 6.5 +/- 0.6% of the filtered load; P < 0.05). However, in the final urine there was a significant increase in the urinary sodium excretion in animals infused with NxP (0.67 +/- 0.14 vs. 0.29 +/- 0.09%; P < 0.05) indicating that natriuresis and probably diuresis was a result of inhibition of Na+ and water transport in the late distal tubule and collecting duct. 5. In conclusion, these experiments demonstrate that circulating factors induced by a reduction in renal mass significantly alter glomerular filtration and tubular Na+ transport.

Tubular hypertrophy due to work load induced by furosemide is associated with increases of IGF-1 and IGFBP-1

We have examined the expression of insulin-like growth factor 1 (IGF-1), IGF binding protein-1 (IGFBP-1), and IGF binding protein-3 (IGFBP-3) in the rat distal nephron during increased cell work load and hypertrophy, induced by the diuretic, furosemide. Furosemide was given for six days to increase distal sodium delivery and uptake. To mitigate salt loss, the animals drank 0.8% NaCl and 0.1% KCl. Control rats were infused with vehicle (0.9% saline) and drank tap water. Furosemide increased urinary volume (13-fold) and sodium excretion (eightfold), and decreased urine osmolarity (fourfold). By immunocytochemistry, staining for IGF-1 and IGFBP-1 was markedly increased in distal convoluted tubules and cortical collecting ducts; both segments also underwent hypertrophy. Increased staining for the peptides was evident early (1 hr, 18 hr) after furosemide, prior to hypertrophy of cells. Whereas transcripts of IGF-1 and IGFBP-3 mRNA showed little or no increase in extracts from furosemide-treated kidney cortices, IGFBP-1 mRNA was increased threefold 18 hours after furosemide. Alterations of IGF-1 and IGFBP-1 were independent of changes in plasma aldosterone, glucocorticoids or arginine vasopressin. That IGFBP-1 mRNA increased threefold without significant changes in IGF-1 mRNA suggests that hypertrophic stimuli might initially induce the synthesis of IGF binding protein followed by the trapping of extracellular IGF-1. The present study raises the possibility of IGF-1 and IGFBP-1 being involved in processes that lead to tubular hypertrophy. IGFBP-1 may regulate these effects by binding to and interaction with IGF-1.

Increased expression of Na+/H+ exchanger in the injured renal tissues of focal glomerulosclerosis in rats

The renal mRNA expression of Na+/H+ exchanger (NHE) and the effects of NHE inhibitor, amiloride, on renal injury were investigated in adriamycin (ADR)-induced glomerulosclerosis model in rats, which progressively developed extensive glomerulosclerosis and interstitial fibrosis. NHE-1 mRNA from the cortex of the ADR rats progressively increased at weeks 4 and 8 and then peaked at week 16, which paralleled with the degree of glomerular sclerosis and interstitial fibrosis. The interstitial fibrosis in the ADR-rats was prevented by a daily administration of amiloride. A simultaneous analysis of the effects of a high salt diet on NHE-1 mRNA expression or renal injury was performed in the ADR rats at weeks 2 and 8. Renal or glomerular hypertrophy was observed in the control or ADR rats fed an 8% NaCl diet at week 2 and 8 compared to a 1% NaCl diet, while the NHE-1 mRNA expression was not up-regulated by an 8% NaCl diet at week 2. At week 8, the NHE-1 mRNA expression or glomerulosclerosis and interstitial fibrosis were enhanced in the ADR rats fed an 8% NaCl diet compared to a 1% NaCl diet. This histological aggravation by an 8% NaCl diet was prevented by a daily administration of amiloride but not by furosemide. In conclusion, the increased NHE-1 mRNA expression and the preventive effects of amiloride on the renal lesions suggest a potential importance of NHE in the progressive process of ADR-nephropathy. The high salt diet had a hypertrophic and destructive effect on kidney or glomeruli in the ADR rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucocorticoid uncoupling of antiogensin II-dependent phospholipase C activation in rat vascular smooth muscle cells

Vascular tone is maintained by both angiotensin II (Ang II) and glucocorticoids, but the effect of glucocorticoids on Ang II function in vascular smooth muscle cells (VSMC) is unclear. To determine the direct influence of glucocorticoids on VSMC Ang II receptor function, the effects of dexamethasone on Ang II receptor binding, Ang II-induced phospholipase C (PLC) activation, and Ang II-dependent cell growth were studied in cultured rat VSMC. Dexamethasone caused concentration- and time-dependent increases in Ang II binding which were prevented by glucocorticoid receptor inhibition with RU 38486. Dexamethasone-induced enhancement of Ang II binding resulted from increased AT1 receptors, as indicated by Northern blot analysis and competitive binding assays. Despite causing increased Ang II receptor number, dexamethasone preincubation prevented Ang II-induced PLC activation, as indicated by phosphatidylinositol 4,5-bisphosphate degradation and inositol trisphosphate formation. When PLC activity was directly measured in VSMC soluble and membrane fractions, Ang II receptor activation caused decreased soluble and increased membrane PLC activity, consistent with the interpretation that Ang II caused cytosol-to-membrane PLC translocation. The effect of Ang II on PLC translocation was prevented by dexamethasone preincubation. Finally, prolonged incubation with dexamethasone and Ang II had additive effects on VSMC hypertrophy. In conclusion, glucocorticoids directly altered Ang II function in VSMC by causing increased Ang II receptor number, Ang II receptor/PLC uncoupling, and enhanced Ang II-dependent hypertrophy.

