Basic properties and potential regulators of the apical K+ channel in macula densa cells

Effects of NKCC2 isoform regulation on NaCl transport in thick ascending limb and macula densa: a modeling study

This study aims to understand the extent to which modulation of the Na(+)-K(+)-2Cl(-) cotransporter NKCC2 differential splicing affects NaCl delivery to the macula densa. NaCl absorption by the thick ascending limb and macula densa cells is mediated by apical NKCC2. A recent study has indicated that differential splicing of NKCC2 is modulated by dietary salt (Schieβl IM, Rosenauer A, Kattler V, Minuth WW, Oppermann M, Castrop H. Am J Physiol Renal Physiol 305: F1139-F1148, 2013). Given the markedly different ion affinities of its splice variants, modulation of NKCC2 differential splicing is believed to impact NaCl reabsorption. To assess the validity of that hypothesis, we have developed a mathematical model of macula densa cell transport and incorporated that cell model into a previously applied model of the thick ascending limb (Weinstein AM, Krahn TA. Am J Physiol Renal Physiol 298: F525-F542, 2010). The macula densa model predicts a 27.4- and 13.1-mV depolarization of the basolateral membrane [as a surrogate for activation of tubuloglomerular feedback (TGF)] when luminal NaCl concentration is increased from 25 to 145 mM or luminal K(+) concentration is increased from 1.5 to 3.5 mM, respectively, consistent with experimental measurements. Simulations indicate that with luminal solute concentrations consistent with in vivo conditions near the macula densa, NKCC2 operates near its equilibrium state. Results also suggest that modulation of NKCC2 differential splicing by low salt, which induces a shift from NKCC2-A to NKCC2-B primarily in the cortical thick ascending limb and macula densa cells, significantly enhances salt reabsorption in the thick limb and reduces Na(+) and Cl(-) delivery to the macula densa by 3.7 and 12.5%, respectively. Simulation results also predict that the NKCC2 isoform shift hyperpolarizes the macula densa basolateral cell membrane, which, taken in isolation, may inhibit the release of the TGF signal. However, excessive early distal salt delivery and renal salt loss during a low-salt diet may be prevented by an asymmetric TGF response, which may be more sensitive to flow increases.

In vivo and ex vivo analysis of tubule function

Analysis of tubule function with in vivo and ex vivo approaches has been instrumental in revealing renal physiology. This work allows assignment of functional significance to known gene products expressed along the nephron, primary of which are proteins involved in electrolyte transport and regulation of these transporters. Not only we have learned much about the key roles played by these transport proteins and their proper regulation in normal physiology but also the combination of contemporary molecular biology and molecular genetics with in vivo and ex vivo analysis opened a new era of discovery informative about the root causes of many renal diseases. The power of in vivo and ex vivo analysis of tubule function is that it preserves the native setting and control of the tubule and proteins within tubule cells enabling them to be investigated in a "real-life" environment with a high degree of precision. In vivo and ex vivo analysis of tubule function continues to provide a powerful experimental outlet for testing, evaluating, and understanding physiology in the context of the novel information provided by sequencing of the human genome and contemporary genetic screening. These tools will continue to be a mainstay in renal laboratories as this discovery process continues and as we continue to identify new gene products functionally compromised in renal disease.

ATP as a mediator of macula densa cell signalling

Within each nephro-vascular unit, the tubule returns to the vicinity of its own glomerulus. At this site, there are specialised tubular cells, the macula densa cells, which sense changes in tubular fluid composition and transmit information to the glomerular arterioles resulting in alterations in glomerular filtration rate and blood flow. Work over the last few years has characterised the mechanisms that lead to the detection of changes in luminal sodium chloride and osmolality by the macula densa cells. These cells are true "sensor cells" since intracellular ion concentrations and membrane potential reflect the level of luminal sodium chloride concentration. An unresolved question has been the nature of the signalling molecule(s) released by the macula densa cells. Currently, there is evidence that macula densa cells produce nitric oxide via neuronal nitric oxide synthase (nNOS) and prostaglandin E(2) (PGE(2)) through cyclooxygenase 2 (COX 2)-microsomal prostaglandin E synthase (mPGES). However, both of these signalling molecules play a role in modulating or regulating the macula-tubuloglomerular feedback system. Direct macula densa signalling appears to involve the release of ATP across the basolateral membrane through a maxi-anion channel in response to an increase in luminal sodium chloride concentration. ATP that is released by macula densa cells may directly activate P2 receptors on adjacent mesangial cells and afferent arteriolar smooth muscle cells, or the ATP may be converted to adenosine. However, the critical step in signalling would appear to be the regulated release of ATP across the basolateral membrane of macula densa cells.

