Adenylate cyclase responsiveness to hormones in various portions of the human nephron

Dimensions of muddy brown granular casts in patients with acute tubular injury

BACKGROUND

The presence of "muddy" brown granular casts (MBGC) in the urine sediment is pathognomonic for acute tubular injury (ATI). Although MBGC have been noted for years, there are no reports regarding their length nor width. The objective of this study was to measure MBGC using images obtained by light microscopy and investigate associations with clinically relevant parameters.

METHODS

Patients with diagnosis of ATI as evidenced by visualization of abundant MBGC (>30% low power fields) were sampled. Bright-field images were measured using ImageJ. Twenty-five patients were included: 44% women; median age 64 yrs; 52% white, 36% black. Mean MBGC width (n = 350) was 34.4 ± 13.1 µm (range: 9 to 110 µm).

RESULTS

Mean MBGC length was 98.7 ± 42.7 µm (range: 33 to 317 µm). Based on a previous report of cortical tubular diameters, MBGC width corresponded well with the median reported range. MBGC width was positively correlated with patient height (ρ=0.41, p=0.04), and length was positively correlated with fractional excretion of sodium (ρ=0.57. p=0.02) and urine chloride concentration (ρ=0.90, p=0.001). Mean MBGC length was negatively correlated with age (ρ=-0.47, p=0.02) and urine phosphate concentration (ρ=-0.72, p=0.03). There were no differences between cases that required renal replacement therapy (RRT, n =10) and those that did not require RRT (n=15).

CONCLUSION

This is the first study reporting dimensions of MBGC from cases with ATI. Clinical implications of these observations require further study.

Cyclic Adenosine Monophosphate Signaling in Chronic Kidney Disease: Molecular Targets and Therapeutic Potentials

Chronic kidney disease (CKD) is characterized by a steady decline in kidney function and affects roughly 10% of the world's population. This review focuses on the critical function of cyclic adenosine monophosphate (cAMP) signaling in CKD, specifically how it influences both protective and pathogenic processes in the kidney. cAMP, a critical secondary messenger, controls a variety of cellular functions, including transcription, metabolism, mitochondrial homeostasis, cell proliferation, and apoptosis. Its compartmentalization inside cellular microdomains ensures accurate signaling. In kidney physiology, cAMP is required for hormone-regulated activities, particularly in the collecting duct, where it promotes water reabsorption through vasopressin signaling. Several illnesses, including Fabry disease, renal cell carcinoma, nephrogenic diabetes insipidus, Bartter syndrome, Liddle syndrome, diabetic nephropathy, autosomal dominant polycystic kidney disease, and renal tubular acidosis, have been linked to dysfunction in the cAMP system. Both cAMP analogs and phosphodiesterase inhibitors have the potential to improve kidney function and reduce kidney damage. Future research should focus on developing targeted PDE inhibitors for the treatment of CKD.

The importance of kidney calcium handling in the homeostasis of extracellular fluid calcium

Extracellular fluid calcium concentration must be maintained within a narrow range in order to sustain many biological functions, encompassing muscle contraction, blood coagulation, and bone and tooth mineralization. Blood calcium value is critically dependent on the ability of the renal tubule to reabsorb the adequate amount of filtered calcium. Tubular calcium reabsorption is carried out by various and complex mechanisms in 3 distinct segments: the proximal tubule, the cortical thick ascending limb of the loop of Henle, and the late distal convoluted/connecting tubule. In addition, calcium reabsorption is tightly controlled by many endocrine, paracrine, and autocrine factors, as well as by non-hormonal factors, in order to adapt the tubular handling of calcium to the metabolic requirements. The present review summarizes the current knowledge of the mechanisms and factors involved in calcium handling by the kidney and, ultimately, in extracellular calcium homeostasis. The review also highlights some of our gaps in understanding that need to be addressed in the future.

Medullary and cortical thick ascending limb: similarities and differences

The thick ascending limb of the loop of Henle (TAL) is the first segment of the distal nephron, extending through the whole outer medulla and cortex, two regions with different composition of the peritubular environment. The TAL plays a critical role in the control of NaCl, water, acid, and divalent cation homeostasis, as illustrated by the consequences of the various monogenic diseases that affect the TAL. It delivers tubular fluid to the distal convoluted tubule and thereby affects the function of the downstream tubular segments. The TAL is commonly considered as a whole. However, many structural and functional differences exist between its medullary and cortical parts. The present review summarizes the available data regarding the similarities and differences between the medullary and cortical parts of the TAL. Both subsegments reabsorb NaCl and have high Na+-K+-ATPase activity and negligible water permeability; however, they express distinct isoforms of the Na+-K+-2Cl−cotransporter at the apical membrane. Ammonia and bicarbonate are mostly reabsorbed in the medullary TAL, whereas Ca2+and Mg2+are mostly reabsorbed in the cortical TAL. The peptidic hormone receptors controlling transport in the TAL are not homogeneously expressed along the cortical and medullary TAL. Besides this axial heterogeneity, structural and functional differences are also apparent between species, which underscores the link between properties and role of the TAL under various environments.

Kidney Is Essential for Blood Pressure Modulation by Dietary Potassium

Eating more potassium may reduce blood pressure and the occurrence of other cardiovascular diseases by actions on various systems, including the vasculature, the sympathetic nervous system, systemic metabolism, and body fluid volume. Among these, the kidney plays a major role in the potassium-rich diet-mediated blood pressure reduction. PURPOSE OF REVIEW: To provide an overview of recent discoveries about the mechanisms by which a potassium-rich diet leads to natriuresis. RECENT FINDINGS: Although the distal convoluted tubule (DCT) is a short part of the nephron that reabsorbs salt, via the sodium-chloride cotransporter (NCC), it is highly sensitive to changes in plasma potassium concentration. Activation or inhibition of NCC raises or lowers blood pressure. Recent work suggests that extracellular potassium concentration is sensed by the DCT via intracellular chloride concentration which regulates WNK kinases in the DCT. High-potassium diet targets NCC in the DCT, resulting in natriuresis and fluid volume reduction, which are protective from hypertension and other cardiovascular problems.