Exogenous epidermal growth factor fails to accelerate functional recovery in the autotransplanted ischemic pig kidney

The reversibility of ischemic renal injury is dependent on epithelial cell regeneration and repopulation of the nephron. Renal cells produce and respond to many growth factors. In the rat, epidermal growth factor (EGF) is mitogenic for tubular cells and accelerates renal recovery after ischemia. We used a pig renal autotransplant model to evaluate the effect of exogenous EGF on renal recovery in a large animal more analogous to man. Group 1 animals underwent left autotransplant after 120 minutes of warm ischemia and received either a single intra-arterial dose of recombinant human EGF (EGF, 10(-7) M.) (N = 11) or vehicle alone (N = 6). Group 2 animals underwent left autotransplant after 72 hours of cold preservation with Collins' solution and received a similar intra-arterial dose plus a subcutaneous dose of EGF (0.5 ml. of 10(-3) M.) (N = 8) or vehicle alone (N = 6). Contralateral nephrectomy was performed in all animals. Daily creatinine measurements revealed no beneficial effect from EGF on recovery of renal function in Group 1 or 2 animals. Studies of EGF on pig proximal tubular cells demonstrated in vitro mitogenesis; autoradiography with 125I-EGF revealed binding of EGF throughout the kidney. Immunohistochemistry showed significant tubular cell proliferation in response to ischemic injury, without further enhancement from EGF. Thus, although exogenous EGF bound to pig kidney cells and stimulated cell proliferation, we were unable to demonstrate a clinically significant acceleration of recovery from ischemic injury.

Can insulin administration cause an acute metabolic acidosis in vivo? An experimental study in dogs

Insulin is the cornerstone of therapy for diabetic ketoacidosis because it causes the rate of ketoacid production to fall; this action takes several hours to occur. Insulin also causes H+ to be transported from the intracellular fluid to the extracellular fluid in vitro. The purpose of this study was to determine if insulin led to the acute export of H+ from the intracellular fluid in vivo. If so, we wished to determine if this also occurred during chronic metabolic acidosis, to quantitate the magnitude of the H+ shift, and to evaluate the mechanisms involved. The administration of low- or high-dose insulin to normal dogs and high-dose insulin to dogs with chronic metabolic acidosis caused the concentration of bicarbonate in plasma to decline by close to 3 mmol/l. The PCO2 fell by close to 15% in all three groups of dogs, so one component of the fall was due to hyperventilation. As the pH of blood did not change, a primary metabolic acidosis also occurred. The fall in bicarbonataemia was not due to net accumulation of organic acids or to a loss of bicarbonate or organic anions in the urine. Taken together, insulin, when given at doses used to treat diabetic ketoacidosis, might induce a significantly greater degree of acidaemia in the extracellular fluid acutely after it is given.

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.

Effects of insulin on renal haemodynamics and the proximal and distal tubular sodium handling in healthy subjects

The effects of insulin on renal haemodynamics and renal sodium handling were studied in 10 healthy males. Using the euglycaemic insulin clamp technique, insulin was infused on separate days resulting in two levels of hyperinsulinaemia (41 +/- 3 and 90 +/- 7 mU/l, respectively). Renal haemodynamics and the proximal and distal tubular sodium handling were studied using inulin, para-amino-hippuric acid, sodium and lithium clearances. Low- and high-dose insulin infusions were followed by a fall in sodium clearance from 1.6 +/- 0.1 ml/min to 1.2 +/- 0.1 and 1.0 +/- 0.1 ml/min, respectively. Both levels of hyperinsulinaemia resulted in increased distal tubular sodium reabsorption. The distal antinatriuretic effect of insulin was associated with dose- and time-dependent decline in proximal tubular sodium reabsorption. The changes in proximal tubular sodium handling occurred without any significant changes in natriuretic factors, such as renal dopamine and plasma atrial natriuretic peptide levels. However, hyperinsulinaemia resulted in time- and dose-dependent increases in renal plasma flow, and renal vasodilatation could, possibly via changes in renal interstitial pressure, have contributed to the fall in the proximal tubular sodium reabsorption. The results also suggest that decreased proximal sodium reabsorption may be a compensatory mechanism counteracting the insulin-induced sodium retention.