Investigation of the Ba2+-sensitive NH4+ transport pathways in the apical cell membrane of primary cultured rabbit MTAL cells

BACKGROUND

Several apical ammonium (NH(4)(+)/NH(3)) transport pathways have been described in medullary thick ascending limb (MTAL) cells. The exact nature and importance of some of these pathways remain controversial.

METHODS

Ammonium transport in primary cultured rabbit MTAL cells was investigated by measuring intracellular pH (pH(i)).

RESULTS

To create physiological conditions, experiments were performed in the symmetrical presence of NH(4)Cl, which acidified the cells to pH(i) 6.89. When blockers of apical NH(4)(+) transport were used, the cells alkalinized due to a decreased NH(4)(+) loading. The following values (pH units) were observed: bumetanide, +0.05; verapamil, +0.04; Ba(2+) and Cs(+), +0.19; tertiapin, +0.09. Tetraethylammonium had no effect. Depolarizing the cells by increasing the K(+) concentration alkalinized the cells by 0.16 pH units. Because NH(4)(+) might enter through nonspecific channels, ammonium pulse experiments were performed: an NH(4)Cl pulse acidified controls as well as depolarized cells. In contrast, when Ba(2+), Cs(+) or tertiapin were present, an NH(4)Cl pulse alkalinized the cells. The pharmacological profile of this apical NH(4)(+) transport pathway correlates with the renal outer medullary K(+) (ROMK) channel. Indirect immunofluorescence showed the presence of the ROMK protein.

CONCLUSION

In these MTAL cells the Ba(2+)-sensitive component of NH(4)(+) transport is predominant and consists of permeation of NH(4)(+) through an apical ROMK-related channel.

Molecular diversity and regulation of renal potassium channels

K(+) channels are widely distributed in both plant and animal cells where they serve many distinct functions. K(+) channels set the membrane potential, generate electrical signals in excitable cells, and regulate cell volume and cell movement. In renal tubule epithelial cells, K(+) channels are not only involved in basic functions such as the generation of the cell-negative potential and the control of cell volume, but also play a uniquely important role in K(+) secretion. Moreover, K(+) channels participate in the regulation of vascular tone in the glomerular circulation, and they are involved in the mechanisms mediating tubuloglomerular feedback. Significant progress has been made in defining the properties of renal K(+) channels, including their location within tubule cells, their biophysical properties, regulation, and molecular structure. Such progress has been made possible by the application of single-channel analysis and the successful cloning of K(+) channels of renal origin.

Current mechanisms of macula densa cell signalling

Macula densa cells couple renal haemodynamics, glomerular filtration and renin release with tubular fluid salt and water reabsorption. These cells detect changes in tubular fluid composition through a complex of intracellular signalling events that are mediated by membrane transport pathways. Increases in luminal fluid sodium chloride concentration result in alterations in cell sodium chloride concentration, cytosolic calcium, cell pH, basolateral membrane depolarization and cell volume. Macula densa signalling then involves the production and release of specific paracrine signalling molecules at their basolateral membrane. Upon moderate increases in luminal sodium chloride concentration macula densa cells release increasing amounts of ATP and decreasing amounts of prostaglandin E(2), thereby increasing afferent arteriolar tone and decreasing the release of renin from granular cells. On the other hand, further increases in luminal concentration stimulate the release of nitric oxide, which serve to prevent excessive tubuloglomerular feedback vasoconstriction. Paracrine signalling by the macula densa cells therefore controls juxtaglomerular function, renal vascular resistance and participates in the regulation of renin release.

Increased intracellular Ca++ in the macula densa regulates tubuloglomerular feedback

BACKGROUND

Tubuloglomerular feedback is initiated by an increase in NaCl at the macula densa lumen, which in turn increases intracellular Ca++. In the present study, we examined the role of increased intracellular Ca++ in tubuloglomerular feedback and the source of the increased Ca++. We hypothesized that an increase in intracellular Ca++ at the macula densa via the basolateral Na+/Ca++ exchanger, caused by an increase in luminal NaCl, initiates Ca++-mediated Ca++ release from intracellular stores, which is essential for tubuloglomerular feedback.

