Na+ and K+ transport by the renal connecting tubule

Trimethoprim inhibits renal H+-K+-ATPase in states of K+ depletion

There is growing consensus that under physiological conditions, collecting duct H+ secretion is independent of epithelial Na+ channel (ENaC) activity. We have recently shown that the direct ENaC inhibitor benzamil acutely impairs H+ excretion by blocking renal H+-K+-ATPase. However, the question remains whether inhibition of ENaC per se causes alterations in renal H+ excretion. To revisit this question, we studied the effect of the antibiotic trimethoprim (TMP), which is well known to cause K+ retention by direct ENaC inhibition. The acute effect of TMP (5 µg/g body wt) was assessed in bladder-catheterized mice, allowing real-time measurement of urinary pH, electrolyte, and acid excretion. Dietary K+ depletion was used to increase renal H+-K+-ATPase activity. In addition, the effect of TMP was investigated in vitro using pig gastric H+-K+-ATPase-enriched membrane vesicles. TMP acutely increased natriuresis and decreased kaliuresis, confirming its ENaC-inhibiting property. Under control diet conditions, TMP had no effect on urinary pH or acid excretion. Interestingly, K+ depletion unmasked an acute urine alkalizing effect of TMP. This finding was corroborated by in vitro experiments showing that TMP inhibits H+-K+-ATPase activity, albeit at much higher concentrations than benzamil. In conclusion, under control diet conditions, TMP inhibited ENaC function without changing urinary H+ excretion. This finding further supports the hypothesis that the inhibition of ENaC per se does not impair H+ excretion in the collecting duct. Moreover, TMP-induced urinary alkalization in animals fed a low-K+ diet highlights the importance of renal H+-K+-ATPase-mediated H+ secretion in states of K+ depletion.NEW & NOTEWORTHY The antibiotic trimethoprim (TMP) often mediates K+ retention and metabolic acidosis. We suggest a revision of the underlying mechanism that causes metabolic acidosis. Our results indicate that TMP-induced metabolic acidosis is secondary to epithelial Na+ channel-dependent K+ retention. Under control dietary conditions, TMP does not per se inhibit collecting duct H+ secretion. These findings add further argument against a physiologically relevant voltage-dependent mechanism of collecting duct H+ excretion.

Principles of human and mouse nephron development

The mechanisms underlying kidney development in mice and humans is an area of intense study. Insights into kidney organogenesis have the potential to guide our understanding of the origin of congenital anomalies and enable the assembly of genetic diagnostic tools. A number of studies have delineated signalling nodes that regulate positional identities and cell fates of nephron progenitor and precursor cells, whereas cross-species comparisons have markedly enhanced our understanding of conserved and divergent features of mammalian kidney organogenesis. Greater insights into the complex cellular movements that occur as the proximal-distal axis is established have challenged our understanding of nephron patterning and provided important clues to the elaborate developmental context in which human kidney diseases can arise. Studies of kidney development in vivo have also facilitated efforts to recapitulate nephrogenesis in kidney organoids in vitro, by providing a detailed blueprint of signalling events, cell movements and patterning mechanisms that are required for the formation of correctly patterned nephrons and maturation of physiologically functional apparatus that are responsible for maintaining human health.

Regulation of distal tubule sodium transport: mechanisms and roles in homeostasis and pathophysiology

Regulated Na⁺ transport in the distal nephron is of fundamental importance to fluid and electrolyte homeostasis. Further upstream, Na⁺ is the principal driver of secondary active transport of numerous organic and inorganic solutes. In the distal nephron, Na⁺ continues to play a central role in controlling the body levels and concentrations of a more select group of ions, including K⁺, Ca⁺⁺, Mg⁺⁺, Cl^-, and HCO₃^-, as well as water. Also, of paramount importance are transport mechanisms aimed at controlling the total level of Na⁺ itself in the body, as well as its concentrations in intracellular and extracellular compartments. Over the last several decades, the transporters involved in moving Na⁺ in the distal nephron, and directly or indirectly coupling its movement to that of other ions have been identified, and their interrelationships brought into focus. Just as importantly, the signaling systems and their components-kinases, ubiquitin ligases, phosphatases, transcription factors, and others-have also been identified and many of their actions elucidated. This review will touch on selected aspects of ion transport regulation, and its impact on fluid and electrolyte homeostasis. A particular focus will be on emerging evidence for site-specific regulation of the epithelial sodium channel (ENaC) and its role in both Na⁺ and K⁺ homeostasis. In this context, the critical regulatory roles of aldosterone, the mineralocorticoid receptor (MR), and the kinases SGK1 and mTORC2 will be highlighted. This includes a discussion of the newly established concept that local K⁺ concentrations are involved in the reciprocal regulation of Na⁺-Cl^- cotransporter (NCC) and ENaC activity to adjust renal K⁺ secretion to dietary intake.

