Studies of renal injury. II. Activation of the glucose transporter 1 (GLUT1) gene and glycolysis in LLC-PK1 cells under Ca2+ stress.

Injury to the renal proximal tubule is common and may be followed by either recovery or cell death. The survival of injured cells is supported by a transient change in cellular metabolism that maintains life even when oxygen tension is reduced. This adaptive process involves the activation of the gene encoding the glucose transporter GLUT1, which is essential to maintain the high rates of glucose influx demanded by glycolysis. We hypothesized that after cell injury increases of cell Ca2+ (Ca2+i) initiate the flow of information that culminates with the upregulation of the stress response gene GLUT1. We found that elevations of Ca2+i caused by the calcium ionophore A23187 activated the expression of the GLUT1 gene in LLC-PK1 cells. The stimulatory effect of Ca2+i on GLUT1 gene expression was, at least in part, transcriptional and resulted in higher levels of GLUT1 mRNA, cognate protein, cellular hexose transport activity, glucose consumption, and lactate production. This response was vital to the renal cells, as its interruption severely increased Ca2+-induced cytotoxicity and cell mortality. We propose that increases of Ca2+i initiate stress responses, represented in part by activation of the GLUT1 gene, and that disruption to the flow of information originating from Ca2+-induced stress, or to the coordinated expression of the stress response, prevents cell recovery after injury and may be an important cause of permanent renal cell injury and cell death.

Evidence that angiotensin-II and potassium collaborate to increase cytosolic calcium and stimulate the secretion of aldosterone.

Angiotensin-II (AII) and potassium (K+) as stimuli of aldosterone secretion enhance each other's stimulatory potential. In the present study we looked for evidence that AII and K+ act through a common mechanism of signal transduction to affect secretion. Bovine adrenal glomerulosa cells were loaded with the calcium (Ca2+) probe aequorin to permit detection over prolonged time periods of the changes in cytosolic Ca2+ that occur in response to AII and K+. Perfusion fractions were collected for simultaneous measurement of aldosterone production rates. AII (10(-7) M) produced an immediate and transient increase in Ca2+, followed by a Ca2+ plateau that remained above baseline for as long as AII was present. An increase in K+ concentration (from 5 to 12 mM) produced a slow and eventually sustained increase in cytosolic Ca2+, which resembled the plateau produced by AII. Nitrendipine (10(-5) M) completely inhibited the secretory response to AII and K+ (during 60-min incubations) and inhibited the typical K+-induced increase in Ca2+. The sustained increase in Ca2+ with AII (the plateau) required extracellular Ca2+ and was proportional to the prevailing extracellular K+ concentration. When glomerulosa cells were incubated with AII, the aldosterone secretory response to K+ was substantially enhanced (P less than 0.001). In summary, stimulation by both AII and K+ resulted in a sustained increase in Ca2+ influx. AII-induced Ca2+ influx was dependent on the ambient K+ concentration. These results indicate that AII and K+ act together to determine the optimal rate of Ca2+ entry, which may then lead to the appropriate secretory rate of aldosterone.

Inhibition of the renal tubular effects of parathyroid hormone on phosphate transport by prostaglandin E2.

The infusion of PGE2 into TPTX dogs abolishes the phosphaturic action of PTH in vivo. This effect occurs only if PGE2 infusion precedes the administration of PTH. The infusion of PGE2 in vivo induces a state of refractoriness of PTH-sensitive adenylate cyclase in renal middle medulla from TPTX dogs in vitro. These results suggest that PGE2 inhibits the biological effects of PTH, in part by altering the events related to PTH activation of renal adenylate cyclase. These effects of PGE2, a local mediator, suggest that the renal tubular cell may modulate its responses to multiple and antagonistic stimuli by altering local renal prostaglandins.

Enhancement of parathyroid hormone-stimulated bone resorption by poly-L-lysine.

