Reductions in renal mass and the nephropathy induced by mercury

The severity of renal injury induced by several graded doses of mercuric chloride and the disposition of mercury were evaluated and compared in control, uninephrectomized (50% NPX), and 75% nephrectomized (75% NPX) rats in an attempt to determine the effect of increased reductions of renal mass on the nephropathy induced by inorganic mercury. Consistent with previously published findings, proximal tubular necrosis (as assessed histopathologically and by the urinary excretion of lactate dehydrogenase (LDH) and total protein) was significantly more severe in 50% NPX rats than in control rats 24 hr after the administration of any of three lowest (1.0, 1.5, or 1.75 micromol/kg) doses of mercuric chloride used in the study. Interestingly, the severity of proximal tubular necrosis in the 75% NPX rats was not greater than that in control rats at these same doses. The reason for this appeared to be due to decreased renal accumulation of mercury, particularly in the renal cortex and outer stripe of the outer medulla. At the highest (8.0 micromol/kg) dose of mercuric chloride used, renal tubular injury was very extensive in all three groups of rats, with the level of injury being greatest in the 50% NPX rats. The injury was so severe in all three groups that acute renal failure was induced within the first 24 hr after the injection of mercury. An important finding that was made at this dose was that the level of blood urea nitrogen (BUN) was significantly greater in the 75% NPX rats than in either the 50% NPX or control rats, which indicates that 75% NPX rats may have entered into acute renal failure sooner than the 50% NPX or control rats. Overall, the findings from the present study indicate that as renal mass is reduced to a level at which renal function is not significantly impaired (50% NPX), the severity of the nephropathy induced by mercury is increased. By contrast, when the reduction of renal mass progresses to a level at which renal function begins to become impaired, the level of proximal tubular injury is not greatly different from that of animals with two kidneys, especially at low nephrotoxic doses of inorganic mercury. In addition, low nephrotoxic doses of inorganic mercury do not appear to affect significantly the reduced glomerular filtration rate in 75% NPX rats. However, it does appear that 75% NPX rats may be at greater risk of entering into acute renal failure at higher toxic doses of inorganic mercury than 50% NPX or control rats.     

Development of a limited sampling approach in pharmacokinetic studies: experience with the antiepilepsy drug tiagabine

To examine the mechanisms involved in the progression of mercury chloride (HgCl2)-induced acute tubular necrosis (ATN), we investigated the histopathological changes and the expression of inducible nitric oxide synthase (iNOS) mRNA and protein in renal cortices of rats at 20 hours after exposure to HgCl2. The expression of iNOS mRNA was significantly augmented in renal cortices of rats with HgCl2-induced acute renal failure (ARF). Likewise, the induction of iNOS protein was observed in damaged proximal tubule epithelial cells of rats with HgCl2-induced ARF. Pretreatment of rats with iNOS inhibitor aminoguanidine, however, suppressed the development of proximal tubule epithelial cell injury and prevented an increase in blood urea nitrogen and serum creatinine as well as resulting in a marked fall in iNOS mRNA and protein in rats with HgCl2-induced ARF. These observations indicate that the induction of iNOS may play a role in the progression of HgCl2-induced ATN through the exacerbation of proximal tubule epithelial cell damage.     

Phospholipid metabolism during renal regeneration after acute tubular necrosis

Renal function, structure, and membrane metabolism were studied during regeneration of proximal tubular cells in rats. A reversible syndrome of nonoliguric acute renal failure was induced by the intravenous administration of a low dose of mercuric chloride (1.0 mg Hg/kg). At day 1 there was a marked increase in serum urea nitrogen concentration (SUN), decrease in food intake, and a zone of proximal tubular cell necrosis in the inner cortex. By day 3 low cuboidal epithelial cells were seen, indicating that regeneration had been initiated despite decreased food intake and increasing SUN. Phospholipid synthesis for new membrane formation in regenerating cells was studied by using [14C] choline as a precursor of phosphorylcholine and cytidine diphosphocholine (CDP-choline), which are intermediates in the synthesis of renal choline-containing phospholipid. The rate of [14C]choline incorporation into phospholipids in inner cortical slices was lowest 1 day after mercury administration, then increased constantly for the next 4 days to reach a maximal value 104% above control. The rate declined slowly for the next 11 days and returned to normal by 28 days. The increased rate represented choline phosphoglyceride synthesis, since degradation was unchanged. The entire increment in choline radioactivity in regenerating tissue 2 and 3 days after mercury administration was in phospholipid or CDP-choline, which suggests that the increased number of choline molecules entering the growing cells were trapped in these two forms. The results indicate that renal regeneration is associated with a specific enhancement of the synthesis of choline-containing phospholipids. This anabolic response of the kidney occurs in the presence of systemic catabolism and progressive renal functional insufficiency.     

