A stretch-activated K+ channel in the basolateral membrane ofXenopus kidney proximal tubule cells

The present study examined whether a basolateral potassium ion (K⁺) channel is activated by membrane-stretching in the cell-attached patch. A K⁺ channel of conductance of 27.5 pS was most commonly observed in the basolateral membrane ofXenopus kidney proximal tubule cells. Channel activity increased with hyperpolarizing membrane potentials [at more positive pipette potentials (V _p)]. Open probability (P _o) was 0.03, 0.13, and 0.21 atV _p values of 0, 40, and 80 mV, respectively. Barium (0.1 mM) in the pipette reducedP _o by 79% at aV _p of 40 mV. Application of negative hydraulic pressure (−16 to −32 cm H₂O) to the pipette markedly activated outward currents (fromP _o=0.01 to 0.75) at aV _p of −80 mV, but not inward currents at aV _p of 80 mV. The size of the activated outward currents (from cell to pipette) did not change by replacing chloride with gluconate in the pipette. These results indicate that a stretch-activated K⁺ channel exists in the basolateral membrane of proximal tubule cells. It may play an important role as a K⁺ exit pathway when the cell membrane is stretched (for example, by cell swelling).     

Transepithelial potential difference in the proximal tubule of Necturus kidney

Summary Transepithelial potential difference (p.d.) was measured in the proximal tubule of Necturus kidney in vivo, by means of microelectrodes filled either with a 3M KCl solution or with a Ringer's solution for amphibians. The average transepithelial p.d., measured with KCl-tips, was: –1.4±2.4 mV (early convolutions), –0.1±2.0 mV (middle convolutions) and +0.1±2.4 mV (straight segment). The corresponding values obtained with Ringer's-filled microelectrodes were –2.3±1.8 mV, –1.3±1.1 mV and +0.1±1.2 mV, respectively. Tip localization into the lumen was ascertained by luminal injection of either oil (KCl electrode measurements) or artificial solutions which produced a measurable shift of transepithelial p.d. (determinations obtained with Ringer's-tips). Transepithelial p.d. in split-drops (mean reabsorptive half time 27.1±2.5 min) was –1.8±1.1 mV. The magnitude of transepithelial p.d. is discussed with respect to an equivalent electrical circuit; it is shown that high transepithelial p.d.'s are inconsistent with the known values of relative conductances of cell membranes in series and shunt pathway, respectively.     

Streaming potentials and diffusion potentials across rabbit proximal convoluted tubule

The streaming potential, defined as the transepithelial potential appearing in the presence of an osmotic water flow, was measured in rabbit kidney proximal convoluted tubules perfused in vitro. The S2 segments studied were dissected from mid-cortical and juxtamedullary portions of the kidney and the streaming potential induced by the addition of raffinose in bath was compared for each tubule with the diffusion potential corresponding to an imposed NaCl gradient in the absence of osmotic gradient. The amplitude of the measured streaming potential was found to vary from positive to negative values (+0.9 to –1.8 mV) according to the location of the dissected tubule: the more juxtamedullary the nephron, the more lumen negative was the streaming potential. This correlated well with the diffusion potentials recorded on the same tubules and the amplitude of the streaming potentials was a close function of the PNa/PCl ratios calculated from these diffusion potentials. This is in agreement with the hypothesis of solute polarization in an unstirred layer as the origin of the streaming potential; a calculation of hydraulic permeability (Pf) of the proximal tubule, taking the role of such an unstirred layer into consideration is proposed.     

Potassium-selective channels in the basolateral membrane of single proximal tubule cells of frog kidney

The membrane potential of proximal tubule cells is dominated by the potassium conductance of the basolateral membrane. In the present paper the nature of this conductance is investigated by the patch-clamp technique. Only one type of K channel was found in the basolateral membranes of freshly isolated proximal cells. In cell-attached patches, the current/voltage relationship is markedly non-linear with much larger inward (30 pS) than outward ( 6 pS) conductances, even in the presence of roughly symmetrical K concentrations. Thus the channels show inward rectification. The determination of the conductance for outward current flow is complicated since the current/voltage curves show an area of negative conductance. Nevertheless, taking the conductance for outward current flow and the density of the channels it is possible to account for all of the previously reported potassium conductance of amphibian proximal tubule cells. The open probability of the channels was found not to depend upon the membrane potential. However, the non-linearity of the current/voltage relationships will confer upon the channel the same voltage dependence as that seen in intact proximal tubules, i.e. the conductance decreases with depolarisation. Incubation of cells in Ringer with no substrates or in the presence of alanine and/or glucose showed no change in the activity of the channels. These findings suggest that, although these channels may represent the basolateral conductance of frog proximal tubule cells, they are not involved in the well-established coupling between transport rate and potassium conductance.This work was supported by the Wellcome Trust     