Poly-L-lysine (PL II; mol wt, 1000-4000) was added to fetal rat bones cultured in a chemically defined medium (BGJ) containing bovine serum albumin in the presence and absence of parathyroid hormone (PTH). Bone resorption was measured by the release of previously incorporated 45Ca. The addition of PL II at concentrations of 3-100 microgram/ml enhanced the stimulation of bone resorption by submaximal doses of PTH but had little effect on 45Ca release from control unstimulated cultures. Higher concentrations of PL II produced inhibition of 45Ca release. Dialysis of PL II did not alter enhancement or inhibition by PL II. PL II did not increase sensitivity to PTH in serum-supplemented cultures. Higher molecular weight PL II preparations were less effective. PL II did not enhance the resorptive response to 1,25-dihydroxyvitamin D, prostaglandin E2, osteoclast-activating factor, or bacterial endotoxin. The mechanism of the selective ability of PL II to enhance the response to low concentrations of PTH is unknown but may be due to the ability of this basic polypeptide to interfere with binding of PTH to sites other than the hormone receptor or to block degradation of PTH by bone.

Dietary phosphate deprivation in women and men: effects on mineral and acid balances, parathyroid hormone and the metabolism of 25-OH-vitamin D.

We evaluated the effects of dietary PO4 restriction on 25-OH-Vitamin D3 metabolism, serum iPTH levels, and mineral balances in healthy women and men. PO4 balances were progressively negative because of fecal losses without sex difference. Turnover of the plasma 25-OH-D pool was increased from 5.8 +/- 0.4 to 12 +/- 1.2 nmol/day; P less than 0.001, despite a fall in serum iPTH of -1.1 +/- 0.3 mulEq/ml; P less than 0.01. In both sexes, net intestinal calcium and magnesium absorption increased in proportion to a more rapid turnover of the plasma 25-OH-D pool, implying increased renal 1,25-(OH)2-D3 production. By contrast, there was a striking sex difference in the response of serum PO4 to dietary PO4 deprivation; the levels falling progressively in women, but remaining at control levels in men. Women demonstrated progressive hypercalciuria and negative Ca balances while in men the increments in intestinal Ca absorption were approximately matched by the increments in urinary Ca excretion so that Ca balances were not different from zero.

Inhibition of aldosterone production by pinacidil in vitro.

Pinacidil, an antihypertensive agent that opens potassium channels, lowers plasma aldosterone levels in hypertensive patients by an unknown mechanism. In the present study, pinacidil's direct effects on production of aldosterone were assessed using isolated cells from bovine adrenal glomerulosa. Pinacidil was found to inhibit aldosterone production, both basally and during stimulation with either potassium, angiotensin II (Ang II), or adrenocorticotropic hormone (p less than 0.001), with half maximal inhibition occurring at 10(-5) M. As assessed by the exclusion of trypan blue from cells, pinacidil did not inhibit secretion through injurious effects on glomerulosa cells. Also, washing of cells previously exposed to pinacidil restored secretory responsiveness. Pinacidil did not alter cytosolic calcium (Ca2+) concentrations when aequorin was used as a photoluminescent indicator of Ca2+ levels, suggesting that pinacidil acted by a non-Ca(2+)-mediated mechanism. Consistent with direct inhibition of the late pathway in steroidogenesis was that pinacidil decreased conversion of pregnenolone and corticosterone to aldosterone. Pinacidil did not block binding of Ang II to its receptor, nor did it appear to affect adrenocorticotropic hormone-receptor binding, since stimulation by cyclic AMP, the post-receptor second messenger of adrenocorticotropic hormone, was also inhibited. In summary, pinacidil inhibited directly the adrenal's production of aldosterone. The mechanism whereby the inhibition occurred was unclear.

The lack of effect of chronic metabolic acidosis on 25-OH-vitamin D metabolism and serum parathyroid hormone in humans.

We evaluated the turnover of the plasma 25-OH-vitamin D pool, acid, and mineral balances in paired balance studies of 6 normal subjects during normal acid base conditions and during stable chronic metabolic acidosis induced by NH4Cl. Positive acid balances and negative Ca balances due to hypercalciuria were observed as previously reported. Plasma 25-OH-D pool turnover averaged 6.1+/-0.4 nmol/day during control and did not change during acidosis (6.5 +/- 0.5 nmol/day) nor were any significant increments in net intestinal absorption of Ca, PO4, or Mg, the physiological expression of vitamin D action, observed during acidosis. In 3 other subjects, repetitive measurements of serum iPTH during 7 control days and 24 days of stable NH4Cl acidosis showed no changes. We interpret the data to support the hypothesis that neither PTH nor vitamin D and its metabolites mediates the increase in net bone resorption that must accompany chronic metabolic acidosis.

