A molecular view of proximal tubular inorganic phosphate (Pi) reabsorption and of its regulation

In recent years, two mammalian proximal tubular brush border membrane Na/Pi cotransporters (types I and II) have been structurally identified by expression cloning techniques. Oocyte expression studies have shown that only the transport characteristics of the type II transporter correspond to the well-known properties of proximal tubular brush border membrane of Pi transport. In studies on physiological regulation by hormonal and non-hormonal factors a direct involvement and determining role of the type II transporter has been documented. Most interestingly, specific membrane retrieval/insertion phenomena participate in acute (minutes/hours) adjustments of brush border membrane Na/Pi cotransport rates; for chronic (hours/days) alterations also specific resynthesis/degradation processes participate. In pathophysiological alterations (e.g. in X-linked hypophosphataemia and in heavy metal-induced nephrotoxicity) the expression of the type II Na/Pi cotransporters is reduced and explains the observed phosphaturia.

Effect of glucocorticoids on neonatal rabbit renal cortical sodium-inorganic phosphate messenger RNA and protein abundance

Administration of glucocorticoids to neonates increases proximal tubule volume absorption by increasing glucose, bicarbonate, and amino acid transport. We have recently demonstrated that glucocorticoids may contribute to the maturational decrease in phosphate transport. This study examines the maturation of NaPi-6 [the regulated proximal tubule sodium-inorganic phosphate (Na-Pi) transporter] mRNA and protein abundance and the mechanism for the decrease in phosphate transport by glucocorticoids. Weaned young rabbits (5 wk) had a 2-fold greater brush border membrane NaPi-6 protein abundance than that measured in adults. Renal cortical NaPi-6 mRNA abundance was comparable in neonates (less than 10 d of age) and adults. Renal brush border membrane vesicles from dexamethasone-treated neonatal rabbits (10 micrograms/100 g of body weight for 4 d) had a lower rate of Na-Pi transport than vehicle-treated controls (46.8 +/- 6.5 versus 71.0 +/- 9.0 pmol 32P/10 s/mg of protein, p < 0.05). Abundance of NaPi-6 protein in brush border membrane vesicles was 3-fold lower in newborn rabbits treated with pharmacologic doses of dexamethasone than in vehicle-treated controls. NaPi-6 mRNA abundance was the same in both groups. NaPi-1, a brush border membrane phosphate transporter which is also an anion channel, mRNA, and protein abundance was not affected by glucocorticoids. These data demonstrate that there is a maturational decrease in NaPi-6 protein abundance and that glucocorticoids decrease neonatal phosphate transport, at least in part, by reducing the number of Na-Pi transporters.

Molecular mechanisms of renal apical Na/phosphate cotransport

In the proximal tubule, sodium-dependent transport of phosphate (Pi) through the brush-border membrane represents the initial step in Pi reabsorption. cDNAs encoding several renal proximal tubule apical Na/Pi cotransport systems have been identified. These Na/Pi cotransporters are subdivided into type I (NaPi-1) and a type II (NaPi-2). Electrophysiological studies reveal that Pi transport by Na/Pi cotransporters is electrogenic. Regulation of proximal Pi reabsorption by dietary Pi intake and parathyroid hormone is primarily due to an alteration of apical type II Na/Pi cotransport; a rapid change of brush-border Na/Pi cotransport most likely occurs via an endo/exocytic mechanism. No evidence for physiological control of type I cotransporters has been obtained. Altered Pi reabsorption as observed in X-linked hypophosphatemia is largely via the type II Na/Pi cotransporter.

Heavy metals inhibit Pi-induced currents through human brush-border NaPi-3 cotransporter in Xenopus oocytes

