Structure-function of recombinant Na/H exchanger regulatory factor (NHE-RF)

Inhibition of the renal brush border membrane (BBM) Na/H exchanger by cAMP-dependent protein kinase, PKA, requires participation of a recently cloned regulatory cofactor, Na/H exchanger-regulatory factor (NHE-RF). As deduced from the cDNA of this 358-amino acid protein, amino acids 11-101 and amino acids 150-241 of the NHE-RF protein share 74% overall homology suggesting duplication of these PDZ containing domains. The serine residues at amino acid position 289 and 340 are considered to be the most likely sites for PKA mediated phosphorylation. To study the structure- function relation between NHE-RF and PKA mediated inhibition of the rabbit BBM Na/H exchanger, the effect of recombinant proteins representing full-length NHE-RF as well as truncated and mutant forms of NHE-RF were determined using a reconstitution assay. The reconstitution assay employed a fraction of rabbit BBM proteins that contains Na/H exchanger activity that is not regulated by PKA. NHE-RF in the presence of ATP and Mg but not PKA, inhibited Na/H exchange activity in a concentration-dependent manner. In the presence of PKA, there was a significant left shift in the dose-response relation such that 10(-12) M NHE-RF inhibited Na/H exchange transport by 30% in the presence but not in the absence of PKA. A recombinant polypeptide representing amino acids 1-151 (Domain I) did not affect Na/H exchange transport in the presence or absence of PKA. A polypeptide representing amino acids 149-358 (Domain II) in the presence of ATP and Mg but not PKA, inhibited Na/H exchange activity in a concentration-dependent manner. In the presence of PKA, there was a left shift in the dose-response relation. 10(-12) M of Domain II polypeptide inhibited transport by 18% in the presence but not in the absence of PKA. Mutation of serine residues 287, 289, and 290 to alanine did not affect the inhibitory effect in the absence of PKA but abolished the left shift in the dose-response relation elicited by PKA. Mutation of serine residues 339 and 340 to alanine were without effect on PKA dependent regulation of Na/H exchange transport. These studies indicate that NHE-RF inhibits basal rabbit renal BBM Na/H exchange activity-an effect which is augmented by PKA. The amino acid sequences in the polypeptide containing only the NH2-terminal PDZ domain of NHE-RF have no intrinsic activity as an inhibitor but appears to be required for the full-length NHE-RF to express its full inhibitory effect on the BBM Na/H exchanger. One or more of the serine residues at positions 287, 289, and/or 290 represent the critical PKA phosphorylation site(s) on the NHE-RF protein that mediates the physiologic effect of cAMP on the renal BBM Na/H exchanger.

The beta2-adrenergic receptor interacts with the Na+/H+-exchanger regulatory factor to control Na+/H+ exchange

Stimulation of beta2-adrenergic receptors on the cell surface by adrenaline or noradrenaline leads to alterations in the metabolism, excitability, differentiation and growth of many cell types. These effects have traditionally been thought to be mediated exclusively by receptor activation of intracellular G proteins. However, certain physiological effects of beta2-adrenergic receptor stimulation, notably the regulation of cellular pH by modulation of Na+/H+ exchanger (NHE) function, do not seem to be entirely dependent on G-protein activation. We report here a direct agonist-promoted association of the beta2-adrenergic receptor with the Na+/H+ exchanger regulatory factor (NHERF), a protein that regulates the activity of the Na+/H+ exchanger type 3 (NHE3). NHERF binds to the beta2-adrenergic receptor by means of a PDZ-domain-mediated interaction with the last few residues of the carboxy-terminal cytoplasmic domain of the receptor. Mutation of the final residue of the beta2-adrenergic receptor from leucine to alanine abolishes the receptor's interaction with NHERF and also markedly alters beta2-adrenergic receptor regulation of NHE3 in cells without altering receptor-mediated activation of adenylyl cyclase. Our findings indicate that agonist-dependent beta2-adrenergic receptor binding of NHERF plays a role in beta2-adrenergic receptor-mediated regulation of Na+/H+ exchange.

cAMP-mediated inhibition of the epithelial brush border Na+/H+ exchanger, NHE3, requires an associated regulatory protein