METHODS

Rabbit afferent arterioles and attached macula densas were simultaneously microperfused in vitro. Tubuloglomerular feedback was induced by increasing macula densa Na+/Cl- from 11/10 mmol/L (low) to 81/80 mmol/L (high) and was measured before and after treatment.

RESULTS

To investigate whether elevations in intracellular Ca++ are required for tubuloglomerular feedback, the calcium ionophore A23187 or the Ca++ chelator BAPTA-AM was added to the macula densa lumen. During the control period, tubuloglomerular feedback decreased afferent arteriole diameter from 18.1 +/- 1.1 microm to 15.3 +/- 0.8 microm. Adding 2 x 10-6 mol/L A23187 to the low NaCl macula densa perfusate induced tubuloglomerular feedback; diameter decreased from 18.0 +/- 1.0 microm to 15.4 +/- 0.9 microm (N = 6; P < 0.01). After adding BAPTA-AM (25 micromol/L) to the macula densa lumen, tubuloglomerular feedback response was completely eliminated. We next studied the source of increased macula densa Ca++ in response to increased NaCl concentration. During the control period, tubuloglomerular feedback decreased afferent arteriole diameter from 18.5 +/- 1.6 microm to 15.3 +/- 1.2 microm (N = 6; P < 0.01). After adding the Na+/Ca++ exchanger inhibitor 2'4'-dichlorobenzamil (10 micromol/L) or KB-R7943 (30 micromol/L) to the bath, the tubuloglomerular feedback response was blocked; however, the afferent arteriole response to angiotensin II or adenosine was not altered. Next, we tested the Ca++-adenosine triphosphatase (ATPase) inhibitor thapsigargin (0.1 micromol/L), which has been reported to inhibit sarcoplasmic reticulum Ca++-ATPase activity and prevent restoration of intracellular Ca++ stores. When thapsigargin was added to the macula densa lumen, it reduced the first tubuloglomerular feedback response by 33% and completely eliminated the second and third tubuloglomerular feedback responses. In the absence of thapsigargin, there was no significant decrease in the tubuloglomerular feedback responses (N = 6). Neither the L-type Ca++ channel blocker nifedipine (25 micromol/L), nor the T-type Ca++ channel blocker pimozide (10 micromol/L), inhibited tubuloglomerular feedback when added to the macula densa lumen.

CONCLUSION

We concluded that (1). increased intracellular Ca++ at the macula densa is required for the tubuloglomerular feedback response; (2). Na+/Ca++ exchange appears to initiate Ca++-mediated Ca++ release from intracellular stores; and (3). luminal L-type or T-type Ca++ channels are not involved in tubuloglomerular feedback.

Calcium-activated nonselective cationic channel in macula densa cells

Patch-clamp experiments in cell-attached (c/a) and inside-out (i/o) configurations were performed to directly observe ionic channels in lateral membranes of macula densa (MD) cells from rabbit kidney. In the presence of 140 mM KCl in the pipette and normal Ringer solution in the bath, we repeatedly observed in c/a and in i/o configurations a 20- to 23-pS channel with a linear current-voltage (I-V) relationship reversing near 0 mV. Ionic replacement in the bath solution clearly indicated a cationic selectivity but with equal permeability for Na+ and K+. Single-channel kinetics was characterized by higher open probability at positive membrane potentials. In i/o experiments, elimination of bath Ca2+ (<or=1 microM) abolished channel activity in a reversible manner. This MD nonselective cationic channel was found to display a certain Ca2+ permeability because single-channel events could be detected when the pipette potential was very negative (-60, -80, and -100 mV) in the presence of 73 mM CaCl2 in the bath solution. The similarities between this channel and some channels of the transient receptor potential family suggest a possible role for this MD basolateral channel in controlling membrane potential and regulating Ca2+ entry during MD cell signaling.