Differential effects of calorie restriction on glomeruli and tubules of the remnant kidney

We have previously shown that 40% calorie restriction (CR) prevents renal injury 21 weeks after 5/6 nephrectomy (Nx) in rats, regardless of whether protein intake was concurrently restricted or not. Growth retardation appeared to be a necessary prerequisite for the protective effects of CR. To further study these mechanisms, we performed 5/6 Nx in male F344 rats and pair-fed them with a control diet (ad lib group) or a high protein diet restricted by 40% so that protein intake was similar, but calorie consumption was reduced (CR group). Four weeks after 5/6 Nx, when glomerulosclerosis had not yet developed, we compared various parameters as follows in both dietary groups and sham operated rats: urinary protein excretion (uPr), GFR (14C inulin clearance), mean nephron GFR (MNGFR; GFR divided by total number of glomeruli), glomerular volume (VG), tubulointerstitial index (TII), a measure of tubular damage kidney weight (kidney wt), kidney IGF-I content by RIA, and IGF-I immunohistochemistry. CR ameliorated the increase of MNGFR, but not glomerular hypertrophy. TII, kidney wt and kidney IGF-I content were increased in the ad lib Nx group; these changes were alleviated by CR. Two weeks after 5/6 Nx, immunohistochemistry for IGF-I showed increased staining in superficial distal nephrons in the ad lib group, and this was also suppressed by CR. The occurrence of tubulointerstitial pathology prior to glomerulosclerosis, and the beneficial effects of CR on all parameters except Vg indicate a dissociation of mechanisms which result in tubular versus glomerular hypertrophy and damage.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal effect of anti-hypertensive drugs depends on sodium diet in the excision remnant kidney model

Angiotensin converting enzyme inhibitors (ACEI) are believed to protect remnant kidney, but all previous studies used the ligation model which causes severe hypertension, and very few have compared drugs in rats having similar control of blood pressure (BP). We compared rats with uremia obtained by 70% excision of total renal mass, a model which causes mild, late hypertension. Study I compared the effects of enalapril (E), cicletanine (C) and placebo (P) in uremic (U) rats fed a 0.50% (normal-high) Na diet. Study II compared the effects of E, C, P, and guanfacine (G) in U rats fed a diet restricted to 0.25% Na (normal-low). In study I, UP rats developed progressive hypertension (140, 146, 160 and 166 mm Hg at 3, 6, 9 and 12 weeks), proteinuria (240 mg/day at 9 and 12 weeks) which were not affected by E or C. The occurrence of end-stage renal disease (ESRD) led to the sacrifice of all rats after three months. All three groups had similar severe renal lesions (over 25% sclerosed glomeruli in 5 of 10 UP, 9 of 14 UE, 7 of 14 UC rats, with huge cystic tubular dilatations). In study II, rats could be sacrificed later (6 months) and had evidence of less severe renal disease. All the drugs tested prevented hypertension throughout the study (P less than 0.001), with lowest values in UE rats. E and G, but not C, reduced proteinuria. Renal damage was reduced with E and G, but not with C, despite similar BP in C and G rats. Thus, in contrast with what was obtained in the ligation model, ACEI affected neither the BP nor the renal lesions of rats made uremic by renal excision and fed a 0.50% Na diet. Moderate Na restriction improved the consequences of nephron loss and restored the anti-hypertensive effect of drugs. However, these drugs had a different effect on renal preservation: it was dramatic with E, good with G, and undetectable with C.

Long-term activation of protein kinase c causes chronic Na/H antiporter stimulation in cultured proximal tubule cells

To examine the role of protein kinase C as a chronic regulator of proximal tubule Na/H antiporter activity, the effect of phorbol 12-myristate 13-acetate (PMA) on the Na/H antiporter was studied in cultured proximal tubule cells. Short-term activation of protein kinase C by 5 min exposure to PMA caused an acute increase in Na/H antiporter activity that was not prevented by cycloheximide or actinomycin D and did not persist 24 h later. Long-term activation of protein kinase C by 2 h exposure to PMA caused a dose-dependent increase in Na/H antiporter activity 24 h later. This latter effect was due to protein kinase C activation in that it was inhibited by sphingosine and was not seen with 4 alpha-PMA, an inactive analogue. The chronic effect of PMA was inhibited by 10 nM actinomycin D or 7 microM cycloheximide. Proximal tubule cells exposed to PMA for 2 h demonstrated a two- to threefold increase in Na/H antiporter mRNA (mRNANa/H) abundance 4 h later. In conclusion, short-term activation of protein kinase C leads to a transient increase in Na/H antiporter activity that is independent of transcription and translation, whereas long-term activation of protein kinase C causes a persistent increase in antiporter activity that is dependent on transcription and translation and is associated with increased mRNANa/H abundance. This latter effect may mediate increased Na/H antiporter activity in a number of chronic conditions.