Heavy metal intoxication with Hg2+, Pb2+ and Cd2+ commonly leads to phosphaturia. In this study, we examined the effects of these heavy metals on Pi-induced currents (Ip) through NaPi-3, the human renal cotransporter for Na+ and Pi. Hg2+ inhibited Ip in a dose- and time-dependent fashion. Hg2+ decreased the extrapolated maximal current but did not alter the apparent affinity for Pi. This inhibition was also observed with the membrane-permeable oxidizing agent 2,2'-dithio-bis(5-nitropyridine) (DTNP) but not with the membrane-impermeable 5,5'-dithiobis(2-nitrobenzoic acid). Hg(2+)- and DTNP-mediated inhibition of Ip was reversible only in the presence of the reducing agent 2,3-dihydroxybutane-1,4-dithiol. Cd2+ and Pb2+ also inhibited Ip. However, while CD2+ did not significantly alter the apparent affinity for Pi, the apparent concentration needed for half-maximal current (Km) for Pi was increased by Pb2+. In contrast to Hg2+, the inhibition of Ip by Cd2+ and Pb2+ was rapidly reversible upon washout. In the presence of the Na(+)-K(+)-adenosinetriphosphatase inhibitor ouabain, Ip was not reduced, and the effects of the heavy metals were maintained. In summary, the three heavy metals Hg2+, Cd2+, and Pb2+ inhibit Ip through the Na+/Pi cotransporter NaPi-3 by distinct mechanisms. Heavy metal-mediated inhibition of NaPi-3 may be responsible for the phosphaturia observed after intoxication with these compounds.

Cellular mechanisms of the age-related decrease in renal phosphate reabsorption

The aging process in humans and in the rat is associated with an impairment in renal tubular reabsorption of Pi and renal tubular adaptation to a low Pi diet. The purposes of the present study were to determine whether changes in the abundance of type II Na-Pi contransporter (NaPi-2) protein and/or mRNA play a role in the age-related decrease in Na-Pi cotransport activity, and to further determine the cellular mechanisms of impaired adaptation to a low Pi diet. In studies performed in 3- to 4-month-old young adult rats and 32-to 16-month-old aged rats we found that there was an age-related twofold decrease in proximal tubular apical brush border membrane (BBM) Na-Pi cotransport activity, which was associated with similar decreases in BBM NaPi-2 protein abundance and renal cortical NaPi-2 mRNA level. Immunohisto-chemistry showed lower NaPi-2 protein expression in the BBM of proximal tubules of superficial, midcortical, and juxtamedullary nephrons. We also found that in response to chronic (7 days) and/or acute (4 hr) feeding of a low Pi diet there were similar adaptive increases in BBM Na-Pi cotransport activity and BBM NaPi-2 protein abundance in both young and aged rats. However, BBM Na-Pi cotransport activity and BBM NaPi-2 protein abundance were still significantly lower in aged rats, in spite of a significantly lower serum Pi concentration in aged rats. The results indicate that impaired expression of the type II renal Na-Pi cotransporter protein at the level of the apical BBM plays an important role in the age-related impairment in renal tubular reabsorption of Pi and renal tubular adaptation to a low Pi diet.

Control of proximal tubular apical Na/Pi cotransport

Two renal proximal tubular apical sodium-dependent transport systems for phosphate (Na/Pi cotransporter) have been identified. Recent studies demonstrated that the abundance of the type II Na/Pi cotransporter in the brush border is critical for the capacity of phosphate reabsorption. Evidence was obtained that a change of apical Na/Pi cotransporters may occur via endo/ exocytic processes and via protein-synthesis-dependent mechanisms involving altered transcription and/or stability of mRNA. In addition, acute control of apical Na/Pi cotransport may occur via the pH dependence and electrogenicity of the Na/Pi cotransporter.

Immunolocalization of Na/SO4-cotransport (NaSi-1) in rat kidney

The proximal tubule is the major site for renal reabsorption of sulphate. A sodium-dependent transport system for sulphate (NaSi-1) has recently been identified from a rat kidney cortex cDNA library. Recent work demonstrated that NaSi-1 mRNA is expressed predominantly in proximal tubules. In the present work expression along the nephron of the Na/SO4-cotransporter NaSi-1 was studied by immunofluorescence. A polyclonal antibody was raised in rabbits against a fusion protein containing a 53-amino-acid polypeptide specific for the NaSi-1 sequence. The anti-NaSi-1 polyclonal antibody specifically detected a 68-kDa protein on Western blots and, by immunofluorescence specific staining, was observed in MDCK cells transfected with the NaSi-1 cotransporter. Using rat kidney cortex slices specific NaSi-1-related immunoreactivity was detected in proximal tubules and was restricted to the apical membrane. No immunoreactivity was observed in the other nephron segments. This was confirmed by Western blot analysis using proximal tubular apical and basolateral membranes isolated by free-flow electrophoresis. The results indicate that the Na/SO4-cotransporter NaSi-1 is expressed in the apical membrane of proximal tubular cells and is therefore likely to be involved in proximal reabsorption of sulphate.