Macula densa cell signaling

Macula densa cells are renal sensor elements that detect changes in distal tubular fluid composition and transmit signals to the glomerular vascular elements. This tubuloglomerular feedback mechanism plays an important role in regulating glomerular filtration rate and blood flow. Macula densa cells detect changes in luminal sodium chloride concentration through a complex series of ion transport-related intracellular events. NaCl entry via a Na:K:2Cl cotransporter and Cl exit through a basolateral channel lead to cell depolarization and increases in cytosolic calcium. Na/H exchange (NHE2) results in cell alkalization, whereas intracellular [Na] is regulated by an apically located H(Na)-K ATPase and not by the traditional basolateral Na:K ATPase. Communication from macula densa cells to the glomerular vascular elements involves ATP release across the macula densa basolateral membrane through a maxi-anion channel. The adaptation of multi-photon microscopy is providing new insights into macula densa-glomerular signaling.

Impaired renal NaCl absorption in mice lacking the ROMK potassium channel, a model for type II Bartter's syndrome

ROMK is an apical K(+) channel expressed in the thick ascending limb of Henle (TALH) and throughout the distal nephron of the kidney. Null mutations in the ROMK gene cause type II Bartter's syndrome, in which abnormalities of electrolyte, acid-base, and fluid-volume homeostasis occur because of defective NaCl reabsorption in the TALH. To understand better the pathogenesis of type II Bartter's syndrome, we developed a mouse lacking ROMK and examined its phenotype. Young null mutants had hydronephrosis, were severely dehydrated, and approximately 95% died before 3 weeks of age. ROMK-deficient mice that survived beyond weaning grew to adulthood; however, they had metabolic acidosis, elevated blood concentrations of Na(+) and Cl(-), reduced blood pressure, polydipsia, polyuria, and poor urinary concentrating ability. Whole kidney glomerular filtration rate was sharply reduced, apparently as a result of hydronephrosis, and fractional excretion of electrolytes was elevated. Micropuncture analysis revealed that the single nephron glomerular filtration rate was relatively normal, absorption of NaCl in the TALH was reduced but not eliminated, and tubuloglomerular feedback was severely impaired. These data show that the loss of ROMK in the mouse causes perturbations of electrolyte, acid-base, and fluid-volume homeostasis, reduced absorption of NaCl in the TALH, and impaired tubuloglomerular feedback.

Novel regulation of cell [Na(+)] in macula densa cells: apical Na(+) recycling by H-K-ATPase

Na-K-ATPase is the nearly ubiquitous enzyme that maintains low-Na(+), high-K(+) concentrations in cells by actively extruding Na(+) in exchange for K(+). The prevailing paradigm in polarized absorbing epithelial cells, including renal nephron segments and intestine, has been that Na-K-ATPase is restricted to the basolateral membrane domain, where it plays a prominent role in Na(+) absorption. We have found, however, that macula densa (MD) cells lack functionally and immunologically detectable amounts of Na-K-ATPase protein. In fact, these cells appear to regulate their cytosolic [Na(+)] via another member of the P-type ATPase family, the colonic form of H-K-ATPase, which is located at the apical membrane in these cells. We now report that this constitutively expressed apical MD colonic H-K-ATPase can function as a Na(H)-K-ATPase and regulate cytosolic [Na(+)] in a novel manner. This apical Na(+)-recycling mechanism may be important as part of the sensor function of MD cells and represents a new paradigm in cell [Na(+)] regulation.

Angiotensin II directly stimulates macula densa Na-2Cl-K cotransport via apical AT(1) receptors

ANG II is a modulator of tubuloglomerular feedback (TGF); however, the site of its action remains unknown. Macula densa (MD) cells sense changes in luminal NaCl concentration ([NaCl](L)) via a Na-2Cl-K cotransporter, and these cells do possess ANG II receptors. We tested whether ANG II regulates Na-2Cl-K cotransport in MD cells. MD cell Na(+) concentration ([Na(+)](i)) was measured using sodium-binding benzofuran isophthalate with fluorescence microscopy. Resting [Na(+)](i) in MD cells was 27.7 +/- 1.05 mM (n = 138) and increased (Delta[Na(+)](i)) by 18.5 +/- 1.14 mM (n = 17) at an initial rate (Delta[Na(+)](i)/Deltat) of 5.54 +/- 0.53 x 10(-4) U/s with an increase in [NaCl](L) from 25 to 150 mM. Both Delta[Na(+)](i) and Delta[Na(+)](i)/Deltat were inhibited by 80% with 100 microM luminal furosemide. ANG II (10(-9) or 10(-12) M) added to the lumen increased MD resting [Na(+)](i) and [NaCl](L)-dependent Delta[Na(+)](i) and caused a twofold increase in Delta[Na(+)](i)/Deltat. Bath (10(-9) M) ANG II also stimulated cotransport activity, and there was no additive effect of simultaneous addition of ANG II to bath and lumen. The effects of luminal ANG II were furosemide sensitive and abolished by the AT(1) receptor blocker candesartan. ANG II at 10(-6) M failed to stimulate the cotransporter, whereas increased cotransport activity could be restored by blocking AT(2) receptors with PD-123, 319. Thus ANG II may modulate TGF responses via alterations in MD Na-2Cl-K cotransport activity.