Myoglobin inhibits proliferation of cultured human proximal tubular (HK-2) cells

Following nephrotoxic injury, renal repair is dependent on tubular regeneration. In the case of myoglobinuric acute renal failure (ARF), persistence of myoglobin within tubular cells, or sublethal injury sustained at the height of exposure to it, might retard this process. To test this hypothesis, a human proximal tubular cell line (HK-2) was cultured for 24 hours in the absence or presence of clinically relevant myoglobin concentrations (0.5, 1, 2, 4 mg/ml). Immediately following myoglobin removal, lethal cell injury (vital dye uptake), lipid peroxidation, and DNA damage (alkaline unwinding assay) were assessed. The extent of cell proliferation was estimated over the next four days by a tetrazolium based (MTT) assay and by determining total intracellular LDH. Myoglobin's effects on protein and DNA synthesis were also assessed (35S-methionine and bromodeoxyuridine incorporation, respectively). Myoglobin induced dose-dependent lipid peroxidation (malondialdehyde generation) and cell death (up to 80% vital dye uptake with the 4 mg/ml challenge). Although 1 mg/ml myoglobin caused no cell death, it induced nearly complete growth arrest. This lasted for approximately three days following myoglobin removal from the media. Neither of two control proteins (albumin; lysozyme) nor a second nephrotoxin (gentamicin; 1 mg/ml) reproduced this effect. The 1 mg/ml myoglobin challenge caused an 80 to 90% depression in protein and DNA synthesis. It also induced significant DNA damage, as assessed by the alkaline unwinding assay (P < 0.01). Iron chelation therapy (deferoxamine) mitigated myoglobin-induced cell killing. However, its addition following myoglobin loading worsened HK-2 outgrowth by exerting a direct anti-proliferative effect. These results indicate that: (1) sublethal myoglobin toxicity can induce transient proximal tubular cell growth arrest, potentially slowing recovery from ARF; (2) this effect correlates with, and could result from, heme-induced DNA damage and a blockade in DNA/protein synthesis; and (3) deferoxamine can inhibit proximal tubular cell proliferation. This possibility needs to be considered in designing clinical trials with DFO for myohemoglobinuric ARF.     

Preventive health practices and perceived health status among women over 50

Changes in the immunoreactivity of laminin have been demonstrated in a variety of tissues undergoing injury-regeneration process and during nephrogenesis. Studies in vitro revealed that laminin is important in many cellular functions such as growth, differentiation, and cell communication. This study examined the changes of laminin in the extracellular matrix (ECM) of proximal straight tubules (PST) in the outer stripe of the outer medulla after renal ischemia, an area where most morphological damage occurred. Anesthetized male Sprague-Dawley adult rats underwent a 30-min temporary occlusion of the left renal artery. The inulin clearance (Cin) of the postischemic left kidneys was significantly decreased at 30 min, 1 day, 2 days, and 5 days after the injury, and at 7 and 10 days after the injury it rose to a level not significantly different from the sham-operated controls. Using immunogold electron microscopy, the density of immunoreactive laminin (gold-conjugated laminin grains/micron2 ECM area) in PST where flattened PST cells were present was decreased at 1 day and 2 days, and at 5 days, a time when the renal function was still depressed, it returned to a level not significantly different from that in controls. These changes were not observed in those PST or proximal convoluted tubules in which morphology was intact. Further studies are needed to determine if these changes in laminin of damaged PST cells have any significant role during their recovery from the ischemic injury.     

Sensitivity of immature regenerating proximal tubular cells in rabbit kidney to mercuric chloride--a light and electron microscopic analysis

The purpose of the present study was to conduct a light and electron microscopic investigation of pathological alterations of the kidney during the recovery phase after injury caused by mercuric chloride toxicity, and the morphological changes in regenerating cells influenced by subsequent mercuric chloride toxicity. The first administration of mercuric chloride to female albino rabbits was performed by intravenous injection at the dose of 1.3 mg/kg, and a second similar injection was given 14 days later. Regenerating cells first appeared 3 days after the first administration and gradually increased thereafter. However, maturation of these cells was incomplete even on the 14th day. After re-administration, the degree of tubular necrosis was significantly reduced compared with that after the initial injection, and most parts of the basement membranes of the degenerated tubules were covered with epithelial cells. Electron microscopy showed that almost all of the cells underwent changes, including disrupted microvilli, irregularly dilated rough endoplasmic reticulum, and dense free polyribosomes arranged as rosettes. Moreover, S-phase cells were found adjacent to the cells. These findings suggested that the regenerating cells were influenced again by the toxic effect of mercuric chloride, but escaped cell necrosis and remained within the reversible stage because of their immaturity and reduced sensitivity to mercuric chloride.     