Insulin increases sodium reabsorption in diluting segment in humans: evidence for indirect mediation through hypokalemia

To examine the mechanism of renal sodium (Na) and potassium (K) retention during insulin infusion, seven healthy volunteers underwent clearance studies without (time control) and with insulin infusion (40 mU bolus, followed by 1 mU/kg/min for 150 min). Maximal free water clearance and fractional lithium clearance (FELi) were used to analyze renal sodium handling. Insulin decreased Na excretion (from 189 +/- 25 to 121 +/- 19 mumol/min, P less than 0.01) and K excretion (from 64 +/- 8 to 19 +/- 1 mumol/min, P less than 0.01), but did not change in glomerular filtration rate. FELi increased from 29.8 +/- 1.9 to 32.3 +/- 1.9% (P less than 0.05), minimal urine osmolality decreased from 59 +/- 3 to 46 +/- 3 mOsm/kg (P less than 0.01), and the diluting segment reabsorption index increased from 88.0 +/- 0.9 to 93.7 +/- 0.9%, P less than 0.01). Insulin also decreased plasma K, from 3.91 +/- 0.08 to 3.28 +/- 0.08 mmol/liter, P less than 0.01. In a third clearance study KCl was infused simultaneously (3.75 mumol/kg/min) to prevent this fall in plasma K. In this study insulin had no effect on Na and K excretion and diluting segment reabsorption, but the rise in FELi remained. In a fourth clearance study NaCl (3.75 mumol/kg/min) instead of KCl was infused together with insulin. This maneuver did not prevent the Na and K retaining effect of insulin, nor any of its effects on renal sodium handling parameters. These data suggest that Na and K retention during insulin infusion are largely secondary to hypokalemia, which causes increased reabsorption in the diluting segment.

Correction of enhanced Na(+)-H+ exchange of rat small intestinal brush-border membranes in streptozotocin-induced diabetes by insulin or 1,25-dihydroxycholecalciferol

Diabetes was induced in rats by administration of a single i.p. injection of streptozotocin (50 mg/kg body wt). After 7 d, diabetic rats were further treated with insulin or 1,25-dihydroxycholecalciferol [1,25(OH)2D3] for an additional 5-7 d. Control, diabetic, diabetic + insulin, and diabetic + 1,25(OH)2D3 rats were then killed, their proximal small intestines were removed, and villus-tip epithelial cells were isolated and used to prepare brush-border membrane vesicles. Preparations from each of these groups were then analyzed and compared with respect to their amiloride-sensitive, electroneutral Na(+)-H+ exchange activity, using 22Na uptake as well as acridine orange techniques. The results of these experiments demonstrated that (a) H+ gradient-dependent 22Na uptake as well as Na+ gradient-dependent transmembrane H+ fluxes were significantly increased in diabetic vesicles compared to their control counterparts, (b) kinetic studies demonstrated that this enhanced 22Na uptake in diabetes was a result of increased maximal velocity (Vmax) of this exchanger with no change in apparent affinity (Km) for Na+, (c) serum levels of 1,25(OH)2D3 were significantly lower in diabetic animals compared with their control counterparts; and (d) insulin or 1,25(OH)2D3 treatment restored the Vmax alterations to control values, without any significant changes in Km, concomitant with significantly increasing the serum levels of 1,25(OH)2D3 in diabetic animals. These results indicate that Na(+)-H+ activity is significantly increased in proximal small intestinal luminal membranes of streptozotocin-induced diabetic rats. Moreover, alterations in the serum levels of 1,25(OH)2D3 may, at least in part, explain this enhanced antiporter activity and its correction by insulin.

Proximal tubular cell sodium concentration in early diabetic nephropathy assessed by electron microprobe analysis

Electron microprobe X-ray analysis techniques were employed in order to assess the changes that occur in proximal tubular cell sodium concentration during the hyperfiltration phase of early diabetes mellitus induced by streptozotocin in Sprague Dawley rats. Intracellular rubidium accumulation following intravenous infusion of rubidium chloride was used as a marker of basolateral Na/K-ATPase activity. The diabetic animals studied had a significantly higher glomerular filtration rate compared with controls [1.44 +/- 0.07 vs. 1.00 +/- 0.07 ml min-1 (100 g body weight)-1; mean +/- SEM, P less than 0.001]. Intracellular Na concentration was significantly higher in diabetic animals (19.5 +/- 0.6 vs. 17.8 +/- 0.4 mmol/kg wet weight; P less than 0.01). Concurrent measurement of Rb demonstrated significantly higher intracellular accumulation in the proximal tubules of diabetic animals compared with control (7.9 +/- 0.5 vs. 5.5 +/- 0.5 mmol/kg wet weight; P less than 0.001). These results indicate that proximal tubular Na/K-ATPase activity is enhanced in the hyperfiltration phase of diabetes mellitus. Since, however, intracellular Na concentration is increased under these conditions, it may be inferred that apical Na entry into proximal tubular cells is stimulated beyond the rate of basal exit during the initial development of hyperfiltration. The reasons for these alterations in cellular Na transport are unclear but similar changes have been implicated in the pathogenesis of cell growth.