Nystatin and valinomycin induce tubuloglomerular feedback

The macula densa expresses a luminal Na(+)-K(+)-2Cl(-) cotransporter and a basolateral Cl(-) conductance. Although it is known that cotransport of Na(+), K(+), and Cl(-) is the first step in tubuloglomerular feedback (TGF), subsequent steps are unclear. We hypothesized that Na(+)-K(+)-2Cl(-) entry via the luminal Na(+)-K(+)-2Cl(-) cotransporter elevates intracellular Cl(-), increases electrogenic Cl(-) efflux across the basolateral membrane, and depolarizes the macula densa, initiating TGF. We perfused afferent arterioles with macula densa attached. The macula densa was perfused with solutions containing either 5 mM Na(+) and 3 mM Cl(-) (low NaCl) or 80 mM Na(+) and 77 mM Cl(-) (high NaCl). When the macula densa perfusate was changed from low to high NaCl, afferent arteriole diameter decreased from 15.8 +/- 0.8 to 13.1 +/- 0.7 mm (P < 0.05). Adding 10 microM furosemide to the macula densa lumen blocked TGF. When nystatin, a group I cation ionophore, was added to the macula densa lumen together with furosemide in the presence of low NaCl, it induced TGF (from 18.0 +/- 1.5 to 15.6 +/- 1.6 mm; P = 0.003). When valinomycin, a K(+)-selective ionophore, was added to the macula densa lumen together with furosemide in the presence of low NaCl containing 5 mM K(+), it did not induce TGF. Subsequent addition of 50 mM KCl to the macula densa perfusate induced TGF (from 21.7 +/- 0.8 to 17.5 +/- 1.3 mm; P = 0.0047; n = 6). Adding 50 mM KCl without valinomycin did not induce TGF. When 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB; 1 microM), a Cl(-) channel blocker, was added to the bath, it blocked TGF induced by high NaCl, but did not block TGF induced by valinomycin plus 50 mM KCl. NPPB did not alter afferent arteriole constriction induced by norepinephrine. We concluded that increased NaCl in the lumen of the macula densa leads to influx of Cl(-) via the Na(+)-K(+)-2Cl(-) cotransporter. The accelerated transport increases intracellular Cl(-). The subsequent exit of Cl(-) across the basolateral membrane via Cl( -) channels in turn leads to depolarization of the macula densa and thereby induces TGF.

Transport and regulatory properties of the apical Na-K-2Cl cotransporter of macula densa cells

NH+4/NH3 fluxes were used to probe apical Na-K-2Cl transport activity of macula densa (MD) cells from rabbit kidney. In the presence of 25 mM NaCl and 5 mM Ba2+, addition of 20 mM NH+4 to the lumen produced a profound intracellular acidification, and approximately 80% of the initial acidification rate was bumetanide sensitive. The NH+4-induced acidification rate was dependent on luminal Cl- and Na+ with apparent affinities of 17 +/- 4 mM (Hill number 1.45) and 1.0 +/- 0.3 mM, respectively. In the presence of saturating luminal NaCl concentration ([NaCl]L), blockade of basolateral Cl- efflux with 10 microM 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) reduced the NH+4-induced acidification rate by 51 +/- 6% (P > 0.01, n = 5). Under similar conditions, dibutyryl-cAMP (DBcAMP) + forskolin increased the NH+4-induced acidification rate by 27%, whereas it produced no detectable effect at low luminal NaCl concentration. Most of the observed DBcAMP + forskolin effect was probably due to the stimulation of the basolateral Cl- conductance, since, in the presence of basolateral NPPB, this activation was changed to a 17.1% and 16.6% inhibition of the NH+4-induced acidification rate observed at high or low [NaCl]L, respectively. We conclude that the cotransporter found in MD cells displays, with respect to other Na-K-2Cl cotransporters, a relatively high affinity for luminal Na+ and luminal Cl- and can be specifically inhibited by increases in intracellular Cl- and cAMP concentrations.