Regulation of the Renal NaCl Cotransporter and Its Role in Potassium Homeostasis

Daily dietary potassium (K+) intake may be as large as the extracellular K+ pool. To avoid acute hyperkalemia, rapid removal of K+ from the extracellular space is essential. This is achieved by translocating K+ into cells and increasing urinary K+ excretion. Emerging data now indicate that the renal thiazide-sensitive NaCl cotransporter (NCC) is critically involved in this homeostatic kaliuretic response. This suggests that the early distal convoluted tubule (DCT) is a K+ sensor that can modify sodium (Na+) delivery to downstream segments to promote or limit K+ secretion. K+ sensing is mediated by the basolateral K+ channels Kir4.1/5.1, a capacity that the DCT likely shares with other nephron segments. Thus, next to K+-induced aldosterone secretion, K+ sensing by renal epithelial cells represents a second feedback mechanism to control K+ balance. NCC’s role in K+ homeostasis has both physiological and pathophysiological implications. During hypovolemia, NCC activation by the renin-angiotensin system stimulates Na+ reabsorption while preventing K+ secretion. Conversely, NCC inactivation by high dietary K+ intake maximizes kaliuresis and limits Na+ retention, despite high aldosterone levels. NCC activation by a low-K+ diet contributes to salt-sensitive hypertension. K+-induced natriuresis through NCC offers a novel explanation for the antihypertensive effects of a high-K+ diet. A possible role for K+ in chronic kidney disease is also emerging, as epidemiological data reveal associations between higher urinary K+ excretion and improved renal outcomes. This comprehensive review will embed these novel insights on NCC regulation into existing concepts of K+ homeostasis in health and disease.

Protein Phosphatase 1 Inhibitor-1 Mediates the cAMP-Dependent Stimulation of the Renal NaCl Cotransporter

BACKGROUND

A number of cAMP-elevating hormones stimulate phosphorylation (and hence activity) of the NaCl cotransporter (NCC) in the distal convoluted tubule (DCT). Evidence suggests that protein phosphatase 1 (PP1) and other protein phosphatases modulate NCC phosphorylation, but little is known about PP1's role and the mechanism regulating its function in the DCT.

METHODS

We used ex vivo mouse kidney preparations to test whether a DCT-enriched inhibitor of PP1, protein phosphatase 1 inhibitor-1 (I1), mediates cAMP's effects on NCC, and conducted yeast two-hybrid and coimmunoprecipitation experiments in NCC-expressing MDCK cells to explore protein interactions.

RESULTS

Treating isolated DCTs with forskolin and IBMX increased NCC phosphorylation via a protein kinase A (PKA)-dependent pathway. Ex vivo incubation of mouse kidney slices with isoproterenol, norepinephrine, and parathyroid hormone similarly increased NCC phosphorylation. The cAMP-induced stimulation of NCC phosphorylation strongly correlated with the phosphorylation of I1 at its PKA consensus phosphorylation site (a threonine residue in position 35). We also found an interaction between NCC and the I1-target PP1. Moreover, PP1 dephosphorylated NCC in vitro, and the PP1 inhibitor calyculin A increased NCC phosphorylation. Studies in kidney slices and isolated perfused kidneys of control and I1-KO mice demonstrated that I1 participates in the cAMP-induced stimulation of NCC.

CONCLUSIONS

Our data suggest a complete signal transduction pathway by which cAMP increases NCC phosphorylation via a PKA-dependent phosphorylation of I1 and subsequent inhibition of PP1. This pathway might be relevant for the physiologic regulation of renal sodium handling by cAMP-elevating hormones, and may contribute to salt-sensitive hypertension in patients with endocrine disorders or sympathetic hyperactivity.

Role of the alternative splice variant of NCC in blood pressure control

The renal thiazide-sensitive sodium-chloride cotransporter (NCC), located in the distal convoluted tubule (DCT) of the kidney, plays an important role in blood pressure regulation by fine-tuning sodium excretion. The human SLC12A3 gene, encoding NCC, gives rise to three isoforms, of which only the third isoform (NCC₃) has been extensively investigated so far. However, recent studies unraveled the importance of the isoforms 1 and 2, collectively referred to as NCC splice variant (NCC_SV), in several (patho)physiological conditions. In the human kidney, NCC_SV localizes to the apical membrane of the DCT and could constitute a functional route for renal sodium-chloride reabsorption. Analysis of urinary extracellular vesicles (uEVs), a non-invasive method for measuring renal responses, demonstrated that NCC_SV abundance changes in response to acute water loading and correlates with patients’ thiazide responsiveness. Furthermore, a novel phosphorylation site at serine 811 (S811), exclusively present in NCC_SV, was shown to play an instrumental role in NCC_SV as well as NCC₃ function. This review aims to summarize these new insights of NCC_SV function in humans that broadens the understanding on NCC regulation in blood pressure control.

Axial and cellular heterogeneity in electrolyte transport pathways along the thick ascending limb

The thick ascending limb (TAL) extends from the border of the inner medulla to the renal cortex, thus ascending through regions with wide differences in tissue solute and electrolyte concentrations. Structural and functional differences between TAL cells in the medulla (mTAL) and the cortex (cTAL) would therefore be useful to adapt TAL transport function to a changing external fluid composition. While mechanisms common to all TAL cells play a central role in the reclamation of about 25% of the NaCl filtered by the kidney, morphological features, Na⁺ / K⁺ -ATPase activity, NKCC2 splicing and phosphorylation do vary between segments and cells. The TAL contributes to K⁺ homeostasis and TAL cells with high or low basolateral K⁺ conductances have been identified which may be involved in K⁺ reabsorption and secretion respectively. Although transport rates for HCO3- do not differ between mTAL and cTAL, divergent axial and cellular expression of H⁺ transport proteins in TAL have been documented. The reabsorption of the divalent cations Ca²⁺ and Mg²⁺ is highest in cTAL and paralleled by differences in divalent cation permeability and the expression of select claudins. Morphologically, two cell types with different cell surface phenotypes have been described that still need to be linked to specific functional characteristics. The unique external environment and its change along the longitudinal axis require an axial functional heterogeneity for the TAL to optimally participate in conserving electrolyte homeostasis. Despite substantial progress in understanding TAL function, there are still considerable knowledge gaps that are just beginning to become bridged.

Heterotrimeric G proteins in the control of parathyroid hormone actions

Parathyroid hormone (PTH) is a key regulator of skeletal physiology and calcium and phosphate homeostasis. It acts on bone and kidney to stimulate bone turnover, increase the circulating levels of 1,25 dihydroxyvitamin D and calcium and inhibit the reabsorption of phosphate from the glomerular filtrate. Dysregulated PTH actions contribute to or are the cause of several endocrine disorders. This calciotropic hormone exerts its actions via binding to the PTH/PTH-related peptide receptor (PTH1R), which couples to multiple heterotrimeric G proteins, including G_s and G_q/11 Genetic mutations affecting the activity or expression of the alpha-subunit of G_s, encoded by the GNAS complex locus, are responsible for several human diseases for which the clinical findings result, at least partly, from aberrant PTH signaling. Here, we review the bone and renal actions of PTH with respect to the different signaling pathways downstream of these G proteins, as well as the disorders caused by GNAS mutations.