The emerging role of aldosterone/mineralocorticoid receptors in the pathogenesis of erectile dysfunction

PURPOSE

Aldosterone is an old hormone that has been discovered for more than fifty years. The clinical application of its receptors' inhibitors, especially spirolactone, has benifited patients for decades worldwide. In this review, we briefly summarized the molecular mechanism of aldosterone/mineralocorticoid receptors (Ald-MRs) signaling in cardiovascular diseases and its emerging role in erectile dysfunction.

METHODS

We searched PubMed, Web of Science, and Scopus for manuscripts published prior to December 2017 using key words " aldosterone " AND " erectile dysfunction " OR " cardiovascular disease " OR " mineralocorticoid receptors ". Related literature and clinical perspectives were collated, summarized and discussed in this review.

RESULTS

The increase of reactive oxygen species production, inhibition of endothelial nitric oxide synthase system, and induction of inflammation are ubiquitous in vascular endothelial cells or vascular smooth muscle cells after the activation of Ald-MRs pathway. In addition, in cardiovascular diseases with over-active Ald-MRs signaling, MRs blockade could reverse the injury and improve the prognosis. Notably, multiple studies have correlated aldosterone and MRs to the pathogenesis of erectile function, while the mechanism is largely unperfectly identified.

CONCLUSION

In conclusion, we summarize the current evidence to highlight the potential role of aldosterone in erectile dysfunction and provide critical insights into the treatment of the disease.

Potassium Homeostasis: The Knowns, the Unknowns, and the Health Benefits

Potassium homeostasis has a very high priority because of its importance for membrane potential. Although extracellular K+ is only 2% of total body K+, our physiology was evolutionarily tuned for a high-K+, low-Na+ diet. We review how multiple systems interface to accomplish fine K+ balance and the consequences for health and disease.

Effect of mineralocorticoid treatment in mice with collecting duct-specific knockout of endothelin-1

Aldosterone increases blood pressure (BP) by stimulating sodium (Na) reabsorption within the distal nephron and collecting duct (CD). Aldosterone also stimulates endothelin-1 (ET-1) production that acts within the CD to inhibit Na reabsorption via a negative feedback mechanism. We tested the hypothesis that this renal aldosterone-endothelin feedback system regulates electrolyte balance and BP by comparing the effect of a high-salt (NaCl) diet and mineralocorticoid stimulation in control and CD-specific ET-1 knockout (CD ET-1 KO) mice. Metabolic balance and radiotelemetric BP were measured before and after treatment with desoxycorticosterone pivalate (DOCP) in mice fed a high-salt diet with saline to drink. CD ET-1 KO mice consumed more high-salt diet and saline and had greater urine output than controls. CD ET-1 KO mice exhibited increased BP and greater fluid retention and body weight than controls on a high-salt diet. DOCP with high-salt feeding further increased BP in CD ET-1 KO mice, and by the end of the study the CD ET-1 KO mice were substantially hypernatremic. Unlike controls, CD ET-1 KO mice failed to respond acutely or escape from DOCP treatment. We conclude that local ET-1 production in the CD is required for the appropriate renal response to Na loading and that lack of local ET-1 results in abnormal fluid and electrolyte handling when challenged with a high-salt diet and with DOCP treatment. Additionally, local ET-1 production is necessary, under these experimental conditions, for renal compensation to and escape from the chronic effects of mineralocorticoids.