Ionic transport in macula densa cells

Recent work has provided substantial insights into functional characteristics of macula densa (MD) cells. Microelectrode and patch-clamp experiments on the rabbit isolated thick ascending limb (TAL)/glomerulus preparation have shown that MD cells possess a furosemide-sensitive Na:K:2Cl cotransporter, an apical 41-pS K+ channel, and a dominant basolateral Cl- conductance. Increasing luminal fluid [NaCl] ([NaCl]L) results in furosemide-sensitive cell depolarization due to a rise in intracellular [Cl-] that stimulates basolateral electrogenic Cl- efflux. Intracellular pH (pHi) measurements show the presence of an apical Na:H exchanger that couples transepithelial Na+ transport to pHi. Experimental results and thermodynamic considerations allow estimation of intracellular [Na+] and [Cl-] ([Na+]i, [Cl-]i) under different conditions. When the Na:K:2Cl cotransporter is equilibrated (or in the presence of furosemide), [Na+]i and [Cl-]i are low (approximately 6 to 7 mM), whereas when the cotransporter is fully activated, [Na+]i and [Cl-]i increase substantially to approximately 70 and 20 mM, respectively. Finally, luminal addition of NH4+ produces cell acidification that depends on NH4+ apical transport rate through the Na:K:2Cl. Using a simple transport model for NH4+, the initial NH4+ influx rate in MD cells is comparable to the corresponding flux in TAL. This challenges the idea that MD cells have a low transport activity but supports our findings about large changes in intracellular concentrations as a function of [NaCl]L.

Juxtaglomerular cell complex in the regulation of renal salt excretion

Luminal NaCl concentration at the macula densa (MD) has the two established effects of regulating glomerular arteriolar resistance and renin secretion. Tubuloglomerular feedback (TGF), the inverse relationship between MD NaCl concentration and glomerular filtration rate (GFR), stabilizes distal salt delivery and thereby NaCl excretion in response to random perturbations unrelated to changes in body salt balance. Control of vasomotor tone by TGF is exerted primarily by NaCl transport-dependent changes in local adenosine concentrations. During long-lasting perturbations of MD NaCl concentration, control of renin secretion becomes the dominant function of the MD. The potentially maladaptive effect of TGF under chronic conditions is prevented by TGF adaptations, permitting adjustments in GFR to occur. TGF adaptation is mechanistically coupled to the end point targeted by chronic deviations in MD NaCl, the rate of local and systemic angiotensin II generation. MD control of renin secretion is the result of the coordinated action of local mediators that include nitric oxide synthase (NOS) and cyclooxygenase (COX) products. Thus vascular smooth muscle cell activation during high MD transport and granular cell activation during low MD transport is achieved by different extracellular mediators. The coordinated regulation of NOS I and COX-2 expression in MD cells and of renin expression in granular cells suggests that control of juxtaglomerular regulation of gene transcription or mRNA metabolism may be another consequence of a chronic alteration in MD NaCl concentration.

Localization of the ROMK protein on apical membranes of rat kidney nephron segments