Demonstration of the functional impact of vasopressin signaling in the thick ascending limb by a targeted transgenic rat approach

The antidiuretic hormone vasopressin (AVP) regulates renal salt and water reabsorption along the distal nephron and collecting duct system. These effects are mediated by vasopressin 2 receptors (V2R) and release of intracellular Gs-mediated cAMP to activate epithelial transport proteins. Inactivating mutations in the V2R gene lead to the X-linked form of nephrogenic diabetes insipidus (NDI), which has chiefly been related with impaired aquaporin 2-mediated water reabsorption in the collecting ducts. Previous work also suggested the AVP-V2R-mediated activation of Na(+)-K(+)-2Cl(-)-cotransporters (NKCC2) along the thick ascending limb (TAL) in the context of urine concentration, but its individual contribution to NDI or, more generally, to overall renal function was unclear. We hypothesized that V2R-mediated effects in TAL essentially determine its reabsorptive function. To test this, we reevaluated V2R expression. Basolateral membranes of medullary and cortical TAL were clearly stained, whereas cells of the macula densa were unreactive. A dominant-negative, NDI-causing truncated V2R mutant (Ni3-Glu242stop) was then introduced into the rat genome under control of the Tamm-Horsfall protein promoter to cause a tissue-specific AVP-signaling defect exclusively in TAL. Resulting Ni3-V2R transgenic rats revealed decreased basolateral but increased intracellular V2R signal in TAL epithelia, suggesting impaired trafficking of the receptor. Rats displayed significant baseline polyuria, failure to concentrate the urine in response to water deprivation, and hypercalciuria. NKCC2 abundance, phosphorylation, and surface expression were markedly decreased. In summary, these data indicate that suppression of AVP-V2R signaling in TAL causes major impairment in renal fluid and electrolyte handling. Our results may have clinical implications.

Alternative splice variant of the thiazide-sensitive NaCl cotransporter: a novel player in renal salt handling

The thiazide-sensitive NaCl cotransporter (NCC) is an important pharmacological target in the treatment of hypertension. The human SLC12A3 gene, encoding NCC, gives rise to three isoforms. Only the third isoform has been extensively investigated. The aim of the present study was, therefore, to establish the abundance and localization of the almost identical isoforms 1 and 2 (NCC1/2) in the human kidney and to determine their functional properties and regulation in physiological conditions. Immunohistochemical analysis of NCC1/2 in the human kidney revealed that NCC1/2 localizes to the apical plasma membrane of the distal convoluted tubule. Importantly, NCC1/2 mRNA constitutes ∼44% of all NCC isoforms in the human kidney. Functional analysis performed in the Xenopus laevis oocyte revealed that thiazide-sensitive 22Na+ transport of NCC1 was significantly increased compared with NCC3. Mimicking a constitutively active phosphorylation site at residue 811 (S811D) in NCC1 further augmented Na+ transport, while a nonphosphorylatable variant (S811A) of NCC1 prevented this enhanced response. Analysis of human urinary exosomes demonstrated that water loading in human subjects significantly reduces the abundance of NCC1/2 in urinary exosomes. The present study highlights that previously underrepresented NCC1/2 is a fully functional thiazide-sensitive NaCl-transporting protein. Being significantly expressed in the kidney, it may constitute a unique route of renal NaCl reabsorption and could, therefore, play an important role in blood pressure regulation.

Vasopressin regulation of sodium transport in the distal nephron and collecting duct

Arginine vasopressin (AVP) is released from the posterior pituitary gland during states of hyperosmolality or hypovolemia. AVP is a peptide hormone, with antidiuretic and antinatriuretic properties. It allows the kidneys to increase body water retention predominantly by increasing the cell surface expression of aquaporin water channels in the collecting duct alongside increasing the osmotic driving forces for water reabsorption. The antinatriuretic effects of AVP are mediated by the regulation of sodium transport throughout the distal nephron, from the thick ascending limb through to the collecting duct, which in turn partially facilitates osmotic movement of water. In this review, we will discuss the regulatory role of AVP in sodium transport and summarize the effects of AVP on various molecular targets, including the sodium-potassium-chloride cotransporter NKCC2, the thiazide-sensitive sodium-chloride cotransporter NCC, and the epithelial sodium channel ENaC.

Physiology and pathophysiology of the renal Na-K-2Cl cotransporter (NKCC2)

The Na-K-2Cl cotransporter (NKCC2; BSC1) is located in the apical membrane of the epithelial cells of the thick ascending limb of the loop of Henle (TAL). NKCC2 facilitates ∼20–25% of the reuptake of the total filtered NaCl load. NKCC2 is therefore one of the transport proteins with the highest overall reabsorptive capacity in the kidney. Consequently, even subtle changes in NKCC2 transport activity considerably alter the renal reabsorptive capacity for NaCl and eventually lead to perturbations of the salt and water homoeostasis. In addition to facilitating the bulk reabsorption of NaCl in the TAL, NKCC2 transport activity in the macula densa cells of the TAL constitutes the initial step of the tubular-vascular communication within the juxtaglomerular apparatus (JGA); this communications allows the TAL to modulate the preglomerular resistance of the afferent arteriole and the renin secretion from the granular cells of the JGA. This review provides an overview of our current knowledge with respect to the general functions of NKCC2, the modulation of its transport activity by different regulatory mechanisms, and new developments in the pathophysiology of NKCC2-dependent renal NaCl transport.

Distal convoluted tubule

The distal convoluted tubule is the nephron segment that lies immediately downstream of the macula densa. Although short in length, the distal convoluted tubule plays a critical role in sodium, potassium, and divalent cation homeostasis. Recent genetic and physiologic studies have greatly expanded our understanding of how the distal convoluted tubule regulates these processes at the molecular level. This article provides an update on the distal convoluted tubule, highlighting concepts and pathophysiology relevant to clinical practice.

Regulation of nephron water and electrolyte transport by adenylyl cyclases

Adenylyl cyclases (AC) catalyze formation of cAMP, a critical component of G protein-coupled receptor signaling. So far, nine distinct membrane-bound AC isoforms (AC1-9) and one soluble AC (sAC) have been identified and, except for AC8, all of them are expressed in the kidney. While the role of ACs in renal cAMP formation is well established, we are just beginning to understand the function of individual AC isoforms, particularly with regard to hormonal regulation of transporter and channel phosphorylation, membrane abundance, and trafficking. This review focuses on the role of different AC isoforms in regulating renal water and electrolyte transport in health as well as potential pathological implications of disordered AC isoform function. In particular, we focus on modulation of transporter and channel abundance, activity, and phosphorylation, with an emphasis on studies employing genetically modified animals. As will be described, it is now evident that specific AC isoforms can exert unique effects in the kidney that may have important implications in our understanding of normal physiology as well as disease pathogenesis.