Renal brush border membrane Na/Pi-cotransport: molecular aspects in PTH-dependent and dietary regulation

Inorganic phosphate (Pi) is reabsorbed in renal proximal tubules in a sodium (Na)-dependent manner involving brush border Na/Pi-cotransporter(s). Regulation of renal Pi-reabsorption, such as by parathyroid hormone (PTH) and/or by dietary Pi-deprivation, involves alterations in the rate of Na/Pi-cotransport. Two structurally different Na/Pi-cotransporters have been identified: type I-transporter and type II-transporter. The related mRNAs and proteins are located in the proximal tubule and in the brush border membrane. In heterologous expression systems type I and type II Na/Pi-cotransporters mediate Na/Pi-cotransport. Characterization of the transport properties suggested that the type II transporter is "responsible' for brush border membrane Na/Pi-cotransport (as observed in isolated vesicles). Administration of PTH to rats resulted in an inhibition of brush border membrane Na/Pi-cotransport (vesicles) and in a reduced brush border membrane content of the type II transporter. Feeding low Pi-diets resulted in an up-regulation of Na/Pi-cotransport (vesicles) and of type II transporter content; only after a prolonged exposure to low Pi-diets (more than 4 hr) was an increase in specific mRNA content observed. Refeeding high Pi diets had the opposite effects on Na/Pi-cotransport activity and on type II transporter protein. It is currently the task of future experiments to define the specific mechanisms leading to protein-synthesis-independent (PTH, acute Pi-deprivation, Pi-refeeding) and to protein-synthesis-dependent (prolonged Pi-deprivation) regulation of the type II Na/Pi-cotransporter.

Renal type II Na/Pi-cotransporter is strongly impaired whereas the Na/sulphate-cotransporter and aquaporin 1 are unchanged in cadmium-treated rats

The cellular mechanisms of cadmium (Cd) nephrotoxicity are poorly understood. In this study we investigated the cellular causes of the Cd-induced phosphaturia in the rat. Compared to controls, Cd-treated rats (2 mg Cd/kg body weight, s.c. for 14 days) showed a marked polyuria, proteinuria and phosphaturia. As studied by the rapid filtration technique in isolated cortical brush-border membrane vesicles (BBMV), Na+-gradient-driven uptake of phosphate ([32Pi]) and of [3H] glucose were markedly decreased in Cd-treated rats, whereas uptake of sulphate ([35S]) remained unchanged. By Western blotting of BBMV proteins and by indirect immunocytochemistry in 4-micron thick frozen fixed kidney sections, using an antibody against the type II Na/Pi-cotransporter (NaPi-2), we found a diminished expression of this protein in the brush-border membrane from Cd-treated rats. How ever, the expression of the water channel aquaporin 1, estimated from the specific antibody staining in brush-border membranes, remained unchanged by Cd. Northern blot analysis showed a strong reduction of 2.7 kb NaPi-2-related mRNA in Cd-affected kidneys. Our data indicate that: (1) Cd may reduce reabsorption of Pi in proximal tubules by affecting the expression of the functional Na/Pi-cotransporters in the luminal membrane, and (2) Cd effects on brush-border transporters are selective.

Expression of a renal type I sodium/phosphate transporter (NaPi-1) induces a conductance in Xenopus oocytes permeable for organic and inorganic anions

Two distinct molecular types (I and II) of renal proximal tubular brush border Na+/Pi cotransporters have been identified by expression cloning on the basis of their capacity to induce Na+-dependent Pi influx in tracer experiments. Whereas the type II transporters (e.g., NaPi-2 and NaPi-3) resemble well known characteristics of brush border Na+/Pi cotransport, little is known about the properties of the type I transporter (NaPi-1). In contrast to type II, type I transporters produced electrogenic transport only at high extracellular Pi concentrations (> or =3 mM). On the other hand, expression of NaPi-1 induced a Cl- conductance in Xenopus laevis oocytes, which was inhibited by Cl- channel blockers [5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) > niflumic acid >> 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid]. Further, the Cl- conductance was inhibited by the organic anions phenol red, benzylpenicillin (penicillin G), and probenecid. These organic anions induced outwardly directed currents in the absence of Cl-. In tracer studies, we observed uptake of benzylpenicillin with a Km of 0.22 mM; benzylpenicillin uptake was inhibited by NPPB and niflumic acid. These findings suggest that the type I Na+/Pi cotransporter functions also as a novel type of anion channel permeable not only for Cl- but also for organic anions. Such an apical anion channel could serve an important role in the transport of Cl- and the excretion of anionic xenobiotics.