The ATP-sensitive, inwardly rectifying K+ channel, ROMK, has been suggested to be the low-conductance ATP-sensitive K+ channel identified in apical membranes of mammalian renal thick ascending limb (TAL) and cortical collecting duct (CCD). Mutations in the human ROMK gene (KIR 1.2) have been identified in kindreds with neonatal Bartter's syndrome. In the present study, we generated polyclonal antibodies raised against both a COOH-terminal (amino acids 252-391) ROMK-maltose binding protein (MBP) fusion protein and an NH2-terminal (amino acids 34-49) ROMK peptide. Affinity-purified anti-ROMK COOH-terminal antibody detected the 45-kDa ROMK protein in kidney tissues and HEK-293 cells transfected with ROMK1 cDNA. The antibody also recognized 85- to 90-kDa proteins in kidney tissue; these higher molecular weight proteins were abolished by immunoabsorption with ROMK-MBP fusion protein and were also detected on Western blots using anti-ROMK NH2-terminal antibody. Immunofluoresence studies using anti-ROMK COOH-terminal antibody showed intense apical staining along the loop of Henle and distal nephron; staining with preimmune and immunoabsorbed serum was negative. When colocalized with distal nephron markers [the thiazide-sensitive cotransporter (rTSC1), the bumetanide-sensitive cotransporter (rBSC1), the vacuolar type H(+)-ATPase, and neuronal nitric oxide synthase (NOS I)], the ROMK protein was found primarily at the apical border of cells in the TAL, macula densa, distal convoluted tubule, and connecting tubule. Within the CCD, the ROMK protein was expressed in principal cells and was absent from intercalated cells. The tubule localization and polarity of ROMK staining are consistent with the distribution of ROMK mRNA and provide more support for ROMK being the low-conductance K+ secretory channel in the rat distal nephron.

Determination of NH4+/NH3 fluxes across apical membrane of macula densa cells: a quantitative analysis

Luminal addition of 20 mM NH4+ produced a rapid acidification of rabbit macula densa (MD) cells from 7.50 +/- 0.06 to 6.91 +/- 0.05 at an initial rate of 0.071 +/- 0.008 pH unit/s. In the luminal presence of 5 microM bumetanide, 5 mM Ba2+ or both, the acidification rate was reduced by 57%, 35% and 93% of control levels. In contrast, intracellular pH (pHi) recovery after removing luminal NH4+ was unaffected by bumetanide and Ba2+ but was sensitive to 1 mM luminal amiloride (71% inhibition). The bumetanide-sensitive acidification rate represents most certainly the NH4+ flux mediated by the apical Na+:K+ (NH4+):2Cl- cotransporter, but the Ba(2+)-sensitive portion does not seem to be associated with the apical K+ channels previously characterized by us. The effects of NH4+ entry across the apical membrane were simulated using a simple model involving five adjustable parameters: apical and basolateral permeabilities for NH4+ and NH3 and a parameter describing a pH-regulating mechanism. The model shows that the apical membrane of MD cells is much more permeable to NH3 than it is to NH4+ and, under control conditions, the apical NH4+ flux appears surprisingly high (11-20 mM/s) and challenges the notion that MD cells present a low intensity of ionic transport.

Characteristics of membrane transport processes of macula densa cells

1. Macula densa (MD) cells are located within the thick ascending limb (TAL) and have their apical surface in contact with tubular fluid and their basilar region in contact with the glomerulus. These cells sense changes in luminal fluid sodium chloride concentration ([NaCl]) and transmit signals resulting in changes in vascular resistance (tubuloglomerular feedback) and renin release. 2. Current efforts have focused on understanding the cellular transport mechanisms of MD cells. Progress in this area has benefited from the use of the isolated perfused TAL-glomerular preparation, which permits direct access to MD cells. 3. Using microelectrodes to measure basolateral membrane potential (VBL) of MD cells, it was found that VBL was very sensitive to changes in luminal fluid [NaCl]. As [NaCl] was elevated from 20 to 150 mmol/L, VBL was found to depolarize by over 30 mV. 4. Basolateral membrane potential measurements were also used to identify an apical Na+:2Cl-:K+ cotransport pathway in MD cells that is the major pathway for NaCl entry into these cells. 5. Other work identified a basolateral chloride channel that is presumed to be responsible for changes in VBL during alterations in luminal [NaCl]. This channel, which is the predominant conductance across the basolateral membrane, may be regulated by intracellular Ca2+ and cAMP. 6. An apical Na+:H+ exchanger in MD cells was detected by measuring changes in intracellular pH using the fluorescent probe 2',7'-bis-(2-carboxyethyl)-5(and-6) carboxyfluorescein. 7. Using patch-clamp techniques, a high density of pH- and Ca(2+)-sensitive K+ channels was observed at the apical membrane of MD cells. 8. Other studies found that, at the normal physiological conditions prevailing at the end of the TAL (luminal [NaCl] of 20-60 mmol/L), reabsorption mediated by MD cells is very sensitive to changes in luminal [NaCl].