Bioengineered 3D human kidney tissue, a platform for the determination of nephrotoxicity

The staggering cost of bringing a drug to market coupled with the extremely high failure rate of prospective compounds in early phase clinical trials due to unexpected human toxicity makes it imperative that more relevant human models be developed to better predict drug toxicity. Drug–induced nephrotoxicity remains especially difficult to predict in both pre-clinical and clinical settings and is often undetected until patient hospitalization. Current pre-clinical methods of determining renal toxicity include 2D cell cultures and animal models, both of which are incapable of fully recapitulating the in vivo human response to drugs, contributing to the high failure rate upon clinical trials. We have bioengineered a 3D kidney tissue model using immortalized human renal cortical epithelial cells with kidney functions similar to that found in vivo. These 3D tissues were compared to 2D cells in terms of both acute (3 days) and chronic (2 weeks) toxicity induced by Cisplatin, Gentamicin, and Doxorubicin using both traditional LDH secretion and the pre-clinical biomarkers Kim-1 and NGAL as assessments of toxicity. The 3D tissues were more sensitive to drug-induced toxicity and, unlike the 2D cells, were capable of being used to monitor chronic toxicity due to repeat dosing. The inclusion of this tissue model in drug testing prior to the initiation of phase I clinical trials would allow for better prediction of the nephrotoxic effects of new drugs.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Anti-edematous effects of tolvaptan in experimental rodent models

PURPOSE

The aim of this study was to evaluate the therapeutic efficacy of tolvaptan, a vasopressin V(2) receptor antagonist, on edema in two rat models: 1) histamine-induced vascular hyperpermeability of the dorsal skin and 2) carrageenan-induced paw edema.

METHODS

In the skin vascular hyperpermeability model, 3 h after oral administration of tolvaptan or the natriuretic agent furosemide, rats were intravenously injected with Evans Blue (EB), followed by intradermal injection of 10 μg of histamine into the dorsal skin. One hour later, blood was collected to measure serum parameters. EB leakage area into the dorsal skin was also measured. Urine was collected for 4 h to determine urine parameters. In the paw edema model, edema was induced by injecting 1% w/v carrageenan into the right hind paw. Paw volume was measured hourly for 5 h. Tolvaptan or furosemide was orally administered 1 h before carrageenan injection.

RESULTS

A single oral dose of tolvaptan (1-10 mg/kg) elicited marked and dose-dependent aquaresis, and improvements in edema. Similar effects were observed with furosemide (30 mg/kg). Tolvaptan tended to elevate the serum sodium level while furosemide caused a significant decrease.

CONCLUSION

Tolvaptan had anti-edematous effects in two different rat models. By increasing free water excretion, tolvaptan may be more advantageous for certain patients than loop diuretics because it does not cause electrolyte loss, and may prevent electrolyte abnormities, such as hyponatremia. These results suggest that tolvaptan has potential clinical benefits for the treatment of edema.

Antinatriuretic effect of vasopressin in humans is amiloride sensitive, thus ENaC dependent

BACKGROUND AND OBJECTIVES

Acute infusion of the potent V2 receptor agonist 1-desamino-8-d-arginine vasopressin (dDAVP) reduces sodium excretion in humans, through an effect attributed to the stimulation of the amiloride sensitive epithelial sodium channel, ENaC, in ex vivo/in vivo experiments. We investigated in humans whether the antinatriuretic effect of dDAVP is sensitive to amiloride, a specific blocker of ENaC.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Forty-eight healthy normotensive adult men were assigned to a high Na/low K (250/40 mmol/d) diet, to suppress aldosterone secretion. dDAVP (4-μg intravenous bolus followed by 4 μg over 2 hours) was administrated before and after a 7-day administration of 20 mg/d amiloride. Urine and blood samples were collected before and at the end of the dDAVP infusion, to measure Na, K, creatinine, and osmolality concentrations.

RESULTS

dDAVP alone decreased the urinary flow rate by 75% and the sodium excretion rate by 19% despite an increase in creatinine clearance by 38 ml/min. Potassium excretion rate was unchanged and the urinary Na/K ratio decreased by 18%. Seven-day amiloride administration had no effect on the dDAVP-induced decrease in the urinary flow rate (-71%) nor on the dDAVP-induced increase in creatinine clearance (+35 ml/min), but it fully prevented the dDAVP-induced decrease in both urinary sodium excretion (+1%) and urinary Na/K ratio (+21%).

CONCLUSIONS

The antinatriuretic effect of dDAVP in humans is amiloride sensitive, and thus is related to the stimulatory effect on ENaC-mediated sodium reabsorption. This test provides a new tool to investigate ENaC function in a clinical setting.

Calcitonin has a vasopressin-like effect on aquaporin-2 trafficking and urinary concentration

The most common cause of hereditary nephrogenic diabetes insipidus is a nonfunctional vasopressin (VP) receptor type 2 (V2R). Calcitonin, another ligand of G-protein-coupled receptors, has a VP-like effect on electrolytes and water reabsorption, suggesting that it may affect AQP2 trafficking. Here, calcitonin increased intracellular cAMP and stimulated the membrane accumulation of AQP2 in LLC-PK1 cells. Pharmacologic inhibition of protein kinase A (PKA) and deficiency of a critical PKA phosphorylation site on AQP2 both prevented calcitonin-induced membrane accumulation of AQP2. Fluorescence assays showed that calcitonin led to a 70% increase in exocytosis and a 20% decrease in endocytosis of AQP2. Immunostaining of rat kidney slices demonstrated that calcitonin induced a significant redistribution of AQP2 to the apical membrane of principal cells in cortical collecting ducts and connecting segments but not in the inner stripe or inner medulla. Calcitonin-treated VP-deficient Brattleboro rats had a reduced urine flow and two-fold higher urine osmolality during the first 12 hours of treatment compared with control groups. Although this VP-like effect of calcitonin diminished over the following 72 hours, the tachyphylaxis was reversible. Taken together, these data show that calcitonin induces cAMP-dependent AQP2 trafficking in cortical collecting and connecting tubules in parallel with an increase in urine concentration. This suggests that calcitonin has a potential therapeutic use in nephrogenic diabetes insipidus.

Calcitonin-stimulated renal Ca2+ reabsorption occurs independently of TRPV5

BACKGROUND

Calcitonin (CT) is known to affect renal Ca(2+) handling. However, it remains unclear how CT affects Ca(2+) transport in the distal convolutions. The aim of this study was to investigate the contribution of the renal epithelial Ca(2+) channel, transient receptor potential vanilloid 5 (TRPV5), to renal Ca(2+) handling in response to CT.