Cluster analysis of consensus water sites in thrombin and trypsin shows conservation between serine proteases and contributions to ligand specificity

Postischemic injury in recipients of 3-7-d-old renal allografts was classified into sustained (n = 19) or recovering (n = 20) acute renal failure (ARF) according to the prevailing inulin clearance. Recipients of optimally functioning, long-standing allografts and living donors undergoing nephrectomy served as functional (n = 14) and structural controls (n = 10), respectively. Marked elevation above control of fractional clearance of dextrans of graded size was consistent with transtubular backleak of 57% of filtrate (inulin) in sustained ARF. No backleak was detected in recovering ARF. To explore a structural basis for backleak, allograft biopsies were taken intraoperatively, 1 h after reperfusion in all recipients, and again on day 7 after transplant in a subset (n = 10). Electron microscopy revealed disruption of both apical and basolateral membranes of proximal tubule cells in both sustained and recovering ARF, but cell exfoliation and tubule basement membrane denudation were negligible. Histochemical analysis of membrane-associated adhesion complexes confirmed an abnormality of proximal but not distal tubule cells, marked in sustained ARF but not in recovering ARF. Staining for the zonula occludens complex (ZO-1) and adherens complex (alpha, beta, and gamma catenins) revealed diminished intensity and redistribution of each cytoskeletal protein from the apico-lateral membrane boundary. We conclude that impaired integrity of tight junctions and cell-cell adhesion in the proximal tubule provides a paracellular pathway through which filtrate leaks back in sustained allograft ARF.     

Potential contribution of endothelin to renal abnormalities in glycerol-induced acute renal failure in rats

We studied the possible participation of endothelin-1 (ET-1) in the pathogenesis of renal damage in glycerol-induced acute renal failure (ARF). Cortical mRNA expression of ET-1 increased, peaking at 10 hr postinjury, but this did not occur in the medulla, plasma concentration and urinary excretion of ET-1 also increased in this model. There was no change in ETA receptor mRNA, whereas the ETB receptor tended to be down-regulated in the kidney after glycerol administration. In situ hybridization study demonstrated that elevated expression of prepro ET-1 was predominantly localized in cells in the proximal tubules of the nephritic kidney. The administration (30-3 mg/kg) of S-0139, (+)-disodium 27-[(E)-3-[2-[(E)-3-carboxylatoacryloylamino]-5-hydroxyphenl ]acrylayl oxy]-3-oxoolean-12-en-28-oate, an ETA selective antagonist, after initiation of insult offered dose-dependent prevention against ARF, demonstrating preventing of renal function impairment and mortality. These findings indicate that ET-1 participates in the pathogenesis of acute tubular injury in glycerol-induced ARF and that ETA antagonist may be useful in the treatment of some types of human ARF.     

A constitutive law for mitral valve tissue

BACKGROUND: Obstructive nephropathy is a primary cause of renal failure in infancy. Chronic unilateral ureteral obstruction (UUO) in the neonatal rat results in reduced renal expression of epidermal growth factor (EGF), renal tubular epithelial (RTE) cell apoptosis and interstitial fibrosis. We wished to determine whether these changes could be prevented by exogenous administration of EGF. METHODS: Thirty-three Sprague-Dawley rats underwent UUO within the first 48 hours of life, and received daily injections of either EGF (0.1 mg/kg/day) or saline (control) for the following seven days, after which obstructed and intact opposite kidneys were removed for study. These were compared to 11 sham-operated rats that received either no injections, EGF injections, or saline injections. Renal cell proliferation was determined by proliferating cell nuclear antigen, apoptosis was measured by the TUNEL technique, and the distribution of vimentin, clusterin, transforming growth factor-beta 1 (TGF-beta 1), and alpha-smooth muscle actin were determined by immunohistochemistry. Tubular dilation, tubular atrophy, and interstitial collagen deposition were quantitated by histomorphometry. RESULTS: Compared to controls, EGF treatment increased RTE cell proliferation in the obstructed kidney by 76%, decreased apoptosis by 80%, and reduced vimentin, clusterin and TGF-beta 1 immunostaining (all P < 0.05). EGF treatment reduced tubular dilation by 50%, atrophic tubules by 30%, and interstitial fibrosis by 50% (all P < 0.05). There was no significant effect of EGF on renal alpha smooth muscle actin distribution. There was no effect of saline or EGF injections on kidneys from sham-operated rats for any of the parameters studied. CONCLUSIONS: We conclude that EGF stimulates RTE cell proliferation and maturation and reduces apoptosis in the neonatal rat kidney subjected to chronic UUO. These effects may contribute to the reduction in tubular dilation, tubular atrophy, and interstitial fibrosis. By preserving renal development, administration of EGF attenuates the renal injury resulting from chronic UUO.     