Renal K+ channels: structure and function

The activity of potassium (K+) channels is intimately linked to several important transport functions in renal tubules. We review recent progress concerning the properties, site along the nephron, and physiological regulation of native K+ channels, and compare their characteristics with those of recently cloned K+ channels. We do not fully cover work on K+ channels in amphibian tubules, cell cultures, and single tubule cells and do not review K+ channels in mesangial cells.

An ATP-regulated, inwardly rectifying potassium channel from rat kidney (ROMK)

With the cloning of ROMK [31] and IRK1 [32], a new family of inwardly rectifying K+ channels has been identified. ROMK channel isoforms are highly and differentially expressed in distal nephron segments of the mammalian kidney. These channels exhibit many of the characteristics of the low conductance, ATP-sensitive K+ channels found in apical membranes of TAL, macula densa, and principal cells that are involved in potassium secretion. Thus ROMK channel isoforms appear to be involved in the formation of these secretory KATP channels. Further characterization of these channels should provide further evidence for their role in the secretory KATP channels and new insights into the function and regulation of these channels.

Activation of Na:2Cl:K cotransport by luminal chloride in macula densa cells

Changes in macula densa intracellular pH (pHi) were used to monitor the direction of flux mediated by the apical Na:2Cl:K cotransporter. At the macula densa, a decrease in luminal [Cl] ([Cl]1) from 60 to 1 mM produced cellular alkalinization secondary to a cascade of events involving a decrease in apical Na:2Cl:K cotransport, a fall in intracellular [Na] ([Na]i) and a stimulation of Na:H exchange. This is supported by the fact that 97% of the change in macula densa pHi with reduction in [Cl]1 was bumetanide-sensitive whereas 92% of this pH change was amiloride-sensitive. We found that, in the presence of 20 mM Na and 5 mM K, a [Cl]1 of 14.3 +/- 2.4 mM (N = 7) produced equilibrium of the apical cotransporter since the pHi obtained under this condition was identical to the pHi found after reducing the net ionic flux to zero with bumetanide. Using this value together with the expected stoichiometry for the bumetanide-sensitive cotransporter, it was estimated that the intracellular [Cl] ([Cl]i) at equilibrium (or in the presence of bumetanide) could be as low as 5 mM. Also, using a Hill number of 2 which is consistent with the present data, the affinity for [Cl]1 was found to be 32.5 mM. Under physiological luminal conditions prevailing at the end of the thick ascending limb (approximately 3.5 mM K, and approximately 25 to 30 mM NaCl), macula densa cells are probably operating close to equilibrium while maintaining a small net reabsorption of Na/K and Cl. Since macula densa cells appear capable of reducing [Cl]i to very low levels, a reabsorptive flux should continue to occur until [NaCl]1 is reduced to 18 mM.

Evidence for apical sodium proton exchange in macula densa cells

These studies were performed to determine if changes in luminal sodium chloride concentration ([NaCl]) might alter macula densa intracellular pH. Isolated thick ascending limbs with attached glomeruli were bathed in a 150 mM NaCl Ringer's solution and perfused in vitro with a 25 mM NaCl solution; N-methyl-D-glucamine cyclamate was used to substitute for NaCl. Macula densa cells were loaded with BCECF and intracellular pH was monitored using a microscope based-dual excitation photometer system. Control intracellular pH for all experiments in which tubules were initially perfused with 25 mM NaCl averaged 7.22 +/- 0.06; N = 28. Increasing luminal [NaCl] from 25 to 150 mM elevated macula densa pH by 0.15 +/- 0.03 (N = 6; P < 0.05) while increasing just luminal [Na] from 25 to 150 mM alkalinized macula densa cells by 0.17 +/- 0.05 (N = 6; P < 0.05). In addition, there was a highly significant linear relationship between luminal [Na] and intracellular pH between 25 and 150 mM NaCl. Other studies were performed to assess the effects of amiloride, an inhibitor of Na:H exchange, on macula densa intracellular pH. Addition of amiloride, to the 25 mM NaCl perfusate acidified macula densa cells by 0.09 +/- 0.03 (N = 6; P < 0.001) and significantly attenuated the increase in pH obtained when luminal [NaCl] was raised from 25 to 150 mM. Other studies evaluated the effects of inhibition of Na:2Cl:K cotransport on macula densa pH.(ABSTRACT TRUNCATED AT 250 WORDS)