In vivo suppression of the renal Na+/Pi cotransporter by antisense oligonucleotides

A 20-mer phosphorothioate oligonucleotide (AS1) was designed to hybridize to the message for the rat kidney sodium phosphate cotransporter NaPi-2 close to the translation initiation site. Single intravenous doses of this oligonucleotide were given to rats maintained on a low phosphorus diet to increase NaPi-2 expression. At 3 days after oligonucleotide infusion, rats receiving 2.5 micromol of AS1 exhibited a reduction in renal NaPi-2 to cyclophilin mRNA ratio by 40% +/- 17%, and rats receiving 7.5 micromol of AS1 exhibited a reduction in NaPi-2 to cyclophilin mRNA ratio by 46% +/- 21%. Reversed-sequence AS1 was without effect. The higher dose of 7.5 micromol of AS1 also reduced the rate of phosphate uptake into renal brush border membrane vesicles and the expression of NaPi-2 protein detected by Western blotting in these vesicles. Reversed sequence AS1 was again without effect on these parameters. These results suggest that systemically infused oligonucleotides can exert antisense effects in the renal proximal tubule.

Renal Na/Pi-cotransporters

Two non-homologous proximal tubular apical Na/Pi-cotransport systems (type I and type II) have been identified thus far by expression cloning. Subsequent studies provided evidence that the type II Na/Pi-cotransporter represents a target for the physiological and pathophysiological regulation of proximal reabsorption of phosphate. The exact role of the type I Na/Pi-cotransporter in proximal Pi-reabsorption and eventually also in the renal handling of other substrates, such as organic anions, is currently less clear and needs further investigation. Evidence was obtained that acute changes of brush border membrane Na/Pi-cotransport involves endo- and exocytic movement of type II Na/Pi-cotransporters. In particular, we elucidated if and how phosphorylation reactions are involved and defined the intracellular structures of the endo/exocytic apparatus involved. At the level of the gene it will be necessary to elucidate its organization in order to understand the mechanisms involved in chronic regulations of Na/Pi-cotransport related to the type II Na/Pi-cotransporter. Furthermore, for structural investigations of these integral membrane proteins, they have to be isolated in sufficient quantities. Thus far the type II cotransporter (NaPi-2) has been expressed in Sf9 insect cells [20], which may eventually allow a purification of this protein.

Renal expression of Na+-phosphate cotransporter mRNA and protein: effect of the Gy mutation and low phosphate diet

The X-linked Gy mutation is closely linked, but not allelic, to Hyp and is characterized by rickets, hypophosphatemia, decreased renal tubular maximum for phosphate (Pi) reabsorption (TmP) and a specific reduction in renal brush-border membrane (BBM) Na+-Pi cotransport. Gy mice, like their normal littermates, respond to a low-Pi diet with an increase in BBM Na+-Pi cotransport, but fail to show an adaptive increase in Tmp. Using an antibody raised against the NH2 terminal peptide of the rat renal-specific Na+-Pi cotransporter (NaPi-2) and a NaPi-2 cDNA probe, we examined the effect of the Gy mutation and low-Pi diet (0.03% Pi) on NaPi-2 protein and mRNA abundance. The reduction in BBM Na+-Pi cotransport in Gy mice (51 +/- 5% of normal, P < 0.05) was associated with a decrease in NaPi-2 protein (46 +/- 12% of normal, P < 0.05) and mRNA abundance (76 +/- 5%, P < 0.05). The low-Pi diet elicited a two- to three-fold increase in Na+-Pi cotransport in both normal and Gy mice that was accompanied by a large increase in NaPi-2 protein (10.2-fold in normal and 16.9-fold in Gy mice) and a modest increase in NaPi-2 mRNA (1.3-fold in both mouse strains, P < 0.05). The present data demonstrate that (1) the renal defect in BBM Pi transport in Gy mice can be ascribed to a deficit in NaPi-2 protein and mRNA abundance, (2) both normal and Gy mice respond to low Pi with an adaptive increase in NaPi-2 protein that exceeds the increase in Na+-Pi cotransport activity and NaPi-2 mRNA, (3) the adaptive increase in NaPi-2 protein and mRNA are not sufficient for the overall increase in TmP following Pi restriction.