METHODS

C57BL/6 mice received a single overnight (16 hr) injection of CT. In addition, TRPV5 knockout (TRPV5(-/-)) mice and their wild type (TRPV5(+/+)) controls, received three bolus injections of CT over a 40 hr study period. All experimental groups were placed in metabolic cages.

RESULTS

C57BL/6 mice received a single bolus injection of CT, which significantly reduced the urinary Ca(2+) excretion. In addition, urinary Na(+) and K(+) excretion also decreased after CT administration. No apparent changes in renal expression of TRPV5, calbindin-D(28K) (CaBP28K) or TRPV6 could be detected between CT- and vehicle-treated mice. To evaluate whether TRPV5 activity is needed for the CT-induced increase in Ca(2+) reabsorption, mice with genetic ablation of TRPV5 (TRPV5(-/-)) were employed. TRPV5(-/-) mice as well as their wild-type (TRPV5(+/+)) controls received three bolus injections of CT over a 40-hr study period. Overnight (16 hrs) as well as the subsequent 24-hr urine was collected. Overnight urinary Ca(2+) excretion was reduced in both TRPV5(-/-) and TRPV5(+/+) mice after a bolus injection of CT. The subsequent 24-hr urinary excretion of Ca(2+) which was collected after the third bolus injection showed no effect of CT on renal Ca(2+) handling in either mice group. Accordingly, CT did not alter the intrarenal protein abundance of TRPV5 and CaBP28K after three bolus injections of CT.

CONCLUSION

CT augments the renal reabsorptive capacity for Ca(2+). This increase is likely to occur independently of TRPV5.

Axial heterogeneity of vasopressin-receptor subtypes along the human and mouse collecting duct

Vasopressin and vasopressin antagonists are finding expanded use in mouse models of disease and in clinical medicine. To provide further insight into the physiological role of V1a and V2 vasopressin receptors in the human and mouse kidney, intrarenal localization of the receptors mRNA was determined by in situ hybridization. V2-receptor mRNA was predominantly expressed in the medulla, whereas mRNA for V1a receptors predominated in the cortex. The segmental localization of vasopressin-receptor mRNAs was determined using simultaneous in situ hybridization and immunohistochemistry for segment-specific markers, including aquaporin-2, Dolichos biflorus agglutinin, epithelial Na channels, Tamm Horsfall glycoprotein, and thiazide-sensitive Na+-Cl−cotransporter. Notably, V1a receptor expression was exclusively expressed in V-ATPase/anion exchanger-1-labeled alpha-intercalated cells of the medullary collecting duct in both mouse and human kidney. In cortical collecting ducts, V1a mRNA was more widespread and detected in both principal and intercalated cells. V2-receptor mRNA is diffusely expressed along the collecting ducts in both mouse and human kidney, with higher expression levels in the medulla. These results demonstrate heterogenous axial expression of both V1a and V2 vasopressin receptors along the human and mouse collecting duct. The restricted expression of V1a-receptor mRNA in intercalated cells suggests a role for this receptor in acid-base balance. These findings further suggest distinct regulation of renal transport function by AVP through V1a and V2 receptors in the cortex vs. the medulla.

Vasopressin-V2 receptor stimulation reduces sodium excretion in healthy humans

In addition to its effect on water permeability, vasopressin, through its V2 receptors (AVPR2), stimulates Na reabsorption in the collecting duct by increasing the activity of the amiloride-sensitive sodium channel ENaC. This study evaluated whether dDAVP (a potent AVPR2 agonist) reduces sodium excretion in healthy humans (n = 6) and in patients with central (C; n = 2) or nephrogenic (N) diabetes insipidus (DI) as a result of mutations of either the aquaporin 2 gene (AQP2; n = 3) or AVPR2 (n = 10). dDAVP was infused intravenously (0.3 microg/kg body wt in 20 min), and urine was collected for 60 min before (basal) and 150 min after the infusion. dDAVP markedly reduced both urine flow rate and sodium excretion in healthy individuals. A reduction in sodium excretion was also observed in CDI and NDI-AQP2 patients but not in NDI-AVPR2 patients. The magnitude of the fall in sodium excretion correlated with the rise in urine osmolality and the fall in urine output but not with the simultaneously observed fall in mean BP. These results suggest that the dDAVP-induced antinatriuresis is due to a direct V2 receptor-dependent stimulation of sodium reabsorption in the collecting duct and is not secondary to a hemodynamic effect. In conclusion, this study reveals a potent V2-dependent antinatriuretic effect of vasopressin in humans. The possibility that an inappropriate stimulation of ENaC by vasopressin might lead to significant sodium retention in chronic situations remains to be determined.

Characterization of three types of calcium channel in the luminal membrane of the distal nephron

We previously reported a dual kinetics of Ca2+transport by the distal tubule luminal membrane of the kidney, suggesting the presence of several types of channels. To better characterize these channels, we examined the effects of specific inhibitors (i.e., diltiazem, an L-type channel; ω-conotoxin MVIIC, a P/Q-type channel; and mibefradil, a T-type channel antagonist) on 0.1 and 0.5 mM Ca2+uptake by rabbit nephron luminal membranes. None of these inhibitors influenced Ca2+uptake by the proximal tubule membranes. In contrast, in the absence of sodium (Na+), the three channel antagonists decreased Ca2+transport by the distal membranes, and their action depended on the substrate concentrations: 50 µM diltiazem decreased 0.1 mM Ca2+uptake from 0.65 ± 0.07 to 0.48 ± 0.06 pmol·µg–1·10 s–1(P < 0.05) without influencing 0.5 mM Ca2+transport, whereas 100 nM ω-conotoxin MVIIC decreased 0.5 mM Ca2+uptake from 1.02 ± 0.05 to 0.90 ± 0.05 pmol·µg–1·10 s–1(P < 0.02) and 1 µM mibefradil decreased it from 1.13 ± 0.09 to 0.94 ± 0.09 pmol·µg–1·10 s–1(P < 0.05); the latter two inhibitors left 0.1 mM Ca2+transport unchanged. Diltiazem decreased the Vmaxof the high-affinity channels, whereas ω-conotoxin MVIIC and mibefradil influenced exclusively the Vmaxof the low-affinity channels. These results not only confirm that the distal luminal membrane is the site of Ca2+channels, but they suggest that these channels belong to the L, P/Q, and T types.Key words: renal calcium transport, calcium channels, diltiazem, mibefradil, ω-conotoxin.