Molecular mechanisms of tubulointerstitial hypertrophy and hyperplasia

Adult kidneys, which are principally composed of tubulointerstitium, do not normally regenerate or expand their working pool of functional cells at a very high rate. Loss of kidney tissue, however, can lead to some compensatory renal enlargement. The catalytic forces initiating such exchanges have not been fully articulated by current experimental endeavors. Increasing evidence, nevertheless, does suggest that factors other than simple changes in renal hemodynamics may be involved in this process. Different cellular elements in the tubulointerstitial microenvironment probably modulate changes in tubular enlargement or size through a complex cytokine network. Autocrine and paracrine stimulation of enlargement by different local growth factors also seem to play a pivotal role. After binding to cellular receptors, these factors activate signal transduction pathways resulting in expression of immediate early genes, which by themselves can synchronize the expression of subsequent genes through the medium of transacting factors. The renal enlargement response can also be modified by endocrine hormones that can activate such genes directly and/or stimulate other adjunctive processes, like receptor expression for the regional binding of growth factors. Furthermore, renal enlargement is under negative feedback of inhibitory factors like TGF beta. It is possible, for example, that special genes exist which are only expressed to arrest enlargement. It has been further suggested that activation of the Na+/H+ antiporter is a common denominator in renal enlargement. Recent findings, however, indicate that the activation of this antiporter is not always necessary, and might rather be a parallel event rather than a key phenomena in tubular enlargement. G0/G1 transition of tubular cells seems to involve similar factors in tubular hypertrophy and hyperplasia. The factors which are responsible for the final determination of the enlargement pattern (hypertrophy vs. proliferation) are unknown. The separation between hypertrophy and hyperplasia, although suggested by striking differences in cellular regulation, may be somewhat artificial, since responses leading to tubular enlargement also exist in circumstances where hyperplasia and hypertrophy are combined events. Recently it has been proposed that growth factors stimulate gluconeogenesis in proximal tubular cells producing hyperplasia, whereas factors inhibiting gluconeogenesis might induce hypertrophy. Whether the common pathway message of this intriguing hypothesis is correct still requires further validation.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Effects of media buffer systems on growth and electrophysiologic characteristics of cultured sweat duct cells

Primary cultures of human reabsorptive sweat duct cells were grown in MCDB 170 medium buffered with either HEPES, bicarbonate, or a mixture of HEPES and bicarbonate buffers. Cultures grown in MCDB media containing bicarbonate seemed to differentiate into a multilayered, keratinized epithelium and began senescing after 1 wk in culture. In contrast, cultures grown in media containing HEPES as the only buffer seemed to undergo a selection process, resulting in the outgrowth of cells that did not multilayer or keratinize extensively for up to 3 or 4 wk in culture. Despite marked differences in growth, cells grown in both bicarbonate and HEPES-buffered media retained electrophysiologic characteristics appropriate to the progenitor. Mean resting potentials were -21.8 +/- 0.8 mV (n = 82), -23.3 +/- 1.3 mV (n = 70) and -18.2 +/- 0.8 mV (n = 82) for duct cells grown in HEPES, bicarbonate, and HEPES-bicarbonate media, respectively. Substitution of Cl- with the impermeant anion gluconate in the bathing medium caused membrane potential depolarization in all media, revealing the presence of a Cl- conductance. Administration of the Na+ conductance inhibitor amiloride hyperpolarized the mean resting potential of cells grown in HEPES medium (-6.8 +/- 0.6 mV, n = 68), bicarbonate medium (-6.9 +/- 0.5 mV, n = 60), and HEPES-bicarbonate medium (-5.9 +/- 0.6 mV, n = 69), demonstrating expression of a Na+ conductance. We observed some but minimal variation with age in any of these conditions.

Transepithelial acidification by cultures of rabbit proximal tubules grown on filters

Transepithelial acidification in the proximal tubule occurs by the simultaneous actions of the Na(+)-H+ exchanger in the brush border and the basolateral Na(+)-HCO3- cotransporter. The presence of these systems has been demonstrated for cultured cells; however, their contributions to the transepithelial movement of acid equivalents has not been confirmed in monolayers. To examine transepithelial acidification by intact cells, tubules were grown on membrane filters. Confluent cultures developed a transepithelial pH gradient within 6 h by decreasing the pH of medium in the apical chamber (6.66 +/- 0.03) while raising the basolateral pH to 7.40 +/- 0.02. Cells maintained on plastic did not acidify the medium during this time. Amiloride (10-100 microM) inhibited development of the gradient only when placed in the top chamber. 4-Acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS; 10-100 microM), which inhibits basolateral Na(+)-HCO3- cotransport, decreased the gradient only when added to the bottom. These results demonstrate that cultured proximal tubule cells can develop a transepithelial pH gradient and that the polarized distribution of the transport systems is maintained in vitro.