Expression cloning and functional characterization of the kidney cortex high-affinity proton-coupled peptide transporter

The presence of a proton-coupled electrogenic high-affinity peptide transporter in the apical membrane of tubular cells has been demonstrated by microperfusion studies and by use of brush border membrane vesicles. The transporter mediates tubular uptake of filtered di- and tripeptides and aminocephalosporin antibiotics. We have used expression cloning in Xenopus laevis oocytes for identification and characterization of the renal high-affinity peptide transporter. Injection of poly(A)+ RNA isolated from rabbit kidney cortex into oocytes resulted in expression of a pH-dependent transport activity for the aminocephalosporin antibiotic cefadroxil. After size fractionation of poly(A)+ RNA the transport activity was identified in the 3.0- to 5.0-kb fractions, which were used for construction of a cDNA library. The library was screened for expression of cefadroxil transport after injection of complementary RNA synthesized in vitro from different pools of clones. A single clone (rPepT2) was isolated that stimulated cefadroxil uptake into oocytes approximately 70-fold at a pH of 6.0. Kinetic analysis of cefadroxil uptake expressed by the transporter's complementary RNA showed a single saturable high-affinity transport system shared by dipeptides, tripeptides, and selected amino-beta-lactam antibiotics. Electrophysiological studies established that the transport activity is electrogenic and affected by membrane potential. Sequencing of the cDNA predicts a protein of 729 amino acids with 12 membrane-spanning domains. Although there is a significant amino acid sequence identity (47%) to the recently cloned peptide transporters from rabbit and human small intestine, the renal transporter shows distinct structural and functional differences.

Molecular mechanisms in the regulation of renal proximal tubular Na/phosphate cotransport

Renal proximal tubular Na/phosphate cotransport is the key target in the control of renal phosphate handling. Using molecular tools for the type II Na/phosphate cotransporter, it could be documented that altered brush border membrane phosphate transport reflects a state of altered brush border expression of the specific transporter molecule. This alteration is either the consequence of specific membrane insertion/retrieval processes and/or reflects states of altered synthesisdegradation of the transporter.

Expression of proximal tubular Na-Pi and Na-SO4 cotransporters in MDCK and LLC-PK1 cells by transfection

Two cD-NAs coding for proximal tubular Na-Pi cotransport (NaPi-2) and Na-SO4 cotransport (NaSi-1) have been transfected by the use of a dexamethasone-inducible vector (pLK-neo) into MDCK and LLC-PK1 cells. By reverse transcription-polymerase chain reaction, expression of corresponding mRNAs was observed after stimulation with dexamethasone only. Similarly, expression of the NaPi-2 protein was detected only after induction with dexamethasone. In transfected Madin-Darby canine kidney (MDCK) cells, dexamethasone induced a large increase of Na-Pi or Na-SO4 cotransport, whereas, in transfected LLC-PK1, cell transport was only minimally expressed. In MDCK cells grown on filter supports, transfected Na-Pi-cotransport activity was equally expressed at both cell surfaces; dual location of expressed NaPi-2 protein was also observed by immunohistochemistry. In contrast, transfected Na-SO4 cotransport activity was predominantly expressed at the apical cell surface of MDCK cells. The results demonstrate that 1), in MDCK cells, the sorting behavior of two proximal tubular cotransport systems seems to be different: apical for Na-SO4 cotransport (NaSi-1) and dual location for Na-Pi cotransport (NaPi-2); and 2) LLC-PK1 cells seem not to be a suitable system to functionally express sodium-dependent cotransport systems for phosphate and sulfate.

Thyroid hormone stimulation of Na/Pi-cotransport in opossum kidney cells

Thyroid hormone (T3), a known stimulator of renal proximal tubular brush border membrane Na-dependent phosphate (Pi) uptake (Na/Pi-cotransport), stimulated Na-dependent Pi transport in opossum kidney (OK) cells. Na/Pi-cotransport was stimulated in a time- and dose-dependent manner with maximal effects (57%) at 24 h and 10(-10) M T3. This stimulation was related to an increase in the apparent capacity (Vmax) of Na/Pi-cotransport. Treatment with T3 had no effect on Na-independent transport of Pi or of L-arginine. The stimulation of Na/Pi-cotransport was paralleled by an increase in the messenger ribonucleic acid (mRNA) encoding the OK cell apical Na/Pi-cotransporter (termed NaPi-4); the mRNA levels related to the activity of Na-independent L-arginine transport (rBAT) were unaffected by T3. Actinomycin D (10(-7) M) completely prevented the stimulatory effect of T3 on OK cell Na/Pi-cotransport and on NaPi-4 mRNA content. In conclusion, T3 stimulates apical Na/Pi-cotransport in OK cells most likely by enhancing its transcription.