A panoramic view of gene expression in the human kidney

To gain a molecular understanding of kidney functions, we established a high-resolution map of gene expression patterns in the human kidney. The glomerulus and seven different nephron segments were isolated by microdissection from fresh tissue specimens, and their transcriptome was characterized by using the serial analysis of gene expression (SAGE) method. More than 400,000 mRNA SAGE tags were sequenced, making it possible to detect in each structure transcripts present at 18 copies per cell with a 95% confidence level. Expression of genes responsible for nephron transport and permeability properties was evidenced through transcripts for 119 solute carriers, 84 channels, 43 ion-transport ATPases, and 12 claudins. Searching for differences between the transcriptomes, we found 998 transcripts greatly varying in abundance from one nephron portion to another. Clustering analysis of these transcripts evidenced different extents of similarity between the nephron portions. Approximately 75% of the differentially distributed transcripts corresponded to cDNAs of known or unknown function that are accurately mapped in the human genome. This systematic large-scale analysis of individual structures of a complex human tissue reveals sets of genes underlying the function of well-defined nephron portions. It also provides quantitative expression data for a variety of genes mutated in hereditary diseases and helps in sorting candidate genes for renal diseases that affect specific portions of the human nephron.

Human cortical distal nephron: distribution of electrolyte and water transport pathways

The exact distributions of the different salt transport systems along the human cortical distal nephron are unknown. Immunohistochemistry was performed on serial cryostat sections of healthy parts of tumor nephrectomized human kidneys to study the distributions in the distal convolution of the thiazide-sensitive Na-Cl cotransporter (NCC), the beta subunit of the amiloride-sensitive epithelial Na channel (ENaC), the vasopressin-sensitive water channel aquaporin 2 (AQP2), and aquaporin 3 (AQP3), the H(+) ATPase, the Na-Ca exchanger (NCX), plasma membrane calcium-ATPase, and calbindin-D28k (CaBP). The entire human distal convolution and the cortical collecting duct (CCD) display calbindin-D28k, although in variable amounts. Approximately 30% of the distal convolution profiles reveal NCC, characterizing the distal convoluted tubule. NCC overlaps with ENaC in a short portion at the end of the distal convoluted tubule. ENaC is displayed all along the connecting tubule (70% of the distal convolution) and the CCD. The major part of the connecting tubule and the CCD coexpress aquaporin 2 with ENaC. Intercalated cells, undetected in the first 20% of the distal convolution, were interspersed among the segment-specific cells of the remainder of the distal convolution, and of the CCD. The basolateral calcium extruding proteins, Na-Ca exchanger (NCX), and the plasma membrane Ca(2+)-ATPase were found all along the distal convolution, and, in contrast to other species, along the CCD, although in varying amounts. The knowledge regarding the precise distribution patterns of transport proteins in the human distal nephron and the knowledge regarding the differences from that in laboratory animals may be helpful for diagnostic purposes and may also help refine the therapeutic management of electrolyte disorders.

Flow cytometric immunodissection of the human distal tubule and cortical collecting duct system

BACKGROUND

In recent years, considerable efforts were drawn to isolate human distal tubule (DT) and collecting duct (CD) cells with more or less success. Here, we present a procedure for isolating human DT cells [thick ascending limb (TAL)/distal convoluted tubule (DCT)] and CD system cells (connecting tubule/initial CD) as separate populations within the same kidney specimen, applying monoclonal antibodies in fluorescence-activated cell sorting (FACS) and culturing them.

METHODS

We tested antibodies directed against the DT/CD system antigens, epithelial membrane antigen (EMA) and L1-cell adhesion molecule (L1-CAM). Segmental and subsegmental expressions were first assessed by using morphologic and histotopographic criteria, and by comparing sections with adjacent sections stained for expression of well-defined distal subsegment-specific markers. Immunoreactive cells were further characterized by dual immunostaining using cell type-specific markers. As a second step, cells obtained by collagenase digestion of normal renal cortical tissue were flow sorted following labeling with aforementioned antibodies and cultured.

RESULTS

EMA expression was found on all cells present in the DT and in the CD system. Its expression was most abundant in TAL and from thereon decreased gradually along the course of the DT and CD system. Flow sorting of all EMA-expressing cells resulted in identification/isolation of DT and CD system cells as a heterogeneous mixture. Flow sorting of only the most strongly EMA-positive cells allowed purification of DT cells only, mainly TAL cells as shown by Tamm-Horsfall protein expression on> 80% of sorted cells. L1-CAM was expressed in only the CD system, and sorting of all L1-CAM-positive cells allowed> 95% purification of CD system cells (connecting tubule/cortical CD). Primary cultures of DT and CD system cells rapidly developed into confluent monolayers, and retained antigenic and functional properties inherent to their segments of origin.

CONCLUSION

Our study presents a procedure for isolating and culturing pure populations of human DT cells and CD system cells as separate populations, using antibodies to the best available markers in FACS.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Proximal tubular phosphate reabsorption: molecular mechanisms

Renal proximal tubular reabsorption of P(i) is a key element in overall P(i) homeostasis, and it involves a secondary active P(i) transport mechanism. Among the molecularly identified sodium-phosphate (Na/P(i)) cotransport systems a brush-border membrane type IIa Na-P(i) cotransporter is the key player in proximal tubular P(i) reabsorption. Physiological and pathophysiological alterations in renal P(i) reabsorption are related to altered brush-border membrane expression/content of the type IIa Na-P(i) cotransporter. Complex membrane retrieval/insertion mechanisms are involved in modulating transporter content in the brush-border membrane. In a tissue culture model (OK cells) expressing intrinsically the type IIa Na-P(i) cotransporter, the cellular cascades involved in "physiological/pathophysiological" control of P(i) reabsorption have been explored. As this cell model offers a "proximal tubular" environment, it is useful for characterization (in heterologous expression studies) of the cellular/molecular requirements for transport regulation. Finally, the oocyte expression system has permitted a thorough characterization of the transport characteristics and of structure/function relationships. Thus the cloning of the type IIa Na-P(i )cotransporter (in 1993) provided the tools to study renal brush-border membrane Na-P(i) cotransport function/regulation at the cellular/molecular level as well as at the organ level and led to an understanding of cellular mechanisms involved in control of proximal tubular P(i) handling and, thus, of overall P(i) homeostasis.