Preincubation in acid medium increases Na/H antiporter activity in cultured renal proximal tubule cells

Chronic acidosis in vivo leads to an increase in proximal tubule Na/H antiporter activity that persists when the transporter is studied out of the acidotic environment. It is presently not clear whether a decrease in extracellular fluid pH alone is sufficient to elicit this adaptation. The present studies examined the effect of acid preincubation on Na/H antiporter activity in cultured proximal tubule cells. Antiporter activity was examined after a 2-day preincubation in control or acid medium, 1 hr after removal from the preincubation fluid. Na/H antiporter activity was assayed as the initial rate of Na-dependent alkalinization after cell acidification in the absence of CO2/HCO3. Preincubation in low [HCO3] acid medium or in high PCO2 acid medium led to increases in amiloride-sensitive Na/H antiporter activity. This adaptation was inhibited by addition of cycloheximide to the preincubation medium. Preincubation of fibroblasts in low [HCO3] acid medium did not lead to increased Na/H antiporter activity but rather caused a small inhibition. These studies demonstrate an adaptation in Na/H antiporter activity elicited by a low pH of the extracellular fluid, which is dependent on protein synthesis, and may be unique to certain H/HCO3-transporting epithelia.

Induction of hypertrophy in cultured proximal tubule cells by extracellular NH4Cl

Ammonia production increases in several models of renal hypertrophy in vivo. The present study was designed to determine whether ammonia can directly modulate the growth of renal cells in the absence of a change in extracellular acidity. In serum-free media NH4Cl (0-20 mM) caused JTC cells and a primary culture of rabbit proximal tubule cells to hypertrophy (increase in cell protein content) in a dose-dependent fashion without a change in DNA synthesis. Studies in JTC cells revealed that the cell protein content increased as a result of both an increase in protein synthesis and a decrease in protein degradation. Total cell RNA content and ribosome number increased after NH4Cl exposure and the cell content of the lysosomal enzymes cathepsin B and L decreased. Inhibition of the Na+/H+ antiporter with amiloride did not prevent the hypertrophic response induced by NH4Cl. The results indicate that ammonia is an important modulator of renal cell growth and that hypertrophy can occur in the absence of functioning Na+/H+ antiport activity.

Peptide-dependent regulation of epithelial nephron functions

It has become evident that the nephron is an important target organ of many of the regulatory peptides; this brief overview will not attempt to consider the vast amount of work on peptide-dependent kidney functions; instead, it will emphasize recent work directed towards understanding intracellular signal pathways between peptide ligand-receptor interaction and expression of physiological transport responses in renal epithelial cells. The awareness that peptide hormones of differing origin, e.g., intestinal and cardiac, share at least some of the signal steps in nephron cells, has stimulated work on nephron segmental analysis of receptor binding, of second messengers, of membrane G proteins, of protein phosphorylation, and of final membrane transport responses, such as peptide-dependent ion channel regulation. Peptides involved in cell growth and differentiation, e.g., growth factors, appear to act through part of the signal pathway shared by other peptides. The peptides selected for the purpose of this review, then, are those that have been linked, by experimental evidence, to intracellular messenger systems in nephron epithelia.

Effect of pH on growth of mouse renal cortical tubule cells in primary culture

We examined the effect of the medium pH on growth of primary cultures of mouse cortical tubule cells grown in defined medium. A significantly higher DNA content was observed within 24 h of lowering medium pH from 7.4 to 6.8 or 7.1 and persisted for the duration of the study. Further studies revealed that either medium acidification or insulin plus prostaglandin E1 nearly doubled uptake of [3H]thymidine in cells deprived of other growth factors for the previous 72-110 h. Moreover, the effects of insulin, prostaglandin E1, and medium acidification on [3H]thymidine uptake of quiescent cells were additive. An alkaline medium pH appeared to have a small but significant effect on cell hypertrophy, since cells exposed to pH 7.4 and 7.7 had a higher protein-to-DNA ratio than cells incubated at a lower pH. Cell pH of monolayers grown on glass slides determined from fluorescence of the carboxyfluorescein analogue 2',7'-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF) was linearly correlated with medium pH, and changes in medium pH resulted in changes in steady-state cell pH of a similar magnitude. Four hours after medium acidification, relative increases in cell Na+ and water content occurred, whereas medium alkalinization led to decreases in cell Na+ and water content. The increases in cell Na+ and cell water content at pH 6.8 could be inhibited by amiloride. We conclude that decreasing the cell pH can be a mitogenic stimulus for renal tubule cells. Medium acidification is accompanied by changes in cell Na+ transport, which may be mediated in part by altered Na+-H+ antiporter activity.