Toxin pharmacology of the large-conductance Ca2+-activated K+ channel in the apical membrane of rabbit proximal convoluted tubule in primary culture

The patch-clamp technique was used to study the toxin pharmacology of the large-conductance Ca²⁺-activated K⁺ channel (BKCa) present in the apical membrane of rabbit proximal convoluted tubules (PCT) in primary culture. Experiments were performed with the inside-out configuration. This channel was very selective for K⁺ against Na⁺ and had a conductance of 180 pS with 140 mmol/l in the pipette and the bath. The action of toxins was studied on the extracellular side of the channel by using the pipette perfusion technique. Experimental conditions were 140 mmol/l KCl in the pipette and 140 mmol/l Nad in the bath. Pipette potential was maintained at 0 mV. Perfusion of crude venom from Leiurus quinquestriatus hebraeus inhibited reversibly the open probability (P _o) in a concentration-dependent fashion (IC₅₀=0.8 mg/l; n=3). The following synthetic or purified toxins were tested: synthetic charybdotoxin (ChTX) IC₅₀=7.3×10^–9 M (n=5); iberiotoxin (IbTX) IC₅₀=5.5×10^–7 mol/l (n=3); and kaliotoxin (KTX) IC₅₀=4.8×10^–7 mol/l (n=3). The suppression of the six first N-terminal amino-acids slightly reduced the affinity of ChTX (IC₅₀=1.2×10^–8 mol/l, n=4). Neither Dendroaspis polylepis venom nor purified dendrotoxin modified P _o even at high concentrations (20 mg/l and 10^–6 mol/l respectively). Apamin, which blocked the small-conductance K⁺ channel in cultured PCT, did not act on BKCa. These results indicate that ChTX is the most efficient known toxin against the epithelial BKCa in primary cultures of PCT. In spite of there being considerable homology of sequence between ChTX, IbTX and KTX, ChTX was about 100 times more effective than the others. Truncated ChTX kept a high affinity for this channel and could be used to obtain a labelled probe.     

Rheogenic sodium-bicarbonate cotransport in the peritubular cell membrane of rat renal proximal tubule

The mechanism of bicarbonate transport across the peritubular cell membrane was investigated in rat kidney proximal tubules in situ by measuring cell pH and cell Na⁺ activity in response to sudden reduction of peritubular Na⁺ and/or HCO ₃ ^– . The following observations were made: 1. sudden peritubular reduction of either ion concentration produced the same transient depolarizing potential response; 2. bicarbonate efflux in response to peritubular reduction of bicarbonate was accompanied by sodium efflux; 3. sodium efflux in response to peritubular sodium removal was accompanied by cell acidification indicating bicarbonate efflux; 4. all aforementioned phenomena were inhibited by SITS (10^–3 mol/l) except for a small SITS-independent sodium efflux and depolarization which occurred in response to peritubular sodium removal and was not accompanied by cell pH changes; 5. bicarbonate efflux and accompanying potential changes in response to reduction of peritubular bicarbonate virtually vanished in sodium-free solutions. From these observations we conclude that bicarbonate efflux proceeds as rheogenic sodium-bicarbonate cotransport with a stoichiometry of bicarbonate to sodium greater than 1. The question which of the charged species of the bicarbonate buffer system moves cannot yet be decided. Attempts to determine the stoichiometry from the SITS-inhibitable initial cell depolarization and from the SITS-inhibitable initial fluxes suggest a stoichiometry of 3 HCO ₃ ^– : 1 Na⁺. In addition to sodium-dependent bicarbonate flux, evidence was obtained for a sodium-independent transport system of acids or bases which is able to regulate cell pH even in sodium-free solutions.     