Pathomorphology of the kidneys in hemolytic-uremic syndrome in children

The kidneys were studied morphologically in 64 cases of hemolytic-uremic syndrome (HUS) in children aged from 17 days to 4 years. Three variants of the kidney alterations are distinguished depending on the duration of acute renal failure (ARF): early, late and delayed alterations. Early alterations were observed if ARF lasted for 1-10 days and were characterized by glomerular thrombotic microangiopathy with disturbance of the nephron proximal part function resultant from microcirculatory damage. Late kidney alterations (11-20 days of ARF) consisted of glomerular thrombi fibrinolysis, development of mesangial cell proliferation and mesangiolysis, proximal tubular epithelium regeneration and the appearance of nephrohydrosis resulting from the distal nephron part obstruction by hyalin cylinders. Delayed alterations (21-62 days of ARF) manifested with predominant atrophic and sclerotic processes. The first variant corresponded to the acute course of HUS while the two last variants to the subacute one.     

Histological diagnosis in aspiration curettage]

Gentamicin sulfate was given to rabbits for four weeks in two dosage regimens, either 4 or 8 mg/kg subcutaneously twice a day. Two of the seven animals in each group regained nearly normal renal function after a transient rise in the level of serum creatinine, but the remainder developed severe renal failure leading to the death of all but one of these rabbits. None of the animals showed significant hyperglycemia, buy glycosuria appeared in all and was significantly correlated with the extent of renal damage. These findings demonstrate gentamicin-induced derangement of proximal tubular function and thus amplify the previously demonstrated histologic changes due to the drug. When the drug was administered twice daily, there was marked nephrotoxicity in rabbits given dosages of gentamicin only slightly higher than those employed clinically.     

Comparison of the inflammatory responses of mice infected with American and Australian Trichinella pseudospiralis or Trichinella spiralis

We have examined the response of the renal insulin-like growth factor (IGF-I) axis to acute ischemic injury in the rat Key findings included a decrease in IGF-I mRNA and peptide levels, a decrease in GH receptor gene plus protein expression and a decrease in the IGF binding proteins except for IGF binding protein I. Administration of GH to compensate for the reduced GH receptor binding corrected the IGF-I mRNA levels suggesting a relative GH deficiency. Interestingly, IGF-I receptor mRNA levels were unchanged while plasma membrane IGF-I receptor number increased two fold. This appeared to be due to a redistribution of receptors to a membrane location. IGF-I receptor autophosphorylation and tyrosine kinase activity were intact despite severe uremia for up to 6 days. We propose that this increase of functional IGF-I receptors following acute tubular necrosis will sensitize the kidney to the administration of exogenous IGF-I.     

Segmental localization and quantitative characteristics of tubulitis in kidney biopsies from patients undergoing acute rejection

The term tubulitis denotes infiltration of the renal tubular epithelium by mononuclear cells. Tubulitis is one of the most reliable signs of acute renal allograft rejection. However, its segmental localization and quantitative characteristics are not precisely known. To investigate this question, formalin-fixed kidney biopsy specimens from 15 patients with transplanted allografts undergoing acute rejection were studied stereologically by identifying cortical tubules with segment-specific markers. The periodic acid-Schiff reaction, peanut lectin, and antibodies against Tamm-Horsfall protein and epidermal cytokeratins, all applied to the same section, were used to identify the profiles of proximal tubules (PTs), distal convoluted tubules (DCTs), distal straight tubules (DSTs), and the cortical collecting system (CCS, connecting tubules and cortical collecting ducts), respectively. Two parameters, the relative intrasegmental length and the average intensity of tubular damage, were determined to describe the degree of tubulitis quantitatively. Tubulitis was most prominent in the DCTs, followed by the CCS. The average intensity of tubulitis was lowest in the DSTs. The results indicate that the PTs are not the main site of tubulitis, despite the fact that they are regarded primary targets of the rejection response.     