Primary cultures of rabbit renal proximal tubule cells: I. Growth and biochemical characteristics

Before the usefulness of a new in vitro model can be ascertained, the model must be properly defined and characterized. This study presents the growth rate and biochemical characteristics of rabbit renal proximal tubule cells in primary culture over a 2-wk culture period. When grown in a hormonally defined, antibiotic-free medium these cells form confluent monolayer cultures within 7 d after plating. Multicellular dome formation, an indicator of transepithelial solute transport, was expressed after confluent cultures were formed. The activity of the cytosolic enzyme, lactate dehydrogenase, and the lysosomal enzyme, N-acetyl-glucosaminidase, increased 14- and 2-fold during the first 8 d of culture, respectively. In contrast, the activity of a brush border enzyme, alkaline phosphatase, decreased 85% within the first 8 d of culture. Release of these enzyme markers into the culture medium, which are routinely used to measure cytotoxicity, stabilized after 8 d in culture. The ratio of cellular protein to DNA changed according to the state of cellular growth. Values rose from 0.035 mg protein/micrograms DNA in preconfluent cultures to 0.059 mg protein/micrograms DNA in confluent cultures. These results document the characteristics of a primary proximal tubule cell culture system for future studies in in vitro toxicology.

Activity of calcium-sensitive phospholipid-dependent protein kinase C following nephron loss

The localization and activity of the calcium-sensitive phospholipid-dependent protein kinase C (PKC) were examined following the loss of 50% of functioning nephron mass. Four hours following unilateral nephrectomy in rats, soluble (100,000 g supernatant) proteins in the contralateral kidney were increased by 11% compared to sham operated controls; the increase was 33% 144 hours following surgery. The specific activity of PKC did not change in the cytosol at any of the time periods examined and averaged 63.9 +/- 8.2 pmol.mg-1.min-1 in unilaterally nephrectomized animals four hours following surgery. Four hours following sham surgery total soluble PKC activity averaged 1667.0 +/- 278.4 pmol.kidney-1.min-1, whereas activity averaged 3067.7 +/- 415.4 pmol.kidney-1.min-1 in animals post-nephrectomy (N = 5, P less than 0.04). Similar data was seen 144 hours following surgery. To examine the PKC activity in plasma membranes of proximal tubular cells, brush border membranes were prepared from rat kidney cortex. Twenty-four hours following unilateral nephrectomy, activity averaged 193.8 +/- 14.9 pmol.mg-1.min-1, while activity in membranes isolated from sham operated animals averaged 76.6 +/- 8.0 pmol.mg-1.min-1 (N = 5, P less than 0.001). Similar data was evident 48 hours following surgery. A small increment in activity was seen in the basolateral membrane preparation 24 hours following unilateral nephrectomy but not at 48 hours. These data indicate that cellular PKC activity increases rapidly following reductions in renal mass, and there are selective increments in the brush border membrane of the proximal tubular cell. The localization of PKC to this membrane may have important consequences for adaptations following nephron loss.

Studies of terminal differentiation of electrolyte transport in the renal proximal tubule using short-term primary cultures

There are several lines of indirect evidence suggesting that the renal tubule cells have not yet reached terminal differentiation at birth. Methods used in cell biology can now be applied to study renal ontogeny. This review describes how primary cultures of proximal tubule cells from rats can be used to investigate developmental changes in Na permeability and Na-K-ATPase-mediated transport.

Epidermal growth factor binding, stimulation of phosphorylation, and inhibition of gluconeogenesis in rat proximal tubule

Epidermal growth factor and insulin share many biological activities, including stimulation of cell proliferation, ion flux, glycolysis, fatty acid and glycogen synthesis, and activation of receptor-linked tyrosine kinase activity. In the kidney, insulin has been shown to regulate transport processes and inhibit gluconeogenesis in the proximal tubule. Since the kidney represents a major source of EGF, the present studies investigated whether proximal tubule contained EGF receptors, whether EGF receptors were localized to apical or basolateral membranes, and whether EGF receptor activation participated in the regulation of an important proximal tubule function, gluconeogenesis. Specific EGF receptors were demonstrated in the basolateral membrane of proximal tubule. Following incubation with 125I EGF, basolateral membranes demonstrated equilibrium binding at 4 degrees C and 23 degrees C. There was 78 +/- 2% specific binding (n = 13). The dissociation constant (Kd) was 1.5 x 10(-9) M and maximal binding was 44 fmol/mg protein. There was ninefold more specific binding to proximal tubule basolateral membrane than to brush border membrane. In basolateral, but not brush border membranes, EGF induced phosphorylation of the tyrosine residues of intrinsic membrane proteins, including a 170 kDa protein, corresponding to the EGF receptor. In the presence of the gluconeogenic substrates, alanine, lactate, and succinate, proximal tubule suspensions synthesized glucose. EGF inhibited glucose production in a concentration-dependent manner over a concentration range of 3 x 10(-11) to 3 x 10(-9) M. In addition, EGF inhibited angiotensin II-stimulated glucose production in the proximal tubule suspensions. EGF did not significantly increase net glucose metabolism nor decrease cellular ATP concentrations. Therefore, these studies demonstrated that rat proximal tubule contained specific receptors for EGF that were localized to the basolateral membrane and linked to tyrosine kinase activity. EGF significantly inhibited proximal tubule glucose production without significantly increasing net glucose consumption.