Collecting duct principal cell transport processes and their regulation

The principal cell of the kidney collecting duct is one of the most highly regulated epithelial cell types in vertebrates. The effects of hormonal, autocrine, and paracrine factors to regulate principal cell transport processes are central to the maintenance of fluid and electrolyte balance in the face of wide variations in food and water intake. In marked contrast with the epithelial cells lining the proximal tubule, the collecting duct is electrically tight, and ion and osmotic gradients can be very high. The central role of principal cells in salt and water transport is reflected by their defining transporters-the epithelial Na(+) channel (ENaC), the renal outer medullary K(+) channel, and the aquaporin 2 (AQP2) water channel. The coordinated regulation of ENaC by aldosterone, and AQP2 by arginine vasopressin (AVP) in principal cells is essential for the control of plasma Na(+) and K(+) concentrations, extracellular fluid volume, and BP. In addition to these essential hormones, additional neuronal, physical, and chemical factors influence Na(+), K(+), and water homeostasis. Notably, a variety of secreted paracrine and autocrine agents such as bradykinin, ATP, endothelin, nitric oxide, and prostaglandin E2 counterbalance and limit the natriferic effects of aldosterone and the water-retaining effects of AVP. Considerable recent progress has improved our understanding of the transporters, receptors, second messengers, and signaling events that mediate principal cell responses to changing environments in health and disease. This review primarily addresses the structure and function of the key transporters and the complex interplay of regulatory factors that modulate principal cell ion and water transport.

Dietary potassium and the renal control of salt balance and blood pressure

Dietary potassium (K(+)) intake has antihypertensive effects, prevents strokes, and improves cardiovascular outcomes. The underlying mechanism for these beneficial effects of high K(+) diets may include vasodilation, enhanced urine flow, reduced renal renin release, and negative sodium (Na(+)) balance. Indeed, several studies demonstrate that dietary K(+) intake induces renal Na(+) loss despite elevated plasma aldosterone. This review briefly highlights the epidemiological and experimental evidences for the effects of dietary K(+) on arterial blood pressure. It discusses the pivotal role of the renal distal tubule for the regulation of urinary K(+) and Na(+) excretion and blood pressure and highlights that it depends on the coordinated interaction of different nephron portions, epithelial cell types, and various ion channels, transporters, and ATPases. Moreover, we discuss the relevance of aldosterone and aldosterone-independent factors in mediating the effects of an altered K(+) intake on renal K(+) and Na(+) handling. Particular focus is given to findings suggesting that an aldosterone-independent downregulation of the thiazide-sensitive NaCl cotransporter significantly contributes to the natriuretic and antihypertensive effect of a K(+)-rich diet. Last but not least, we refer to the complex signaling pathways enabling the kidney to adapt its function to the homeostatic needs in response to an altered K(+) intake. Future work will have to further address the underlying cellular and molecular mechanism and to elucidate, among others, how an altered dietary K(+) intake is sensed and how this signal is transmitted to the different epithelial cells lining the distal tubule.

Acute inhibition of NCC does not activate distal electrogenic Na+ reabsorption or kaliuresis

Na(+) reabsorption from the distal renal tubule involves electroneutral and electrogenic pathways, with the latter promoting K(+) excretion. The relative activities of these two pathways are tightly controlled, participating in the minute-to-minute regulation of systemic K(+) balance. The pathways are interdependent: the activity of the NaCl cotransporter (NCC) in the distal convoluted tubule influences the activity of the epithelial Na(+) channel (ENaC) downstream. This effect might be mediated by changes in distal Na(+) delivery per se or by molecular and structural adaptations in the connecting tubule and collecting ducts. We hypothesized that acute inhibition of NCC activity would cause an immediate increase in Na(+) flux through ENaC, with a concomitant increase in renal K(+) excretion. We tested this using renal clearance methodology in anesthetized mice, by the administration of hydrochlorothiazide (HCTZ) and/or benzamil (BZM) to exert specific blockade of NCC and ENaC, respectively. Bolus HCTZ elicited a natriuresis that was sustained for up to 110 min; urinary K(+) excretion was not affected. Furthermore, the magnitude of the natriuresis was no greater during concomitant BZM administration. This suggests that ENaC-mediated Na(+) reabsorption was not normally limited by Na(+) delivery, accounting for the absence of thiazide-induced kaliuresis. After dietary Na(+) restriction, HCTZ elicited a kaliuresis, but the natiuretic effect of HCTZ was not enhanced by BZM. Our findings support a model in which inhibition of NCC activity does not increase Na(+) reabsorption through ENaC solely by increasing distal Na(+) delivery but rather by inducing a molecular and structural adaptation in downstream nephron segments.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

Purinergic activation of Ca2+-permeable TRPV4 channels is essential for mechano-sensitivity in the aldosterone-sensitive distal nephron