Plasma 1,25-(OH)2-vitamin D concentrations and net intestinal calcium, phosphate, and magnesium absorption in humans.

We evaluated the relationship between plasma concentrations of the renal hormone 1,25-(OH)2-vitamin D and net intestinal absorption of Ca, PO4, and Mg in vitamin D-replete patients eating similar diets, who had undetectable, normal or elevated plasma 1,25-(OH)2-D levels, Net intestinal Ca absorption was positively correlated to plasma 1,25-(OH)2-D concentrations: percentage dietary Ca absorbed = 10 + 0.17 x plasma total 1,25-(OH)2-3, pmole/liter, r = + 0.58; P less than 0.001. By contrast, there was no significant correlation between PO4 or Mg absorption and plasma 1,25-(OH)2-D concentrations. Moreover, significant quantities of PO4 and Mg were absorbed in the absence of detectable plasma 1,25-(OH)2-D. We conclude that net intestinal Ca absorption is critically dependent upon the availability of the renal hormone 1,25-(OH)2-D in vitamin D-replete humans when dietary Ca intake is normal. By contrast, other factors must play a dominant role in regulating net intestinal PO4 and Mg absorption.

Spontaneous hypercalcemia in patients undergoing dialysis. Etiologic and therapeutic considerations.

Ten dialysis-treated patients with hypercalcemia (11.5 +/- 0.3 mg/dl, mean +/- SE) due to renal osteodystrophy were compared with 30 control dialysis-treated patients who were not hypercalcemic (9.5 +/- 0.1 mg/dl). The hypercalcemic patients were more disabled than the control patients. Fifty percent of the hypercalcemic patients and 37 percent of the control patients had a mineralization defect (p greater than 0.6). In the control group, intact parathyroid hormone level was significantly higher in patients with osteitis fibrosa than in those with osteomalacia (247 +/- 39 pg/ml versus 60 +/- 20 pg/ml, respectively, p less than 0.005) whereas in the hypercalcemic patients, parathyroid hormone measurements did not discriminate between these two types of bone disease. Osteomalacia was more severe and bone aluminum staining was stronger in the hypercalcemic patients than in the control patients (2.02 +/- 0.47 versus 0.35 +/- 0.11 mm/mm2 tissue area, p less than 0.001). The mean serum calcium level fell from 11.2 +/- 0.2 mg/dl to 10.5 +/- 0.3 mg/dl (p less than 0.01) in eight hypercalcemic patients treated with 24,25-dihydroxyvitamin D. It is concluded that hypercalcemia in patients undergoing dialysis is associated with an increase in bone aluminum level, and with more severe osteomalacia. Intact parathyroid hormone levels are useful for predicting bone histomorphometric parameters but only when hypercalcemia is not present. The drug, 24,25-dihydroxyvitamin D, was effective in lowering the serum calcium level.

Muscle group-specific regulation of GLUT 4 glucose transporters in control, diabetic, and insulin-treated diabetic rats.

Insulin resistance in diabetic rats involves pretranslational suppression of the GLUT 4 glucose transporter in muscle. Because the capacity for insulin-mediated glucose transport varies as a function of muscle group, we hypothesized that GLUT 4 was differentially expressed and regulated by diabetes in a muscle-specific manner. We studied control (C), streptozocin (STZ)-induced diabetic (D), and insulin-treated diabetic (Tx) rats and examined the following muscles that vary in fiber composition: soleus (type I fibers), gastrocnemius (mixed type IIa > IIb), vastus lateralis and rectus abdominis (type IIb > IIa), and cardiac muscle. In C animals, these muscles exhibited significant differences in the baseline expression of GLUT 4. Relative GLUT 4 content was highest in cardiac muscle, intermediate in soleus, and significantly lower in gastrocnemius, rectus abdominis, and vastus lateralis (1.8:1.0:0.6). The impact of diabetes and insulin therapy on GLUT 4 expression also varied as a function of muscle group. After four weeks of diabetes, GLUT 4 levels were reduced by approximately 50% in cardiac muscle, soleus, and gastrocnemius. In contrast, GLUT 4 content in rectus abdominis and vastus lateralis was similar to that in control rats. Exogenous insulin treatment of diabetic rats increased GLUT 4 content in soleus, cardiac muscle, and gastrocnemius, but had no effect in either vastus lateralis or rectus abdominis. Temporal effects of diabetes and insulin treatment were also examined in different skeletal muscle. Soleus showed a significant decrease in GLUT 4 content as early as 2 days with a further decrease at 4 weeks; rectus abdominis showed little change at either time point.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of timing of estradiol benzoate administration upon synchronization of ovulation in suckling Nelore cows (Bos indicus) treated with a progesterone-releasing intravaginal device.