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)

Patterns of mRNA expression during early cell growth differ in kidney epithelial cells destined to undergo compensatory hypertrophy versus regenerative hyperplasia

An increase in cell size and protein content is characteristic of cells undergoing hypertrophy and of replicating cells prior to DNA synthesis. Cell enlargement in the two situations could be regulated by similar early events with an interruption of the cell cycle occurring in hypertrophy, or the two processes could be uncoupled. In vivo models were used to compare hypertrophy induced by unilateral nephrectomy and hyperplasia induced by folic acid injection in rabbit renal cortical cells. Within 48 hr, cell volume increased in both groups but the number of cells in the cell cycle and DNA synthesis was increased only after folic acid. Patterns of mRNA expression of the following three groups of cell cycle-dependent genes were analyzed: (i) protooncogenes (c-fos, c-myc, and c-Ha-ras), (ii) structural protein genes (vimentin and beta-actin), and (iii) transport protein genes (Na+, K+-ATPase, ADP-ATP translocase, and calcyclin). mRNAs for all genes, except calcyclin and c-Ha-ras, were detected in controls. Folic acid generally induced rapid, transient increases in mRNA levels, but after unilateral nephrectomy, expression of most mRNAs showed a gradual, progressive increase. These data indicate that gene expression in the early stages of cell enlargement differs in cells destined to undergo proliferation vs. hypertrophy. The term "sustained message amplification" is proposed to describe the hypertrophied cell.

Insulin-like growth factor 1 stimulates renal epithelial Na+ transport

Insulin-like growth factor 1 (IGF1) stimulates vectorial Na+ transport in a classical model of the mammalian distal nephron, the toad urinary bladder. Net mucosal to serosal Na+ flux is stimulated by concentrations of IGF1 as low as 0.1 nM, and the response is maximal at 10 nM. Na+ transport increases within minutes of the serosal addition of IGF1, reaches a maximum in 2-3 h, and is sustained for at least 5 h. Neither the initial nor the sustained response to IGF1 is dependent on a new protein synthesis. The IGF1 response is inhibited by a concentration of amiloride (10(-5) M) that is known to specifically block the conductive apical Na+ channel but that has little effect on the Na+-H+ antiporter. Further studies will be necessary to establish a role for this growth factor in normal renal epithelial function, but it is possible that the natriferic and growth-stimulatory effects of IGF1 are intimately related.

Intracellular pH regulation in resting and contracting segments of rat mesenteric resistance vessels

1. The pH-sensitive dye 2',7'-bis-(2-carboxyethyl)-5 (and -6)-carboxyfluorescein (BCECF) was used to measure intracellular pH (pHi) in segments of rat resistance vessels (internal diameter about 200 microns) with the vessels mounted in a myograph for simultaneous measurements of isometric contraction. 2. BCECF loaded slowly into the vessels over 1 h and did not affect the maximal contractility of the vessels. There was a loss of dye with time which, however, was very slow when the segments were only excited for 2 s/min, suggesting that the loss was mainly due to dye bleaching with only a very slow leak. 3. The ratio of the emissions (at 540 nm) with excitation at 495 and 450 nm was calibrated in terms of pH using the K+-H+ ionophore nigericin. This calibration gave a pHi value of 7.15 +/- 0.02 (n = 20), suggesting that hydrogen ions are not in electrochemical equilibrium in these vascular smooth muscles which have a membrane potential of about -60 mV. 4. Addition of 10 mM-NH4Cl caused a transient alkalinization and wash-out of 10 mM-NH4Cl a transient acidification. Increasing CO2 with maintained bicarbonate caused a rapid acidification followed by an incomplete recovery. Removal of CO2 and bicarbonate (HEPES-buffered solution) with constant extracellular pH caused a transient alkalinization but steady-state pHi was not significantly altered. 5. In bicarbonate-free buffer the Na+-H+ exchange blocker 5-(N-ethyl-N-isopropyl) amiloride (EIPA) and sodium-free conditions caused a slow acidification. In bicarbonate buffer (PSS) EIPA had no detectable effect after 10 min but the anion exchange blocker diisothio-cyanatostilbenedisulphonic acid (DIDS) caused a small acidification over that time course. 6. The rate of recovery after an acid load was about 50% lower in HEPES buffer compared to PSS and it was inhibited by EIPA. In PSS amiloride and EIPA each had a small inhibitory effect on the pH recovery after an acid load. DIDS also inhibited the recovery from an acid load in PSS and this effect was additive to that of EIPA. DIDS and EIPA also had additive inhibitory effects on the 22Na+ influx stimulated by the acid loading, while in HEPES buffer DIDS had no effect on either pH recovery or 22Na+ influx. These results suggest that a Na+-H+ exchange and an influx of bicarbonate coupled to sodium influx are of importance for pHi control in these vessels.(ABSTRACT TRUNCATED AT 400 WORDS)