Properties of electrogenic Pi transport by a human renal brush border Na+/Pi transporter

Inorganic phosphate (Pi) induced an inward current (IP) in Xenopus oocytes expressing the human renal Na+/Pi cotransporter NaPi-3. At 100mM Na+, Pi-transport was independent of the holding potential and resulted in an apparent Km of 0.08 mM; lowering the Na+ concentration to 50 mM resulted in an increase of the apparent Km to 0.22 mM at -50 mV and to 0.31 mM at -90 mV. In contrast, the apparent Km for Na+ was not significantly influenced by the holding potential. A decrease of the pH from 7.8 to 6.8 resulted in a decrease of IP at 50 mM Na+, but not at 150 mM Na+. Arsenate induced inward currents through NaPi-3 and decreased the apparent Km in measurements of IP. Phosphonoformic acid itself induced no currents, but inhibited Pi-induced currents with an apparent Ki of 3.6 mM. In summary, NaPi-3 displays characteristic Na+/Pi cotransporter properties with relevant interactions with arsenate (transport substrate) and phosphonoformic acid (inhibitor). Monovalent and divalent Pi both appear to be transported by NaPi-3.

Transport characteristics of a murine renal Na/Pi-cotransporter

A complementary deoxyribonucleic acid (cDNA) corresponding to a murine renal cortical Na/phosphate-(Na/Pi-) cotransporter was isolated and its transport properties characterized by electrophysiological techniques after expression in Xenopus laevis oocytes. A Na-dependent inward movement of positive charges ("short-circuit current") was observed upon superfusion with Pi (and with arsenate). Increasing the Na concentration led to a sigmoidal elevation in Pi-induced short-circuit current; the apparent Michaelis constant, Km, (around 40 mM Na) was increased by lowering the pH of the superfusate but was not influenced by altering the Pi concentration. Increasing the Pi (and arsenate) concentration led to a hyperbolic elevation in Na-dependent short-circuit current (apparent Km for Pi at 100 mM Na was around 0.1 mM; apparent Km for arsenate was around 1 mM); lowering the Na concentration decreased the apparent affinity for Pi. The Pi-induced short-circuit current was lower at more acidic pH values (at pH 6.3 it was about 50% of the value at pH 7.8); this pH dependence was similar if the Pi concentration was calculated in total, or if distinction was made between its mono- and divalent forms. Thus, the pH dependence of Na-dependent Pi transport (total Pi) may not be related primarily to a pH-dependent alteration in the availability of divalent Pi, but includes also a competitive interaction of Na with protons. The effect of Pi and Na concentration on the apparent Km values for Na or Pi, respectively, provides evidence for an ordered interaction of "cosubstrate" (Na first) and "substrate" (Pi or arsenate second).

Protein kinase C consensus sites and the regulation of renal Na/Pi-cotransport (NaPi-2) expressed in XENOPUS laevis oocytes

Renal brush border membrane sodium/phosphate (Na/Pi)-cotransport activity is inhibited by hormonal mechanisms involving activation of protein kinases A and C. The recently cloned rat renal Na/Pi-cotransporter (NaPi-2) contains several protein kinase C but no protein kinase A consensus sites [17, 20]. In the present study we have expressed wild type and polymutant (protein kinase C consensus sites removed) NaPi-2-transporters in Xenopus laevis oocytes. The expression of transport function as well as the basic transport properties were unaffected by the removal of the consensus sites. Pharmacological activation of protein kinase C with phorbol 12,13-didecanoate (PDD) led to a time-dependent inhibition of expressed wild type Na/Pi-cotransport function; simultaneous exposure to staurosporine (0.3) prevented the PDD induced (50 nM) inhibition. The kinase-C-mediated inhibition was not prevented by the removal of the protein kinase C consensus sites. Pharmacological activation of protein kinase A (dibutyryl adenosine 3':5':cyclic monophosphate (cAMP)/forskolin) had no effect on wild type NaPi-2-induced oocyte Na/Pi-cotransport. It is concluded that the protein-kinase-C-mediated regulation of expressed Na/Pi-cotransport does not involve the predicted consensus sites. The involvement of "cryptic" phosphorylation sites and/or of a phosphorylated "regulatory" protein is discussed.