The present study investigated how the timing of the administration of estradiol benzoate (EB) impacted the synchronization of ovulation in fixed-time artificial insemination protocols of cattle. To accomplish this, two experiments were conducted, with EB injection occurring at different times: at withdrawal of the progesterone-releasing (P4) intravaginal device or 24h later. The effectiveness of these times was compared by examining ovarian follicular dynamics (Experiment 1, n=30) and conception rates (Experiment 2, n=504). In Experiment 1, follicular dynamics was performed in 30 Nelore cows (Bos indicus) allocated into two groups. On a random day of the estrous cycle (Day 0), both groups received 2mg of EB i.m. and a P4-releasing intravaginal device, which was removed on Day 8, when 400 IU of eCG and 150 microg of PGF were administered. The control group (G-EB9; n=15) received 1mg of EB on Day 9, while Group EB8 (G-EB8; n=15) received the same dose a day earlier. Ovarian ultrasonographic evaluations were performed every 8h after device removal until ovulation. The timing of EB administration (Day 8 compared with Day 9) did affect the interval between P4 device removal to ovulation (59.4+/-2.0 h compared with 69.3+/-1.7h) and maximum diameter of dominant (1.54+/-0.06 acm compared with 1.71+/-0.05 bcm, P=0.03) and ovulatory (1.46+/-0.05 acm compared with 1.58+/-0.04 bcm, P<0.01) follicles. In Experiment 2, 504 suckling cows received the same treatment described in Experiment 1, but insemination was performed as follows: Group EB8-AI48 h (G-EB8-AI48 h; n=119) and Group EB8-AI54 h (G-EB8-AI54 h; n=134) received 1mg of EB on Day 8 and FTAI was performed, respectively, 48 or 54 h after P4 device removal. Group EB9-AI48h (G-EB9-AI48 h; n=126) and Group EB9-AI54 h (G-EB9-AI54 h; n=125) received the same treatments and underwent the same FTAI protocols as G-EB8-AI48 h and G-EB8-AI54 h, respectively; however, EB was administered on Day 9. Conception rates were greater (P<0.05) in G-EB9-AI54 h [63.2% (79/125) a], G-EB9-AI48 h [58.7% (74/126) a] and G-EB8-AI48 h [58.8% (70/119) a] than in G-EB8-AI54 h [34.3% (46/134) b]. We concluded that when EB administration occurred at device withdrawal (D8), the interval to ovulation shortened and dominant and ovulatory follicle diameters decreased. Furthermore, when EB treatment was performed 24h after device removal, FTAI conducted at either 48 or 54 h resulted in similar conception rates. However, EB treatment on the same day as device withdrawal resulted in a lesser conception rate when FTAI was conducted 54 h after device removal.

Renal injury: similarities and differences in male and female rats with the metabolic syndrome.

The metabolic syndrome is complicated by nephropathy in humans and rats, and males are more affected than females. We hypothesized that female rats had reduced expression of glomerular oxidized low-density lipoprotein (oxLDL) receptor 1 (LOX-1), attendant glomerular oxidant injury, and renal inflammation. Three groups, obese males (OM), obese females (OF), and lean males (LM) of first-generation (F(1)) hybrid rats derived from the Zucker fatty diabetic (ZDF) strain and the spontaneous hypertensive heart failure rat (SHHF/Gmi-fa) were studied from 6 to 41 weeks of age. OM had severe renal oxidant injury and renal failure. Their glomeruli expressed the LOX-1, and exhibited heavier accumulation of the lipid peroxide 4-hydroxynonenal (4-HNE). OM had compromised mitochondrial enzyme function, more renal fibrosis, and vascular leakage. Younger LM, OM, and OF ZS (ZDF/SHHF F(1) hybrid rat) rats, studied from 6 to 16 weeks of age, showed that unutilized renal lipids were comparable in OM and OF, although young OM had worse nephropathy and inflammation. In conclusion, glomerular LOX-1 expression is coupled to deposits of 4-HNE and glomerulosclerosis in OM. We presume that LOX-1 enhances glomerular uptake of oxidized lipids and renal inflammation, causing greater oxidant stress and severe glomerulosclerosis. In OF, renal protection from lipid oxidants appears to be conferred by blunted glomerular LOX-1 expression and renal inflammation.