Mechanical forces are known to induce increases of [Ca²⁺]_i in the aldosterone-sensitive distal nephron (ASDN) cells to regulate epithelial transport. At the same time, mechanical stress stimulates ATP release from ASDN cells. In this study, we combined ratiometric Fura-2 based monitoring of [Ca²⁺]_i in freshly isolated split-opened ASDN with targeted deletion of P2Y2 and TRPV4 in mice to probe a role for purinergic signaling in mediating mechano-sensitive responses in ASDN cells. ATP application causes a reproducible transient Ca²⁺ peak followed by a sustained plateau. Individual cells of the cortical collecting duct (CCD) and the connecting tubule (CNT) respond to purinergic stimulation with comparative elevations of [Ca²⁺]_i. Furthermore, ATP-induced Ca²⁺-responses are nearly identical in both principal (AQP2-positive) and intercalated (AQP2-negative) cells as was confirmed using immunohistochemistry in split-opened ASDN. UTP application produces elevations of [Ca²⁺]_i similar to that observed with ATP suggesting a dominant role of P2Y2-like receptors in generation of [Ca²⁺]_i response. Indeed, genetic deletion of P2Y2 receptors decreases the magnitude of ATP-induced and UTP-induced Ca²⁺ responses by more than 70% and 90%, respectively. Both intracellular and extracellular sources of Ca²⁺ appeared to contribute to the generation of ATP-induced Ca²⁺ response in ASDN cells. Importantly, flow- and hypotonic-induced Ca²⁺ elevations are markedly blunted in P2Y2 −/− mice. We further demonstrated that activation of mechano-sensitive TRPV4 channel plays a major role in the sustained [Ca²⁺]_i elevation during purinergic stimulation. Consistent with this, ATP-induced Ca²⁺ plateau are dramatically attenuated in TRV4 −/− mice. Inhibition of TRPC channels with 10 µM BTP2 also decreased ATP-induced Ca²⁺ plateau whilst to a lower degree than that observed with TRPV4 inhibition/genetic deletion. We conclude that stimulation of purinergic signaling by mechanical stimuli leads to activation of TRPV4 and, to a lesser extent, TRPCs channels, and this is an important component of mechano-sensitive response of the ASDN.

Characteristic clinical and biochemical profile of recurrent calcium-oxalate nephrolithiasis in patients with metabolic syndrome

BACKGROUND

Metabolic syndrome is a risk factor for nephrolithiasis. This study was performed to evaluate the clinical and biochemical profile of calcium-oxalate nephrolithiasis in stone formers with metabolic syndrome.

METHODS

A total of 526 recurrent stone formers, 184 of them with metabolic syndrome, and 214 controls were examined on a free diet and after a sodium-restricted diet (sodium intake < 100 mmol/24 h).

RESULTS

On free diet, stone formers with metabolic syndrome showed higher sodium excretion [mean (95% confidence interval), 196 (176-218) vs 160 (150-168) mmol/24 h; P < 0.01] and lower citrate excretion [2.23 (1.99-2.58) vs 2.84 (2.51-3.17) mmol/24 h; P < 0.01] compared to controls, whereas stone formers without metabolic syndrome showed higher calcium and oxalate excretion [5.43 (5.01-5.82) vs 3.58 (2.84-4.19) and 0.34 (0.32-0.36) vs 0.26 (0.20-0.31)m mmol/24 h for calcium and oxalate, respectively; P < 0.01] and lower citrate excretion [2.18 (1.98-2.38) vs 2.84 (2.51-3.17) mmol/24 h; P < 0.01] compared to controls. The ion activity product of urinary calcium-oxalate salts was similar between stone formers with and without metabolic syndrome [1.41 (1.31-1.59) vs 1.40 (1.35-1.45); P > 0.05]. After the test diet, this index was lower in diet-compliant stone formers with metabolic syndrome compared to diet-compliant stone formers without metabolic syndrome [1.15 (1.10-1.21) vs 1.39 (1.31-1.45); P < 0.01].

CONCLUSIONS

The biochemical profiles and responses to the sodium-restricted diet were significantly different between stone formers with metabolic syndrome and those without. Dietary habits play a central role in the pathogenesis of nephrolithiasis in stone formers with metabolic syndrome.

Distal potassium handling based on flow modulation of maxi-K channel activity

PURPOSE OF REVIEW

Studies on the mechanisms of distal K+ secretion have highlighted the importance of the renal outer-medullary K+ (ROMK) and maxi-K channels. This review considers several human disorders characterized by hypokalemia and hyperkalemia, as well as mouse models of these disorders, and the mechanisms by which ROMK and maxi-K may be dysregulated.