Axial heterogeneity of sodium-bicarbonate cotransport in proximal straight tubule of rabbit kidney

Intracellular microelectrodes were used to investigate rheogenic Na⁺(HCO ₃ ^– ) _n cotransport in different segments of isolated proximal straight tubule (PST) of rabbit kidney. In the first portion (S2 segment) the peritubular cell membrane potentialV _b averaged –46.0, SE±1.3 mV (n=20), while in the terminal portion (S3 segment) it averaged –68.3, SE±2.5 mV (n=10). This difference may reflect different modes of anion permeation across the peritubular cell membrane. In S2 segments, sudden 101 reduction of bath HCO ₃ ^– concentration caused a fast transient cell depolarization, V _b=–45.8, SE±1.2 mV (n=33) as expected from the presence of Na⁺(HCO ₃ ^– ) _n contransport. As the puncture site moved further distally, V _b declined and gradually changed its time course by superposition of a slower secondary depolarization. In this region the transient cell depolarization could be recuperated by inhibiting the peritubular K⁺ conductance with Ba²⁺ (1 mmol/l). In S3 segments, however, the HCO ₃ ^– -dependent transient cell depolarization was completely lost both in the absence and presence of Ba²⁺. In addition, sudden reduction of bath Na⁺ concentration did not acidify the cell, as it did in the S2 segment. The data indicate that the expression of Na⁺(HCO ₃ ^– ) _n cotransport in the peritubular cell membrane gradually diminishes towards the end of the S2 segment and is lost in the S3 segment.     

Solvent drag of sucrose during absorption indicates paracellular water flow in the rat kidney proximal tubule

Single convoluted proximal tubules of the rat kidney were lumen perfused in situ with isosmotic solutions containing C¹⁴-sucrose and H³-inulin as tracers, to evaluate whether the extracellular marker sucrose is entrained by water during proximal tubular reabsorption. Inulin was used as volume marker. The absorptive rate was varied by using as luminal perfusion fluids either a solution made up of (in mmole/l) 120 NaCl, 5 glucose, 25 NaHCO₃ and altering the perfusion rate, or a solution containing 110 NaCl and 70 raffinose.J ^S, the net sucrose efflux is found to be a function of the net volume flow,J _V, such that atJ _V=0,J ^S is very small and at high rates ofJ _V,J ^S is over 60-fold the value observed at lowJ _V values. In addition, the transported to luminal sucrose concentrations decreased withJ _V in a hyperbolic manner.Unstirred layers affect the diffusive component ofJ ^S, but only to a small extent. Therefore, the large remaining dependency ofJ ^S withJ _V must be due to drag of sucrose by water, within the paracellular pathway. This leads to the conclusion that water flows through the paracellular pathway during absorption in the rat proximal tubule, in addition to transcellular water flow. Using equations for molecular sieving and the measured value of _s for sucrose of 0.76–0.91, it is calculated that the pathway where entrainment of solute by water occurs must be 1.0–1.1 nm wide. This calculation is only tentative since _s depends on the as yet unknown relative contribution of transcellular and paracellular pathways to transepithelial water osmotic permeability.     

Use of the pH-sensitive dye BCECF to study pH regulation in cultured human kidney proximal tubule cells

Summary This protocol describes the use of the pH-sensitive, intracellularly trapped dye 2′,7′-bis (2-carboxyethyl), 5 (and -6) carboxyfluorescein (BCECF), to characterize the pH regulating mechanisms in cultured human kidney proximal tubule cells. This is a reliable method for intracellular pH measurements and is applicable to single cells, cell suspensions, and confluent cultures. This study was supported in part by a Rapid Advisory Group grant from the Veterans Administration, Washington, DC.     

Na+ selective channels in the apical membrane of rabbit late proximal tubules (pars recta)

Using the patch-clamp technique, Na⁺ selective channels were observed in the luminal membrane of rabbit straight proximal tubule segments. In the cell-attached configuration (NaCl-Ringers in pipette and bath) influx of Na⁺ ions from the pipette into the cell through fluctuating channels was observed was observed. The current-voltage curve of these Na⁺ channels yielded a zero-current potential of 84.3±30.9 mV (n=10), reflecting the electrochemical driving force for Na⁺ influx under resting conditions. The single channel conductance was 12.0±2.1 pS (n=13). In inside-out oriented cell-excised patches the single channel conductance was not significantly different with NaCl-Ringers on both sides. At clamp potentials ranging from +50 mV to –50 mV the single channel current was ohmic and channel kinetics were independent of the voltage. With KCl-Ringers on the bath side (corresponding to cell interior), the zero current potential was 62±19 mV (n=4), indicating a high selectivity of Na⁺ over K⁺ ions. Addition of 10^–5 mol/l amiloride to the bathing solution decreased the mean channel open time slightly. This effect was more pronounced with 10^–4 mol/l amiloride, whereas the single channel conductance was unaffected by the diuretic. 10^–3 mol/l amiloride caused a complete block of the channel. It is concluded that amiloride sensitive Na⁺ channels, with similar properties to those observed in tight epithelia, contribute to Na⁺ reabsorbtion in the straight portion of proximal tubules.     