A non-aversive approach to reducing hospital absconding in a head-injured adolescent boy

Human envenomation caused by bee or wasp stings has been reported to cause acute renal failure (ARF), usually due to acute tubular necrosis (ATN), as a frequent complication. The pathogenetic mechanisms of ATN occurring in these accidents are still unclear. In the present study, female Wistar rats weighing 150-200 g were injected intravenously with Africanized bee venom at a dose of 0.4 microl/100 g body weight and used in functional and light microscopy studies. The animals were divided into two groups: the early group was studied 3-8 h after inoculation, and the late group was studied 24-30 h thereafter. The animals showed ARF characterized by reduction of glomerular filtration rate with increasing levels of plasma creatinine. They also showed increased fractional sodium and potassium excretions, suggesting changes in the proximal portion of the nephron. The water transport through collecting tubules was reduced, with consequent diuresis, indicating functional changes in the distal portion of the nephron. These functional changes were more marked in the early group, with recovery tending to occur after 24 h. Albuminuria was also observed in this group. Light microscopy showed ATN mainly in cortex and outer medulla, with isolated necrosis in cells or small groups of cells and cast formation in the distal and collecting tubules. After 24 h frequent mitotic figures were found in the tubular epithelium. The observed ARF was due to ATN which in turn was probably caused by multiple effects, mainly hemodynamic changes secondary to cardiotoxicity and systemic vasodilation caused by the venom, myohemoglobinuria, and the direct action of the venom on tubular cells.     

Decreased expression of mitochondrial-derived H2O2 and hydroxyl radical in cytoresistant proximal tubules

Increased production of reactive oxygen metabolites (ROM) can contribute to the initiation phase of nephrotoxic and ischemic acute renal failure (ARF). However, whether altered ROM expression also exists during the maintenance phase of ARF has not been adequately assessed. Since diverse forms of tubular injury can initiate a "cytoresistant state," this study tested whether a down-regulation of ROM expression might develop in the aftermath of acute tubular damage, potentially limiting renal susceptibility to further attack. To test this hypothesis, rats were subjected to either mild myohemoglobinuria (glycerol injection) or bilateral ureteral obstruction and 24 hours later, cytoresistant proximal tubular segments (PTS) were isolated to assess ROM expression. PTS from sham operated rats were used to establish normal values. Both sets of cytoresistant PTS manifested approximately 75% reductions in H2O2 levels, as assessed by the phenol red/horseradish peroxidase technique (P < 0.01 to 0.001). A 40% reduction in hydroxyl radical (.OH) levels was also observed (salicylate trap method), thereby substantiating decreased oxidant stress in cytoresistant PTS. Catalase, glutathione peroxidase, and free iron levels were comparable in control and cytoresistant PTS, suggesting that decreased H2O2 production (such as by mitochondria) was the cause of the decreased oxidant stress. To test this latter hypothesis, H2O2 expression by control and cytoresistant PTS was assessed in the presence of respiratory chain inhibitors. Although site 1 and site 3 inhibition markedly suppressed H2O2 production in control PTS, they had no impact on H2O2 production in cytoresistant PTS, implying that production at these sites was already maximally suppressed. Correlates of the decreased mitochondrial H2O2 production were improvements in cell energetics (increased ATP/ADP ratios with Na ionophore treatment) and approximately 40 to 90% increases in PTS/renal cortical glutathione content. We conclude that: (1) proximal tubule H2O2/.OH expression can be downregulated during the maintenance phase of ARF; (2) this seemingly reflects a decrease in mitochondrial ROM generation; and (3) the associated improvements in glutathione content and/or cellular energetics could conceivably contribute to a post-injury cytoresistant state.     