RECENT FINDINGS

Analysis of knockout mice lacking ROMK, a model for type II Bartter's syndrome, has shown a role for maxi-K in distal K+ secretion. Knockout mice lacking either the alpha or beta1 subunits of maxi-K also show deficits in flow-dependent K+ secretion. Analysis of transgenic and knock-in mouse models of pseudohypoaldosteronism type II, in which mutant forms of with-no-lysine kinase 4 are expressed, suggests ways in which ROMK and maxi-K may be dysregulated to result in hyperkalemia. Modeling studies also provide insights into the role of Na+ delivery vs. flow in K+ secretion.

SUMMARY

The importance of both ROMK and maxi-K to distal K+ secretion is now well established, but the relative role that each of these two channels plays in normal and diseased states has not been definitively established. Analysis of human and animal model data can generate hypotheses for future experiments.

K+ secretion in the rat kidney: Na+ channel-dependent and -independent mechanisms

Renal Na(+) and K(+) excretion was measured in rats with varying dietary K(+) intake. The requirement for channel-mediated distal nephron Na(+) reabsorption was assessed by infusing the animals with the K(+)-sparing diuretic amiloride via osmotic minipumps. At infusion rates of 2 nmol/min, the concentration of amiloride in the urine was 38 microM, corresponding to concentrations of 9-23 microM in the distal tubular fluid, sufficient to block >98% of Na(+) transport through apical Na(+) channels (ENaC). With a control K(+) intake (0.6% KCl), amiloride reduced K(+) excretion rates (U(K)V) from 0.85 +/- 0.15 to 0.05 +/- 0.01 micromol/min during the first 2 h of infusion, suggesting that distal nephron K(+) secretion was completely dependent on the activity of Na(+) channels. When K(+) intake was increased by feeding overnight with a diet containing 10% KCl, amiloride reduced U(K)V from 7.5 +/- 0.7 to 1.3 +/- 0.1 micromol/min despite an increased plasma K(+) of 9 mM, again suggesting a major but not exclusive role for the Na(+) channel-dependent pathway of K(+) secretion. The maximal measured rates of amiloride-sensitive K(+) excretion correspond well with estimates based on apical K(+) channel activity in distal nephron segments. However, when the animals were adapted to the high-K(+) diet for 7-9 days, the diuretic decreased U(K)V less, from 6.1 +/- 0.6 to 3.0 +/- 0.8 micromol/min, indicating an increasing fraction of K(+) excretion that was independent of Na(+) channels. This indicates the upregulation of a Na(+) channel-independent mechanism for secreting K(+).

Placental oxidative stress in malnourished rats and changes in kidney proximal tubule sodium ATPases in offspring

1. Intrauterine malnutrition has been linked to the development of adult cardiovascular and renal diseases, which are related to altered Na(+) balance. Here we investigated whether maternal malnutrition increases placental oxidative stress with subsequent impact on renal ATP-dependent Na(+) transporters in the offspring. 2. Maternal malnutrition was induced in rats during pregnancy by using a basic regional diet available in north-eastern Brazil. Placental oxidative stress was evaluated by measuring thiobarbituric acid-reactive substances, which were 35-40% higher in malnourished dams (MalN). Na(+) pumps were evaluated in control and prenatally malnourished rats (at 25 and 90 days of age). 3. Identical Na(+)/K(+)-ATPase activity was found in both groups at 25 days (approximately 150 nmol P(i)/mg per min). However, although Na(+)/K(+)-ATPase increased by 40% with growth in control rats, it remained constant in pups from MalN. 4. In juvenile rats, the activity of the ouabain-insensitive Na(+)-ATPase was higher in MalN than in controls (70 vs 25 nmol P(i)/mg per min). Nevertheless, activity did not increase with kidney and body growth: at 90 days, it was 50% lower in MalN than in controls. The maximal stimulation of the Na(+)-ATPase by angiotensin (Ang) II was 35% lower in MalN than in control rats and was attained only with a much higher concentration of the peptide (10(-10) mol/L) than in controls (10(-14) mol/L). 5. Protein kinase C activity, which mediates the effects of AngII on Na(+)-ATPase was only one-third of normal values in the MalN group. 6. These results indicate that placental oxidative stress may contribute to fetal undernutrition, which leads to later disturbances in Na(+) pumps from proximal tubule cells.