In situ characterization of oxalate transport across the basolateral membrane of the proximal tubule

 Oxalate transport across the contraluminal membrane of the proximal tubule was studied in vivo using the ”capillary stopped flow microperfusion method” (Pflügers Arch 400:250–256, 1984). Cellular uptake of oxalate was characteristic of a carrier-mediated transport process (J _max = 1.6 ± 0.6 pmol/s per cm proximal tubular length, K _m = 2.03 ± 0.77 mmol/l). Sulphate inhibited oxalate transport in a dose-dependent manner (K _i-value = 1.53 ± 0.38 mmol/l). Sulphate transport across the basolateral membrane was also characteristic of a carrier- mediated transport process (J_max = 1.83 ± 0.56 pmol/s per cm proximal tubular length, K _m = 1.37 ± 0.57 mmol/l). Oxalate inhibited the sulphate transport in a dose-dependent manner (K _i = 2.06 ± 0.82 mmol/l). No significant differences were found between the K _i values and the K _m values of the two substances, indicating that oxalate and sulphate are transported by the same carrier across the basolateral membrane of the proximal tubule. Oxalate transport was not dependent on the extracellular sodium or potassium concentration. Bicarbonate competitively inhibited the oxalate transport. Chloride significantly inhibited the oxalate transport, but not dose dependently. It is, therefore, suggested that oxalate is transported into the cell of the proximal tubule in exchange for sulphate or bicarbonate. The dose-independent inhibition by chloride is suggested to be mediated by the coupling of the sulphate (bicarbonate)/oxalate exchanger with the chloride/bicarbonate exchanger at the basolateral membrane of the proximal tubule. This, furthermore, suggests that the transport of oxalate or sulphate across the basolateral membrane might be indirectly coupled with the reabsorption of chloride at this membrane side. Received: 5 August 1997 / Accepted: 8 January 1998     

Sexual dimorphism of proximal straight tubular cells in mouse kidney

The proximal straight tubular epithelium of the mouse kidney exhibited sexual dimorphism in conventional paraffin sections stained with periodic acid Schiff (PAS) in our preliminary observation. The purpose of this study was to clarify the sex‐dependent structural features in the proximal straight tubular cells of the mouse kidney, and to clarify the effects of sex hormones on this portion of the renal tissue. The mice used in this study were divided into intact, orchiectomized, ovariectomized, testosterone‐treated and estradiol‐treated groups. The kidneys of these animals were examined by histological, cytological and cytochemical (for acid phosphatase reaction) procedures. In the proximal straight tubular epithelium of intact adult mice, PAS staining of the brush border in females was more intense than that in males. Furthermore, PAS‐positive granules were observed in the cytoplasm of females only. Orchiectomy changed the male‐specific features to that of the females, and treatment with testosterone induced the male‐specific features. Ovariectomy and estradiol treatment showed no effects. Ultrastructurally, PAS‐positive granules were observed as electron‐dense myelinoid bodies, and these contained acid phosphatase‐positive matrix. The present study demonstrated apparent sexual dimorphism and effects of testosterone on PAS staining and PAS‐positive granules in the proximal straight tubule cells in normal mice. In addition, the association of PAS‐positive granules and lysosomes was suggested by cytological and cytochemical examination. Anat Rec 255:316–323, 1999. © 1999 Wiley‐Liss, Inc.     

Primary culture of proximal tubule cells in defined medium

Summary A method is described for culturing primary rabbit kidney epithelial cells which express proximal tubule functions. First, rabbit kidney proximal tubules are purified in a sterile manner. Then the tubules are put into serum free culture medium supplemented with insulin, transferrin and hydrocortisone. After one week, confluent monolayers of epithelial cells are obtained which can be used for investigations of interest.     