Prostaglandin E2 antagonizes the renal effects of parathyroid hormone but not those mediated by a cyclic AMP analog.

An infusion of PGE2 resulted in the development of refractoriness to the phosphaturic effects of PTH in the chronically TPTX dog. The antagonism of PGE2 to the actions of PTH was not observed when the phosphaturia was stimulated by the cAMP analog, 8-bromo cAMP. We propose that the development of antagonism to the actions of PTH by PGE2 is localized to cellular events proximal to the generation of tubular cAMP. This antagonism by PGE2 could be an effective regulatory mechanism of hormonal action at the local level.

Effect of ACTH on PTH-stimulated bone resorption in vitro.

In vitro demonstration of PTH effects requires hormone concentrations greater than the "physiological" concentrations reported by radioimmunoassay or cytochemical assays. This discrepancy could be the result of binding or destruction of PTH at nonbiologically active sites. In the present study, ACTH was found to have no effect by itself on bone resorption, but addiction of ACTH to bone cultures together with low concentrations of PTH resulted in a specific enhancement of PTH-stimulated bone resorption. This effect was not observed when bone resorption was stimulated by PGE2 and 1,25(OH)2D3, and it was blocked by human serum. The effect of ACTH is similar to the enhancement in PTH-stimulated bone resorption by poly-l-lysine [7]. We suggest that the amplification of PTH stimulation was the result of displacement of PTH from nonbiologically active sites, making more PTH available for binding to its biologically active receptor. An alternative explanation for our results was that ACTH prevented degradation of PTH by bone-derived proteolytic enzymes. Thus the sensitivity of bioassays for PTH could be improved by adding ACTH.

Overexpression of GLUT2 gene in renal proximal tubules of diabetic Zucker rats.

Renal tubular reabsorption of glucose is substantially increased in humans and rats with diabetes mellitus. The influx of luminal glucose is mediated by Na+/glucose cotransporter system and glucose efflux from tubules to interstitium by facilitative glucose transporters (GLUT). In Zucker diabetic rats, GLUT2 protein levels of renal proximal tubules were higher than in control litter mates: 9.67 +/- 1.95 versus 4.72 +/- 1.55 (P = 0.0073). In the same proximal tubules, diabetes was associated with minor decreases in GLUT1 protein levels: 1.96 +/- 0.37 for diabetics and 2.37 +/- 0.34 for controls (P = 0.12). Na+/glucose cotransporter system protein levels were similar in both groups, whereas Na+/K+ ATPase levels were slightly decreased in diabetic rats, but the difference was not statistically significant. In this report, it is suggested that in long-term uncontrolled diabetes, GLUT2 transporters are overexpressed in renal tubules. This adaptation promotes low-affinity, high-capacity glucose efflux. The higher number of high K(m) GLUT2 ensures that glucose reabsorption is increased by promoting glucose efflux, which could be rate-limiting in the face of hyperglycemia.

Studies of renal injury. I. Gentamicin toxicity and expression of basolateral transporters.

Gentamicin nephrotoxicity may arise in part from alterations in the expression of genes critical for renal proximal tubule metabolism. We tested the hypothesis that gentamicin suppressed the gene expression of the Na+/Ca2+ exchanger (NaCaX), glucose transporter 1 (GLUT1) and alpha 1-subunit of Na(+)-K(+)-ATPase (alpha 1-NKA) in renal tubules. The products of these genes mediate Na(+)-dependent Ca2+ efflux, glucose efflux and influx, and ATP-dependent Na+ efflux across tubular basolateral membranes, respectively. After 10 days of gentamicin intoxication (40 mg/kg ip, twice daily), levels of mRNAs encoding NaCaX and the cognate protein declined. GLUT1 mRNA levels increased, although GLUT1 protein levels were also reduced. Moreover, whereas alpha 1-NKA mRNA levels remained unchanged, alpha 1-NKA protein levels were also reduced. We suggest that the higher GLUT1 mRNA level is part of the stress response to tubular injury. However, regardless of the mRNA level, the most consistent effect of gentamicin was reduction of specific protein levels. We propose that failure to translate high levels of mRNA into proportionally high levels of protein, as in the case of GLUT1, may attenuate the expression of stress response gene products, and thus diminish the possibility of recovery in gentamicin intoxication.