Regulated sodium transport in the renal connecting tubule (CNT) via the epithelial sodium channel (ENaC)

The aldosterone-sensitive distal nephron (ASDN) includes the late distal convoluted tubule 2, the connecting tubule (CNT) and the collecting duct. The appropriate regulation of sodium (Na(+)) absorption in the ASDN is essential to precisely match urinary Na(+) excretion to dietary Na(+) intake whilst taking extra-renal Na(+) losses into account. There is increasing evidence that Na(+) transport in the CNT is of particular importance for the maintenance of body Na(+) balance and for the long-term control of extra-cellular fluid volume and arterial blood pressure. Na(+) transport in the CNT critically depends on the activity and abundance of the amiloride-sensitive epithelial sodium channel (ENaC) in the luminal membrane of the CNT cells. As a rate-limiting step for transepithelial Na(+) transport, ENaC is the main target of hormones (e.g. aldosterone, angiotensin II, vasopressin and insulin/insulin-like growth factor 1) to adjust transepithelial Na(+) transport in this tubular segment. In this review, we highlight the structural and functional properties of the CNT that contribute to the high Na(+) transport capacity of this segment. Moreover, we discuss some aspects of the complex pathways and molecular mechanisms involved in ENaC regulation by hormones, kinases, proteases and associated proteins that control its function. Whilst cultured cells and heterologous expression systems have greatly advanced our knowledge about some of these regulatory mechanisms, future studies will have to determine the relative importance of the various pathways in the native tubule and in particular in the CNT.

Fluorescence isolation of mouse late distal convoluted tubules and connecting tubules: effects of vasopressin and vitamin D3 on Ca2+ signaling

The renal late distal convoluted tubules and connecting tubules are sites for the fine regulation of Na+ and Ca2+ reabsorption. The role of these segments in Na+ and K+ homeostasis is possibly underestimated, as the tubules are technically difficult to isolate in sufficient numbers and purity for functional analysis. To overcome these difficulties, we have developed a transgenic mouse model expressing enhanced green fluorescent protein in late distal convoluted tubules and connecting tubules. Enhanced green fluorescent protein expression was driven by the promoter for the transient receptor potential subfamily V, member 5. Confocal fluorescence microscopy allowed detection of enhanced green fluorescent protein in living, isolated late distal convoluted tubules and connecting tubules and in the initial cortical collecting ducts. Enhanced green fluorescent protein expression was validated by double- and triple-fluorescence immunolabeling with specific tubule markers. Freshly isolated late distal convoluted tubules and connecting tubules increased their intracellular Ca2+ levels in response to the V2 receptor-specific agonist deamino-Cys,d-Arg8-vasopressin (2 × 10−10 M) after 1 min of superfusion. In addition, both late distal convoluted tubules and connecting tubules displayed a concentration-dependent intracellular Ca2+ response to 1α,25-dihydroxyvitamin D3 (range 10−10 to 10−8 M). This suggests that 1α,25-dihydroxyvitamin D3 can act through a nongenomic signaling pathway in these tubules. In conclusion, the transgenic mouse model, expressing enhanced green fluorescent protein, is suitable for rapid isolation of viable late distal convoluted tubules, connecting tubules, and initial cortical collecting ducts and provides an ideal tool for a more exhaustive functional characterization of these segments.

Dietary K regulates ROMK channels in connecting tubule and cortical collecting duct of rat kidney

The activity of ROMK channels in rat kidney tubule cells was assessed as tertiapin-Q (TPNQ)-sensitive current under whole cell clamp conditions. With an external K(+) concentration of 5 mM and an internal K(+) concentration of 140 mM and the membrane potential clamped to 0 mV, TPNQ blocked outward currents in principal cells of the cortical collecting duct (CCD) outer medullary collecting duct and connecting tubule (CNT). The apparent K(i) was 5.0 nM, consistent with its interaction with ROMK. The TPNQ-sensitive current reversed at voltages close to the equilibrium potential for K(+). The currents were reduced when the pipette solution contained ATP. In the CCD, the average TPNQ-sensitive outward current (I(SK)) was 476 +/- 48 pA/cell in control animals on a 1% KCl diet. I(SK) increased to 1,255 +/- 140 pA when animals were maintained on a high-K (10% KCl) diet for 7 days and decreased to 314 +/- 46 pA after 7 days on a low-K (0.1% KCl) diet. In the CNT, I(SK) was 360 +/- 30 pA on control, 1,160 +/- 110 on high-K, and 166 +/- 16 pA on low-K diets. The results indicate that ROMK channel activity is highly regulated by dietary K in both the CCD and the CNT.