NHE3 is the Na+/H+ exchanger located on the intestinal and renal brush border membrane, where it functions in transepithelial Na+ absorption. The brush border Na+ absorptive process is acutely inhibited by activation of cAMP-dependent protein kinase, but the molecular mechanism of this inhibitory effect is poorly understood. We have identified two regulatory proteins, E3KARP and NHERF, that interact with NHE3 to enable cAMP to inhibit NHE3. The two regulatory proteins are structurally related, sharing approximately 50% identity in amino acid sequences. It has been previously shown that when NHE3 is transfected into PS120 fibroblasts or Caco-2 cells, cAMP failed to inhibit NHE3 activity. Northern blot analysis showed that both PS120 and Caco-2 cells lacked the expression of both E3KARP and NHERF. In contrast, other cell lines in which cAMP inhibits NHE3, including OK, CHO, and LLC-PK1 cells, expressed NHERF-related regulatory proteins. To determine their functions in cAMP-dependent inhibition of NHE3, E3KARP and NHERF were transfected into PS120/NHE3 fibroblasts. Transfection in PS120/NHE3 fibroblasts with either NHERF or E3KARP reconstituted cAMP-induced inhibition of NHE3, resulting in 25-30% inhibition in these cells.

Reconstitution of human red blood cell Na/H and Na/Na exchange transport

Na/H and Na/Na exchange transport was measured using human erythrocyte membrane proteins solubilized with octyl glucoside and reconstituted into voltage clamped soybean phospholipid membrane vesicles. The uptake of Na in exchange for either H or Na was: 1) 8 to 10 times higher in proteoliposomes that contained erythrocyte proteins than in proteoliposomes that contained heat denatured proteins or in liposomes that contained no proteins; 2) not affected by ouabain, bumetanide, or 4.4'-diisothiocyanostilbene-2-2'-disulfonic acid (DIDS); 3) inhibited by amiloride, 5-(n-ethyl-n-isopropyl)amitoride (EIPA), and 5-(n-ethyl-1-n-isobutyl)amiloride (MIA) but not phenamil; and 4) inhibited by lithium (Li) in a concentration-dependent manner. Incubation of erythrocyte proteins with a low concentration of immobilized trypsin resulted in a significant increase (52%) in Na/Na transport, but no change was seen in Na/H transport. A higher concentration of trypsin increased Na/H transport by more than 2.5 times but did not increase Na/Na transport further. Examination of these studies indicates that, as assayed in reconstituted proteoliposomes that contained erythrocyte proteins, there is a differential response between Na/H and Na/Na transport to trypsin.

Characterization of a protein cofactor that mediates protein kinase A regulation of the renal brush border membrane Na(+)-H+ exchanger

Activation of cAMP-dependent protein kinase A inhibits the renal proximal tubule brush border membrane Na(+)-H+ exchanger by a process involving participation of a regulatory cofactor (NHE-RF) that is distinct from the transporter itself. Recent studies from this laboratory reported a partial amino acid sequence of this putative cofactor (Weinman, E. J., D. H. Steplock, and S. Shenolikar. 1993. J. Clin. Invest. 92:1781-1786). The present experiments detail the structure of the NHE-RF protein as determined from molecular cloning studies. A codon-biased oligonucleotide probe to a portion of the amino acid sequence of the putative cofactor was used to isolate a 1.9-kb cDNA from a rabbit renal library. The encoded protein is 358 amino acids in length and is rich in proline residues. Search of existing data bases indicates that NHE-RF is a unique protein. Using a reticulocyte lysate, the cDNA translated a product of approximately 44 kD, which was recognized by an affinity-purified polyclonal antibody to NHE-RF. Potential phosphorylation sites for protein kinase A are present. The mRNA for the protein is expressed in kidney, proximal small intestine, and liver. Reverse transcription/PCR studies in the kidney indicate the presence of mRNA for NHE-RF in several distinct nephron segments including the proximal tubule.