Interference by prostaglandin E2 with the phosphaturic effects of nonhormonal agents.

The purpose of the present study was to evaluate the ability of prostaglandin (PG) E2 to alter phosphate excretory patterns induced by the nonhormonally mediated phosphaturic maneuvers, bicarbonate loading (BIC), volume expansion (VE) and acetazolamide administration (Az). Acute clearance studies were performed in chronically thyroparathyroid-ectomized dogs. The dosage of PGE2 utilized was free of excretory or hemodynamic effects. After BIC, percentage of phosphate excretion (%EP) increased from 4.8 to 14.0%, (P less than .05). In contrast, BIC preceded by PGE2 infusion produced no phosphaturia. Additionally, PGE2 blunted the increases in Na and HCO3 excretion associated with BIC. VE (5-6% b.wt.) increased %EP from 6.0 to 14.2%, (P less than .01). However, VE after PGE2 treatment did not result in a phosphaturia. Az increased %EP from 4.7 to 28.5%, (P less than .02). When PGE2 was infused before Az, %EP rose from 1.9 to 7.1%, (P less than .02) in response to Az. However, the increment in %EP over base line was reduced from 23.8 to 3.6% by PGE2 pretreatment (P less than .005). PGE2 did not alter the effects of VE or Az on sodium or bicarbonate excretion. Thus, PGE2 blunts or abolishes the phosphaturic effect of each of these maneuvers independent of its action on sodium or bicarbonate transport. We conclude that PGs have a specific action on phosphate transport events induced by certain nonhormonal factors, revealing a hitherto unappreciated role for these agents in modulating tubular reabsorptive processes.

The renal sodium-calcium exchanger.

The functional expression of the renal sodium-calcium exchanger has been amply documented in studies on renal cortical basolateral membranes. In perfused renal tubules, other investigators have shown sodium-calcium exchange activity in the proximal convolution of the rat and in the distal convolution, the connecting tubule, and the collecting tubule of the rabbit. In rat proximal tubules, we found that the sodium-calcium exchanger is an important determinant of cytosolic calcium homeostasis, since inhibition of sodium-dependent calcium efflux mode caused a large accumulation of tubular calcium. In membranes from rat proximal tubules sodium-calcium activity was high, and in intact proximal tubules, the tubular sodium-calcium exchanger exhibited a high affinity for cytosolic calcium and had a substantial transport capacity, which may be absolute requirements for the maintenance of stable cytosolic calcium in proximal tubules.

Na(+)-Ca2+ exchanger of rat proximal tubule: gene expression and subcellular localization.

The activity of the Na(+)-Ca2+ exchanger, a membrane transporter that mediates Ca2+ efflux, has been described in amphibian and mammalian renal proximal tubules. However, demonstration of cell-specific expression of the Na(+)-Ca2+ exchanger in proximal renal tubules has been restricted to functional assays. In this work, Na(+)-Ca2+ exchanger gene expression in rat proximal tubules was characterized by three additional criteria: functional assay of transport activity in membrane vesicles derived from proximal tubules, expression of specific Na(+)-Ca2+ exchanger protein detected on Western blots, and determination of specific mRNA encoding Na(+)-Ca2+ exchanger protein on Northern blots. A new transport activity assay showed that proximal tubule membranes contained the highest Na(+)-Ca2+ exchanger transport activity reported in renal tissues. In dog renal proximal tubules and sarcolemma, a specific protein of approximately 70 kDa was detected, whereas in rat proximal tubules and sarcolemma, the specific protein approximated 65 kDa and was localized to the basolateral membrane. On Northern blots, a single 7-kb transcript isolated from rat proximal tubules, whole kidney, and heart hybridized under high-stringency conditions with rat heart cDNA. These data indicate that Na(+)-Ca2+ exchanger protein expressed in rat proximal tubule is similar, if not identical, to the cardiac protein. We suggest that the tubular Na(+)-Ca2+ exchanger characterized herein represents the Na(+)-Ca2+ exchanger described in functional assays of renal proximal tubules.