Pseudohypoparathyroidism. New insights into an old disease

The GNAS1 gene (chromosome 20q13.3) encodes the alpha subunit of the stimulatory G protein (Gs alpha) and at least three additional, alternatively spliced transcripts, XL alpha s, NESP55, and the antisense transcript AS. Gs alpha transcripts seem to be derived exclusively, at least in the renal cortex, from the maternal allele. XL alpha s and AS are transcribed only from the paternal allele, and NESP55 is transcribed only from the maternal allele. Numerous GNAS1 mutations have been identified in PHP-Ia and pPHP. Patients with either disorder show skeletal and developmental defects now referred to as AHO. Owing to paternal imprinting, that is, inactivation of the paternal allele, which may be tissue- or cell-specific, resistance toward PTH and, often, other hormones is only observed in patients with PHP-Ia. Patients with PHP-Ib show PTH-resistant hypocalcemia and hyperphosphatemia but no AHO. The abnormal regulation of mineral ion homeostasis is paternally imprinted, such as in PHP-Ia/pPHP kindreds, Gs alpha activity/protein is normal in fibroblasts and blood cells, and no GNAS1 mutations have been identified. Recent linkage studies have mapped the genetic defect responsible for PHP-Ib to chromosome 20q13.3, making it likely that mutations in distinct regions of the GNAS1 gene are the cause of at least three different forms of PHP.

Luminal and contraluminal action of 1-34 and 3-34 PTH peptides on renal type IIa Na-P(i) cotransporter

Parathyroid hormone (PTH) inhibits proximal tubular reabsorption of P(i) by retrieval of type IIa Na-P(i) cotransporters (NaPi-IIa) from the brush-border membrane (BBM). We analyzed by immunohistochemistry whether PTH analogs, signaling through either protein kinase A (PKA) and C (PKC; 1-34 PTH) or only PKC (3-34 PTH), elicit in rat kidney in vivo or in the perfused murine proximal tubule in vitro a retrieval of NaPi-IIa and whether pharmacological agonists or inhibitors of these kinases are able to either mimic or interfere with these PTH effects. Treatment with either 1-34 or 3-34 PTH downregulated NaPi-IIa in rat kidney. In isolated murine proximal tubules 1-34 PTH was effective when added to either the apical or basolateral perfusate, whereas 3-34 PTH acted only via the luminal perfusate. These effects were mimicked by an activation of PKA with 8-bromoadenosine 3',5'-cyclic monophosphate or PKC with 1, 2-dioctanoylglycerol. The luminal action of both PTH peptides was blocked by inhibition of the PKC pathway (calphostin C), whereas the basolateral effect of 1-34 PTH was completely abolished by inhibiting both pathways (H-89 and calphostin C). These results suggest that 1) NaPi-IIa can be internalized by cAMP-dependent and -independent signaling mechanisms; 2) functional PTH receptors are located in both membrane domains; and 3) apical PTH receptors may preferentially initiate the effect through a PKC-dependent mechanism.

ATP directly enhances calcium channels in the luminal membrane of the distal nephron

Calcium (Ca(2+)) transport by the distal tubule (DT) luminal membrane is regulated by the parathyroid hormone (PTH) and calcitonin (CT) through the action of messengers, protein kinases, and ATP as the phosphate donor. In this study, we questioned whether ATP itself, when directly applied to the cytosolic surface of the membrane could influence the Ca(2+) channels previously detected in this membrane. We purified the luminal membranes of rabbit proximal (PT) and DT separately and measured Ca(2+) uptake by these vesicles loaded with ATP or the carrier. The presence of 100 microM ATP in the DT membrane vesicles significantly enhanced 0.5 mM Ca(2+) uptake from 0.57 +/- 0.02 to 0.71 +/- 0.02 pmol/microg per 10 sec (P < 0. 01) in the absence of Na(+) and from 0.36 +/- 0.03 to 0.59 +/- 0.01 pmol/microg per 10 sec (P < 0.01) in the presence of 100 mM Na(+). This effect was dose dependent with an EC(50) value of approximately 40 microM. ATP action involved the high-affinity component of Ca(2+) transport, decreasing the Km from 0.08 +/- 0.01 to 0.04 +/- 0.01 mM (P< 0.02). Replacement of the nucleotide by the nonhydrolyzable ATPgammas abolished this action. Because ATP has been reported to be necessary for cytoskeleton integrity, we also investigated the effect of intravesicular cytochalasin on Ca(2+) transport. Inclusion of 20 microM cytochalasin B decreased 0.5 mM Ca(2+) uptake from 0.33 +/- 0.01 to 0.15 +/- 0.01 pmol/microg per 10 sec (P< 0.01). However, when both 100 microM ATP and 20 microM cytochalasin were present in the vesicles, the uptake was not different from that observed with ATP alone. Neither ATP nor cytochalasin had any influence on Ca(2+) uptake by the PT luminal membrane. We conclude that the high-affinity Ca(2+) channel of the DT luminal membrane is regulated by ATP and that ATP plays a crucial role in the integrity of the cytoskeleton which is also involved in the control of Ca(2+) channels within this membrane.

Effects of water balance, diet and antidiuretic-hormone administration on the renal excretion of water

Enuresis is the result of multifactorial processes. Enuretic patients often exhibit an abnormal diurnal rhythm of plasma vasopressin in addition to high nocturnal urine production. Renal function is considered to be a core factor in influencing the volume of fluid delivered to the bladder. Animal studies have suggested that the amount of fluid delivered to the bladder is dependent upon the state of hydration and/or the amount of protein present in the animal's diet. The state of hydration, or diuresis, may also influence the permeability of the terminal collecting ducts to water and urea and the hydro-osmotic response of the kidney to desmopressin. Multiple agents, including vasopressin, glucagon, calcitonin, parathyroid hormone, beta-adrenergic agonist, insulin, angiotensin II, prostaglandins and calcium and magnesium ions influence sodium transport in the thick ascending limb, indicating that all of these factors may potentially play a role in enuresis.

Dopamine-1 receptor coupling defect in renal proximal tubule cells in hypertension

The ability of the dopamine-1 (D1)-like receptor to stimulate adenylyl cyclase (AC) and phospholipase C (PLC), inhibit sodium transport in the renal proximal tubule (RPT), and produce natriuresis is attenuated in several rat models of hypertension. Since the inhibitory effect of D1-like receptors on RPT sodium transport is also reduced in some patients with essential hypertension, we measured D1-like receptor coupling to AC and PLC in cultures of human RPT cells from normotensive (NT) and hypertensive (HT) subjects. Basal cAMP concentrations were the same in NT (n=6) and HT (n=4). However, the D1-like receptor agonist fenoldopam increased cAMP production to a greater extent in NT (maximum response=67+/-1%) than in HT (maximum response=17+/-5%), with a potency ratio of 105. Dopamine also increased cAMP production to a greater extent in NT (32+/-3%) than in HT (14+/-3%). The fenoldopam-mediated increase in cAMP production was blocked by SCH23390 (a D1-like receptor antagonist) and by antisense D1 oligonucleotides in both HT and NT, indicating action at the D1 receptor. The stimulatory effects of forskolin and parathyroid hormone-related protein of cAMP accumulation were not statistically different in NT and HT, indicating receptor specificity and an intact G-protein/AC pathway. The fenoldopam-stimulated PLC activity was not impaired in HT, and the primary sequence and expression of the D1 receptor were the same in NT and HT. However, D1 receptor serine phosphorylation in the basal state was greater in HT than in NT and was not responsive to fenoldopam stimulation in HT. These studies demonstrate the expression of D1 receptors in human RPT cells in culture. The uncoupling of the D1 receptor in both rats (previously described) and humans (described here) suggests that this mechanism may be involved in the pathogenesis of hypertension; the uncoupling may be due to ligand-independent phosphorylation of the D1 receptor in hypertension.