Effects of caffeine on the metabolic and catecholamine responses to exercise in 5 and 28 degrees C

A controversy presently exists concerning the ability of albumin to inhibit the tubular reabsorption of low-molecular-weight (M(r)) proteins in experimental renal diseases leading to massive proteinuria. We have examined the urinary excretion of albumin and of 2 low-M(r) proteins, beta 2-microglobulin and cystatin C, in rats treated with toxins affecting primarily the glomerulus (puromycin amino-nucleoside and Adriamycin) or the tubule (mercuric chloride and maleic acid). Above a threshold of 100 mg/24 h, albuminuria induced by puromycin aminonucleoside (50 mg/kg) and Adriamycin (5 mg/kg) was associated with a marked increase in the urinary excretion of beta 2-microglobulin and cystatin C peaking at more than 100-fold the baseline levels. These glomerulotoxins did not affect the urinary excretion of the tubular enzyme N-acetyl-beta-D-glucosaminidase. This pattern of effects was completely different from that induced by mercuric chloride (2 mg/kg) and maleic acid (400 mg/kg) which increased the excretion of both N-acetyl-beta-D-glucosaminidase and low-M(r) proteins in rats with albuminuria values below 100 mg/24 h. These results strongly support the hypothesis that at high filtered loads, albumin decreases the tubular uptake of low-M(r) proteins most likely by competition for a common transport mechanism.     

Distribution of epidermal growth factor receptor and ligands during bronchiolar epithelial repair from naphthalene-induced Clara cell injury in the mouse

Clara cells are primary targets for metabolically activated pulmonary toxicants because they contain an abundance of the cytochrome P450 monooxygenases required for generation of toxic metabolites. The factors that regulate bronchiolar regeneration after Clara cell injury are not known. Previous studies of naphthalene-induced bronchiolar injury and repair in the mouse have shown that epithelial cell proliferation is maximal 1 to 2 days after injury and complete 4 days after injury. Proliferation is followed by epithelial re-differentiation (4 to 14 days). In this study, mice were treated with the environmental pollutant naphthalene to induce massive Clara cell injury. The distribution and abundance of three growth-regulatory peptides (epidermal growth factor receptor (EGFR), epidermal growth factor (EGF), and transforming growth factor (TGF)-alpha) was determined immunochemically during repair of this acute bronchiolar injury. EGFR and its ligands were detected at low levels in cells throughout the lung including peribronchiolar interstitial cells, blood vessels, and conducting airway epithelium. Immediately after naphthalene injury (1 to 2 days), EGFR, EGF, and TGF-alpha are expressed in increased abundance in squamous epithelial cells of the injury target zone, distal bronchioles. These immunopositive squamous cells are detected in clumps in the distal bronchioles at the time when cell proliferation is maximal. EGFR protein expression is decreased slightly 4 to 7 days after injury and continues to decrease below control levels of abundance 14 to 21 days after injury. This down-regulation of EGFR is not reflected in a corresponding decrease in EGF and TGF-alpha protein expression, indicating that control of cell proliferation is regulated at the receptor level. Co-localization of EGFR and bromodeoxyuridine-positive proliferating cells in the same bronchiole indicates that EGFR is up-regulated within the proliferative microenvironment as well as in specific proliferating cells within the injury target zone. The coincident localization within terminal bronchioles of EGFR, EGF, and TGF-alpha to groups of squamous epithelial cells 2 days after naphthalene injury suggests that these peptides are important in up-regulating cell proliferation after Clara cell injury in the mouse.     

Evidence for distal tubular inhibition of calcium efflux by nisoldipine in the SHR rat

Nisoldipine, a calcium channel blocking agent, is known to have antihypertensive, renal tubular and hemodynamic effects. The present studies were designed to examine the effects of this drug on the renal tubular transport of calcium in 12 saline-loaded SHR rats. Calcium-45 was infused into three different nephron segments: early proximal, late proximal and early distal sites with or without nisoldipine. Calcium efflux averaged 93.6 +/- 4.9 and 90.5 +/- 8.7% after early and late proximal administration, respectively, indicating that the proximal tubule and the loop of Henle are highly efficient in transporting calcium out of the tubule. In distal nephron segments, calcium transport was limited to 41.1 +/- 4.8% of the amount delivered to these tubules. Nisoldipine inhibited the efflux of simultaneously infused calcium. This apparent inhibitory effect occurred predominantly in distal nephron segments where the drug reduced calcium efflux from 41.1 +/- 4.8 to 22.5 +/- 2.7%, indicating a 45.3% reduction in net calcium reabsorption. The results are consistent with the interpretation that nisoldipine-induced reduction in the tubular efflux of calcium was secondary to a direct inhibition of voltage-sensitive, L-type calcium channels or to a blunting of the rate of phosphorylation of channel proteins by protein kinase C in the distal tubular epithelial cells.