CAMP-mediated inhibition of the renal brush border membrane Na+-H+ exchanger requires a dissociable phosphoprotein cofactor

Prior studies have suggested that protein kinase A (PKA)-mediated inhibition of the rabbit renal brush border membrane (BBM) Na(+)-H+ exchanger involves a regulatory protein that is distinct from the transporter. This putative regulatory protein was purified by column chromatography and SDS-PAGE, and a partial primary amino acid sequence was determined. An affinity-purified polyclonal antibody to a synthetic peptide representing a sequence of the protein recognized a polypeptide of 55 kD in BBM but not in basolateral membrane. The antibody immunoprecipitated a PKA substrate of a similar molecular mass from detergent-solubilized BBM proteins. When assayed after reconstitution, PKA in the presence of ATP and Mg2+ did not inhibit Na(+)-H+ exchange transport in a fraction of solubilized BBM proteins eluting from an anion exchange column between 0.2 and 0.4 M NaCl (fraction B). Coreconstitution of fraction B with the immunoprecipitated 55-kD protein restored the inhibitory effect of PKA (change = 42%, P < 0.05). By contrast, Na(+)-H+ exchange transport in total solubilized BBM proteins was inhibited 25% (P < 0.05) by PKA, ATP, and Mg2+. This effect was abolished by immunodepletion of the cAMP regulatory protein (change = +5%, P = NS). These findings provide evidence that the regulation of renal BBM Na(+)-H+ exchange transport by PKA is affected by repletion and depletion of a specific protein. This suggests that PKA-mediated inhibition of the renal BBM Na(+)-H+ exchanger requires participation of a regulatory protein that is distinct from the transporter itself.

Accumulation of aliphatic amines in gastric juice of acute renal failure patients. Possible cause of hypergastrinemia associated with uremia

In this study we analyzed by gas chromatographic headspace analysis the composition and concentration of gastrin-stimulatory volatile aliphatic amines in the gastric juice of healthy subjects and acute renal failure patients. We demonstrated that although these aliphatic amines are present in the gastric juice of normal subjects in trace amounts, they accumulate in the gastric juice of uremic subjects. This 30-40-fold elevation in gastric juice amine concentration agreed favorably with the 40-50-fold augmentation in serum gastrin levels in acute renal failure, with a significant association (r = 0.87) existing between these two parameters. It was also determined that a 2-hr hemodialysis procedure resulted in a modest nonparallel decline in both gastric amine and serum gastrin levels. These results support the hypothesis that the accumulation of volatile aliphatic amines in the gastric juice of uremic individuals may induce an activation of the antral G cells, resulting in hypergastrinemia.

Increased Na/H antiport activity and abundance in uremic red blood cells

Alterations in red blood cell sodium (Na) transport have been described in chronic renal failure. This study examines the possible impact of uremia on two ouabain-insensitive pathways, the Na/H antiporter and the Cl-/NaCO3- anion exchanger. The Vmax of Na/H antiporter measured as Na influx driven by outward H gradient in acid loaded red blood cells was significantly higher in uremic red blood cells versus controls (60.5 +/- 16.5 vs. 24.5 +/- 5.4 mmol/liter cells/hr, P < 0.025). This increase in activity was associated with an increased abundance of the Na/H antiporter as determined by immunologic analysis using an affinity purified polyclonal antibody to the human NHE-1 isoform of the antiporter. By contrast, the activity of the anion exchanger measured as the DIDS-sensitive lithium (Li) influx was similar in uremic versus control red blood cells (2.10 +/- 0.18 vs. 2.14 +/- 0.20 mmol/liter cells/hr). These experiments, when considered in conjunction with prior studies showing normal Na/Li countertransport in uremia indicate that there is a selective increase in the number of functional Na/H antiporters in uremic red blood cells and that Na/Li countertransport measurements may not be a valid marker for Na/H antiporter activity in red blood cells in patients requiring dialysis for end-stage renal failure.

Identification of the human NHE-1 form of Na(+)-H+ exchanger in rabbit renal brush border membranes

To study the relation between the human Na(+)-H+ exchanger (NHE-1) and the renal brush border membrane (BBM) Na(+)-H+ exchanger, polyclonal antibodies to synthetic peptides representing a putative external (Ab-E) and an internal cytosolic domain (Ab-I) of human NHE-1 were generated in rabbits. Western immunoblot analyses indicated that both antibodies recognized a 97-kD protein in rabbit renal BBM but not basolateral membranes (BLM). Octyl glucoside-extracted rabbit renal BBM proteins also contained the 97-kD polypeptide as did a fraction eluted from an anion-exchange column with 0.2 M NaCl (fraction A). A fraction eluting between 0.2 and 0.4 M NaCl (fraction B) did not contain this protein. Prior reconstitution studies have indicated that Na(+)-H+ exchange activity is higher significantly in fraction B than fraction A. Administration of NH4Cl for 3-7 d to rabbits, a stimulus known to increase renal BBM Na(+)-H+ exchange activity, did not result in a change in expression of the 97-kD protein in either renal BBM or BLM. The results indicate that affinity-purified polyclonal antibodies to two separate domains of the human Na(+)-H+ exchanger recognize a 97-kD protein in rabbit renal BBM but not BLM. The dissociation between recognition of the 97-kD protein using antibodies and the majority of functional Na(+)-H+ exchange activity after chromatographic fractionation of solubilized BBM proteins and in native BBM after administration of NH4Cl suggest that rabbit renal BBM contains more than one form of Na(+)-H+ exchanger.