The effect of shear stress on the basolateral membrane potential of proximal convoluted tubule of the rat kidney

As consequence of glomerular filtration the viscosity of blood flowing through the efferent arteriole increases. Recently, we found that shear stress modulates proximal bicarbonate reabsorption and nitric oxide (NO·) was the chemical mediator of this effect. In the present work, we found that agonists of NO· production affected basolateral membrane potential (V _blm) of the proximal convoluted tubule (PCT) epithelium. Using paired micropuncture experiments, we perfused peritubular capillaries with solutions with different viscosity while registering the V _blm. Our results showed that a 50% increment in the viscosity, or the addition of bradykinin (10⁻⁵ M) to the peritubular perfusion solution, induced a significant and similar hyperpolarization of the V _blm at the PCT epithelium of 6 ± 0.7 mV (p < 0.05). Both hyperpolarizations were reverted by l-NAME (10⁻⁴ M). Addition of 2,2′-(hydroxynitrosohydrazino) bis-ethanamine (NOC-18) 3 × 10⁻⁴ M to the peritubular perfusion solution induced a hyperpolarization of the same magnitude of that high viscosity or bradykinin. These results strongly suggest the involvement of NO· in the effect of high viscosity solutions. This effect seems to be mediated by activation of channels as glybenclamide (5 × 10⁻⁵ M) added to peritubular solutions induced a larger depolarization of the V _blm with high viscosity solutions. Acetazolamide (5 × 10⁻⁵ M) added to high viscosity solutions induced a larger hyperpolarization (8 ± 1 mV; p < 0.05), suggesting that depolarizing current due to exit across the basolateral membrane damps the hyperpolarizing effect of high viscosity. Considering that Na⁺ and consequently water reabsorption is highly dependent on electrical gradient, the present data suggest that the endothelium of kidney vascular bed interacts in paracrine fashion with the epithelia, affecting V _blm and thus modulating PCT reabsorption.     

胰岛素影响肾脏近曲小管上皮细胞多巴胺D_5受体的机制研究

目的研究胰岛素对肾脏近曲小管上皮(RPT)细胞多巴胺D_5受体作用的机制。方法本研究在以往研究基础上,以WKY大鼠的RPT细胞为研究对象,研究胰岛素对D_5受体的作用及其机制。免疫组织化学法观察在胰岛素(10~(-7)mol/L,24 h)作用下,RPT细胞的D_5受体表达改变;同时,利用免疫印迹观察酪氨酸激酶抑制剂genistein,PI3K抑制剂wortmannin,PKC抑制肽19-31等对该效应的影响。结果胰岛素可升高WKY大鼠的RPT细胞D_5受体表达;genistein、wortmannin以及PKC抑制肽19-31可以分别阻断胰岛素对D_5受体的作用。结论胰岛素通过其受体对RPT细胞D_5受体具有调节作用,该作用可能通过酪氨酸激酶、PI3K以及PKC途径发挥一系列生理效应。     

Effects of membrane potential changes on electrical cell-to-cell coupling in proximal tubule

Single proximal convoluted tubules (PCT) of Necturus kidney were impaled with three microelectrodes in the sequence M1, M2, M3. M1 was used for injecting short DC current pulses, M2 for recording peritubular membrane potential, V, and M3 for injecting longer DC current steps and thereby shifting V to a new baseline potential, V. We define the p.d. changes at M2 due to M1-induced pulses as V and V (for baselines V and V, respectively). Our objective was to test whether V was equal to V. The main finding is that when V depolarized by 10 to 80 mV V/V remained close to 1.00. Care was taken to ensure that this apparent stability of the pulse ratio was not due (i) to opposite changes of apical and basolateral membrane conductances (g(A) and g(B) respectively), (ii) to changes of the sum g(A)+g(B) compensated for by changes of the cell-to-cell junctional conductance, g(j), or (iii) to a distortion of the V/V ratio as a function of interelectrode distance, masking voltage-dependent changes of cell membrane conductances. Hyperpolarization of V produced gradual electrical uncoupling between cells as V became increasingly negative, by a mechanism yet to be determined.Supported by GRECO 24 (CNRS).     

The effect of phenylalanine on the electrical properties of proximal tubule cells in the frog kidney