Acute renal failure in cancer patients

Compromise of kidney function in patients with serious medical illnesses, including cancer, is a serious and potentially fatal complication. In cancer patients, the types of renal injury that may occur are more varied than in patients with other underlying illnesses, and definition of the etiology of the decline in renal function may have important therapeutic and prognostic implications. This review article discusses the clinical manifestations and the diagnostic workup of insults to the kidney in patients with cancer. Therapy (excluding dialysis) for each type of renal insufficiency is briefly reviewed.

Effects of maleic acid on renal phosphorus transport: role of dietary phosphorus

The effects of maleic acid on renal phosphate (Pi) transport were examined by clearance and brush-border membrane vesicle (BBMV) transport studies. In normal rats, maleic acid 50 mg.kg body wt-1.h-1 increased the phosphaturia (P less than 0.001). Intraperitoneal administration of a similar dose of maleic acid decreased the BBMV uptake of Pi but not glucose. In rats fed a low-phosphate diet (0.03%), the maleic acid-induced phosphaturia was blunted, but the inhibitory effect on the BBMV transport of Pi persisted. In chronic parathyroidectomized rats fed a low-phosphate diet, where the filtered load of Pi was higher than in the previous groups, the phosphaturia was abolished, but the inhibition of the BBMV transport of Pi was sustained. Both the in vitro incubation of BBMVV and in vivo administration of maleic acid were associated with a competitive inhibition of Pi transport. These studies indicate that the maleic acid-induced phosphaturia is expressed at the apical membrane entry step of Pi, and the enhanced distal tubular reabsorption accounts for the lack of phosphaturia in dietary Pi deprivation.

Identification of a putative Na(+)-H+ exchanger regulatory cofactor in rabbit renal BBM

Previous in vitro studies with detergent-solubilized rabbit renal brush-border membrane (BBM) proteins have suggested that adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase A (PKA)-mediated inhibition of the Na(+)-H+ exchanger requires the presence of 42-kDa cofactor that is distinct from the exchanger itself. We sought to determine whether there was a protein in native rabbit renal BBM vesicles that has characteristics similar to that of the 42-kDa cofactor. Incubation of native BBM vesicle proteins with a hypotonic phosphorylation solution containing purified catalytic subunit of PKA resulted in phosphorylation of a number of BBM proteins, including a protein with an apparent molecular weight that was similar but not identical to that of the 42-kDa cofactor obtained from anion-exchange column chromatography of n-octyl glucoside-extracted BBM proteins. The identity between the BBM vesicle protein and the 42-kDa cofactor was established by phosphopeptide maps and radioiodinated peptide maps. These results indicate that native BBM vesicles contain a number of proteins that are phosphorylated by PKA when the PKA and ATP are present inside the vesicle space. One of these proteins appears to be identical to the 42-kDa protein that, as previously suggested by in vitro studies, acts as a regulatory cofactor mediating the inhibitory effect of PKA on the renal Na(+)-H+ exchanger.

Identification of calcium-calmodulin multifunctional protein kinase II in rabbit kidney

Recent studies have demonstrated that calcium/calmodulin-dependent multifunctional protein kinase II (CaM-KII) inhibits the reconstituted Na(+)-H+ exchanger from the brush border membrane of proximal convoluted tubule of the rabbit kidney. The present studies were undertaken to evaluate the physiological relevance of this finding by establishing the presence of CaM-KII in rabbit kidney and proximal convoluted tubule cells by Northern RNA hybridization analysis to demonstrate the messenger RNA (mRNA) for CaM-KII and by a selective enzymatic assay of CaM-KII using a synthetic peptide substrate. A single 4.9 Kb mRNA was observed on hybridization of total RNA from rabbit kidney cortex and medulla and from an enriched suspension of rabbit kidney proximal convoluted tubules with a cDNA for rat brain CaM-KII. An enzyme assay using a synthetic peptide substrate representing the site phosphorylated by CaM-KII on glycogen synthase demonstrated calcium-calmodulin dependent protein kinase activity in both rabbit kidney cortex (specific activity of 662 +/- 127 nmol.min-1.mg protein-1) and proximal tubule cells (546 +/- 77 nmol.min-1.mg protein-1). These data establish the presence of CaM-KII in the rabbit kidney, and suggest a role for this enzyme in the control of renal electrolyte transport.

Regulation of renal Na(+)-H+ exchanger by cAMP-dependent protein kinase

Octyl glucoside-extracted rabbit renal brush-border membrane (BBM) proteins were sequentially fractionated using anion exchange chromatography, and the fractions were tested for Na(+)-H+ exchange activity, amiloride sensitivity, and the effect of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA) after reconstitution into artificial lipid vesicles. Compared with the initial protein extract, an anionic protein fraction eluting with 0.2-0.4 M NaCl (fraction B) demonstrated increased Na(+)-H+ exchange activity. Fraction B also demonstrated sensitivity to inhibition by amiloride but was not regulated by PKA. Co-reconstitution of fraction B with a BBM protein fraction highly enriched in a 42-kDa polypeptide restored the inhibitory response to PKA. These experiments suggest that, as assayed in a solubilized and reconstituted system, the Na(+)-H+ exchanger contains a dissociable PKA regulatory component, possibly a polypeptide of 42 kDa.

Regulation of the renal Na+-H+ exchanger by protein phosphorylation

Starting from observations in intact cells and extending to studies in native membranes and solubilized membrane proteins, a significant body of evidence has been accumulated to indicate that some of the short-term regulatory influences on the Na+-H+ exchanger in the apical membrane of the proximal convoluted tubule act via protein phosphorylation mediated by specific protein kinases. Protein phosphorylation mediated by PKA inhibits the Na+-H+ exchanger while that mediated by PKC stimulates activity. The effect of PKA and PKC on the Na+-H+ exchanger in native membranes and in solubilized brush border membrane proteins appears to be consistent with most of the published observations in intact cells. Further studies using solubilized, renal brush border membrane proteins indicated that protein phosphorylation mediated by CaM-kinase II inhibited the activity of the Na+-H+ exchanger. The physiologic significance of this observation in intact cells remains to be determined. It is hoped that the types of experimental approaches outlined in this review will yield additional insights into the structure of the Na+-H+ exchanger and to a clearer understanding of its physiologic regulation.

Effect of limited trypsin digestion on the renal Na+-H+ exchanger and its regulation by cAMP-dependent protein kinase

The Na+-H+ exchanger from solubilized rabbit renal brush border membranes is inhibited by cAMP-dependent protein kinase (PKA) mediated protein phosphorylation. To characterize this inhibitory response and its sensitivity to limited proteolysis, the activity of the transporter was assayed after reconstitution of the proteins into artificial lipid vesicles. Limited trypsin digestion increased the basal rate of proton gradient-stimulated, amiloride-inhibitable sodium uptake in reconstituted proteoliposomes and blocked the inhibitory response to PKA-mediated protein phosphorylation. To determine if the inhibitory response to PKA-mediated protein phosphorylation could be restored to the trypsin-treated solubilized proteins, nontrypsinized solubilized brush border membrane proteins were separated by column chromatography. The addition of small molecular weight polypeptides, fractionated on Superose-12 FPLC (Ve = 0.7), to trypsinized solubilized brush border membrane proteins restored the inhibitory response to PKA-mediated protein phosphorylation. Similarly, the addition of the 0.1 M NaCl fraction from an anion exchange column, Mono Q-FPLC, also restored the inhibitory response to PKA. Both protein fractions contained a common 42-43 kDa protein which was preferentially phosphorylated by PKA. These results indicate that limited trypsin digestion dissociates the activity of the renal Na+-H+ exchanger from its regulation by PKA. It is suggested that trypsin cleaves an inhibitory component of the transporter and that this component is the site of PKA-mediated regulation. Phosphoprotein analysis of fractions that restored PKA regulation raises the possibility that a polypeptide of 42-43 kDa is involved in the inhibition of the renal Na+-H+ exchanger by PKA-mediated protein phosphorylation.