The present study was designed to elucidate the effects of sodium-coupled transport on the electrical properties of proximal tubule cells in the isolated perfused frog kidney. Cable analysis techniques have been employed to determine the resistance of the luminal and peritubular cell membranes in parallel (R _m) and the apparent ratio of the luminal over the peritubular cell membrane resistance (VDR). Furthermore, the sensitivity of the potential difference across the peritubular cell membrane (PD_pt) to 6-fold increases of peritubular potassium concentration (PD_k) was taken as a measure of the relative potassium conductance of this membrane. In the absence of luminal phenylalanine, PD_pt amounts to –60±1 mV (n=90),R _m to 36±3 k cm (n=22), VDR to 1.81±0.14 (n=20), and PD_k to 15.0±0.9 mV (n=25). The application of 10 mmol/l phenylalanine replacing 10 mmol/l raffinose leads to a rapid (within 30 s) depolarisation of PD_pt to 50±5% of its control value and to a delayed (within 12 min) recovery to 95±5% of control. The rapid depolarisation is associated with a decline ofR _m and VDR, indicating a decrease mainly of the luminal cell membrane resistance. During recovery of PD_pt there is a parallel increase of VDR and a further decline ofR _m pointing to a decline of the basolateral cell membrane resistance. PD_k is decreased during rapid depolarisation but increases again during the recovery phase. Thus, phenylalanine initially decreases but then increases above control the apparent potassium conductance. Removal of phenylalanine leads to a transient hyperpolarisation and increased apparent potassium conductance. If a cell is depolarised by current injection into a neighbouring cell, a similar decrease of PD_k is observed which shows also a similar recovery (partial repolarisation) despite continued injection of constant current. The data point to a potential-dependent peritubular K⁺-conductance (of the inwardly rectifying type) and to a regulatory increase within some ten minutes, when the cell is depolarised either by sodium entry across the luminal cell membrane or by current injection into a neighbouring cell.     

The effect of phenylalanine on intracellular pH and sodium activity in proximal convoluted tubule cells of the frog kidney

The present study was performed to test the influence of sodium coupled transport of neutral substrates on intracellular pH and sodium activity in proximal tubules of the amphibian kidney. To this end, kidneys of rana esculenta have been isolated and perfused both through the portal vein (peritubular capillaries) and the aorta (luminal perfusate). The potential difference across the peritubular membrane of proximal tubule cells has been redorced with conventional (PD_pt) as well as with sodium (PD_na) and hydrogen ion (PD_h) selective microelectrodes continuously before during and after the luminal application of 10 mmol/l phenylalanine, replacing 10 mmol/l raffinose. PD_b and PD_na allowed the calculation of intracellular pH (pH_i) and sodium activity (Na_i), respectively. In the absence of phenylalanine in the tubule lumen, PD_pt approximates –57.5±2.3 mV (n=27), pH_i 7.73±0.04 (n=14, extracellular pH 7.77), and Na_i 13.3±0.9 mmol/l (n=13, extracellular sodium activity 74 mmol/l). Within 1 min the luminal application of phenylalanine leads to a depolarisation of PD_pt by +32±2 mV, as well as an increase of pH_i by 0.24±0.04 and of Na_i by 5.2±1.0 mmol/l. At 8 min from luminal application of phenylalanine, Na_i plateaus 5±1 mmol/l above control value, PD_pt increases again to a value of +12±2 mV below and pH_i decreases to a value 0.04±0.07 above their respective control values. All changes are fully reversed after removal of phenylalanine from the tubule lumen. The steady state of intracellular sodium activity might be explained by an extrusion of sodium via the sodium/potassium-ATPase, which approaches the entry across the luminal membrane, the intracellular alkalinisation is probably due to the reduced exit of bicarbonate across the peritubular cell membrane following the depolarisation of PD_pt.     

Zinc uptake by proximal cells isolated from rabbit kidney: Effects of cysteine and histidine

The aim of this study was to characterize the mechanisms of zinc transport in proximal cells isolated from rabbit kidney cortex. Uptakes of ⁶⁵Zn were assessed under initial rate conditions, after 0.5 min of incubation. The kinetic parameters obtained at 20°C were a K _m of 15.0±1.5 M, a J _max of 208.0±8.4 pmol min^–1 (mg protein)^–1, and an unsaturable constant of 0.259±0.104 (n=8). Cadmium competitively inhibited the zinc uptake, with a K _i value of 13.0±2.8 M, while zinc competitively inhibited ¹⁰⁹Cd uptake by isolated cells. Cysteine and histidine stimulated zinc transport at an amino acidzinc molar ratio ranging from 11 to 81. This stimulation was not observed in the absence of a sodium gradient. At a molar ratio greater than 161 (i.e., 400 M cysteine or histidine and 25 M Zn), there was evidence of inhibition. These data suggest that zinc enters renal proximal cells (a) as a free ion via a saturable carrier-mediated process or an unsaturable pathway and (b) complexed with cysteine or histidine, by means of a sodium/amino acid cotransport mechanism.