Solubilization and reconstitution of renal brush border Na+-H+ exchanger

In order to permit future characterization and possible isolation of the Na+-H+ exchanger from the apical membrane of proximal tubular cells, studies were performed to solubilize and reconstitute this transporter. Rabbit brush border membranes were prepared by a magnesium aggregation method, solubilized with the detergent octyl glucoside, and reconstituted into artificial phospholipid vesicles. In the presence of a pH gradient (pHin 6.0, pHout 8.0), the uptake of 1 mM 22Na+ into the proteoliposomes was five- to sevenfold higher than into liposomes. Amiloride (2 mM) inhibited proton gradient-stimulated uptake of sodium by 50%. As compared to proton gradient conditions, the uptake of sodium was lower in the absence of a pH gradient but was significantly higher when the outside and inside pH was 6.0 than 8.0. The Ka for sodium in reconstituted proteoliposomes studied under pH gradient conditions was 4 mM. The uptake of sodium in proteoliposomes prepared from heat-denatured membrane proteins was significantly decreased. These studies demonstrate that proteoliposomes prepared from octyl glucoside-solubilized brush border membrane proteins and asolectin exhibit proton gradient-stimulated, amiloride-inhibitable, electroneutral uptake of sodium. The ability to solubilize and reconstitute the Na+-H+ exchanger from the apical membrane of the proximal tubule will be of value in isolating and characterizing this transporter.

Reconstitution of cAMP-dependent protein kinase regulated renal Na+-H+ exchanger

Studies were performed to determine if the Na+-H+ exchanger, solubilized from renal brush border membranes from the rabbit and assayed in reconstituted artificial proteoliposomes, could be regulated by cAMP-dependent protein kinase. Octyl glucoside solubilized renal apical membrane proteins from the rabbit kidney were phosphorylated by incubation with ATP and highly purified catalytic subunit of cAMP-dependent kinase. 22Na+ uptake was determined subsequently after reconstitution of the proteins into proteoliposomes. cAMP-dependent protein kinase resulted in sustained protein phosphorylation and a concentration-dependent decrease in the amiloride-sensitive component of pH gradient-stimulated sodium uptake. The inhibitory effect of cAMP-dependent protein kinase demonstrated an absolute requirement for ATP and was blocked by the specific protein inhibitor of this kinase. cAMP-dependent protein kinase also inhibited 22Na+ uptake in the absence of a pH gradient (pHin 6.0, pHout 6.0) and the inhibitory effect was blocked by the specific inhibitor of the kinase. Solubilized membrane proteins exhibited little endogenous protein kinase or protein phosphatase activity. These studies indicate that Na+-H+ exchange activity of proteoliposomes reconstituted with proteins from renal brush border membranes is inhibited by phosphorylation of selected proteins by cAMP-dependent protein kinase. These findings also indicate that the regulatory components of the Na+-H+ exchanger remain active during the process of solubilization and reconstitution of renal apical membrane proteins.

Regulation of reconstituted renal Na+/H+ exchanger by calcium-dependent protein kinases

Studies were performed to determine the effect of protein phosphorylation mediated by calcium-calmodulin-dependent multifunctional protein kinase II and calcium-phospholipid-dependent protein kinase on Na+/H+ exchange activity. Proteins from the apical membrane of the proximal tubule of the rabbit kidney were solubilized in octyl glucoside and incubated in phosphorylating solutions containing the protein kinase. 22Na+ uptake was determined subsequently after reconstitution of the proteins into proteoliposomes. Calcium-calmodulin-dependent multifunction protein kinase II inhibited the amiloride-sensitive component of proton gradient-stimulated Na+ uptake in a dose-dependent manner. The inhibitory effect of this kinase had an absolute requirement for calmodulin, Ca2+, and ATP. Calcium-phospholipid-dependent protein kinase stimulated the amiloride-sensitive component of proton gradient-stimulated Na+ uptake in a dose-dependent manner. The stimulating effect of this kinase had an absolute requirement for ATP, Ca2+, and an active phorbol ester. These experiments indicate that Na+/H+ exchange activity of proteoliposomes reconstituted with proteins from renal brush-border membranes are inhibited by protein phosphorylation mediated by calcium-calmodulin-dependent multifunctional protein kinase II and stimulated by that mediated by calcium-calmodulin-dependent protein kinase.