Second messenger production in avian medullary nephron segments in response to peptide hormones

We examined the sites of peptide hormone activation within medullary nephron segments of the house sparrow (Passer domesticus) kidney by measuring rates of hormone-induced generation of cyclic nucleotide second messenger. Thin descending limbs, thick ascending limbs, and collecting ducts had baseline activity of adenylyl cyclase that resulted in cAMP accumulation of 207 +/- 56, 147 +/- 31, and 151 +/- 41 fmol. mm-1. 30 min-1, respectively. In all segments, this activity increased 10- to 20-fold in response to forskolin. Activity of adenylyl cyclase in the thin descending limb was stimulated approximately twofold by parathyroid hormone (PTH) but not by any of the other hormones tested [arginine vasotocin (AVT), glucagon, atrial natriuretic peptide (ANP), or isoproterenol, each at 10(-6) M]. Thick ascending limb was stimulated two- to threefold by both AVT and PTH; however, glucagon and isoproterenol had no effect, and ANP stimulated neither cAMP nor cGMP accumulation. Adenylyl cyclase activity in the collecting duct was stimulated fourfold by AVT but not by the other hormones; likewise, ANP did not stimulate cGMP accumulation in this segment. These data support a tubular action of AVT and PTH in the avian renal medulla.

Isolation of proximal and distal tubule cells from human kidney by immunomagnetic separation. Technical note

After collagenase digestion and Percoll density gradient centrifugation of human renal tissue, tubular epithelial cells of the proximal and the distal segments were isolated with an immunomagnetic method using MACS microbeads. To enrich proximal tubular (PT) cells we used a monoclonal antibody (mAb) against aminopeptidase M (APM, CD 13), specific of the proximal tubule. Distal tubular (DT) cells were isolated through a mAb recognizing Tamm-Horsfall glycoprotein (THG), a specific antigen for the thick ascending limb and the early distal convoluted tubule. Cells of the proximal primary isolate were histochemically strongly positive for aminopeptidase M (98.6%), however, cells of the distal portion were negative (98.7%). Ultrastructural analysis of PTC primary isolates revealed highly preserved brush border microvilli, well-developed endocytosis apparati and numerous mitochondria, whereas DTC primary isolates showed smaller cells with basolateral invaginations and less apical microvilli. Characterization by immunofluorescence indicated the coexpression of cytokeratin and vimentin, whereas staining for desmin, smooth muscle actin, a fibroblast-specific marker and von Willebrand factor was negative. Cultured PT and DT cells displayed different adenylate cyclase responsiveness to hormonal stimulation. PTH (10(-6) M) increased cAMP production in distal cells up to 32.8-fold of the basal level and in proximal only up to 3.5-fold (10(-8) M, DT 14.4x and PT 2.25x). Calcitonin stimulated adenylate cyclase in DT in a dose dependent fashion (10(-6) M, 4.3x; 10(-8) M, 2.25x), whereas only a low calcitonin response was found in PT cells (10(-6) M, 1.6x; 10(-8) M, 1.4x). AVP (10(-6) M) activated the distal cAMP-production only up to 1.9x of the basal level, but the proximal cAMP-production was negligible (only 1.3x the basal level). The data of this study indicate the proximal and distal tubule origin of the cultured cells that were isolated according to their segment-specific antigens.

Expression and functional analysis of water channels in a stably AQP2-transfected human collecting duct cell line

In this study, we describe the establishment of a stably transfected epithelial cell line with the cDNA for the rat aquaporin 2 (AQP2). To this end, we used a human cell line (HCD) derived from the cortical collecting duct and having characteristics of principal cells (Prié, D., Friedlander, G., Coureau, C., Vandewalle, A., Cassigena, R., and Ronco, P. M. (1995) Kidney Int. 47, 1310-1318). The HCD cells were first screened for the constitutive expression of AQPs. By Western blot analysis, we found a low expression of immunoreactive AQP2 and AQP4 proteins. In contrast, transfected cells (clone CD8) probed with AQP2 antiserum expressed an intense 29-kDa protein on immunoblot in addition to a broad band between 35-45 kDa corresponding to the glycosylated form of the protein, indicating that full maturity of the protein is attained in transfected cells. Immunofluorescence demonstrated that AQP2 was located in intracellular vesicles. After vasopressin stimulation, the staining redistributed from an intracellular site to the apical pole of the cells, an effect similar to that described on collecting duct principal cells in vivo (Sabolic, I., Katsura, T., Verbavatz, J. M., and Brown, D. (1995) J. Membr. Biol. 143, 165-175) and in perfused tubules (Nielsen, S., Chou, C. L., Marples, D., Christensen, E. I., Kishore, B. K., and Knepper, M. A. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 1013-1017). The redistribution of AQP2 in CD8 cells was accompanied by an approximately 6-fold increase in osmotic water permeability coefficient (Pf), which was inhibited by 0.3 m HgCl2. These data indicate that functional vasopressin-sensitive water channels are expressed in transfected cells. The stably transfected cells represent a suitable model to unravel by direct experimental approach the intracellular signals involved in the translocation of AQP2 to the apical plasma membrane in the presence of vasopressin.

Isolation and characterisation of human proximal tubular cells derived from kidney cortical segments

1. Human renal proximal tubular cells (HPTC) were isolated by collagenase digestion and purified following filtration and isopycnic Percoll density centrifugation. This method used cortical tissue obtained from surgical nephrectomies and was both rapid and simple, providing a preparation of cells with high viability (> 93 +/- 3%) and recovery (16 +/- 7 x 10(6) cells g-1 of cortical tissue). 2. Characterisation of the isolated cells showed that, in terms of morphology, enzyme profile, transport systems and hormonal responsiveness, they were > 95% proximal tubular. The transport systems obeyed Michaelis-Menten kinetics, with the kinetic parameters of the glucose transport system (Km = 2.5mM, Vmax = 7.7 nmol min-1 mg-1 protein) suggesting a higher proportion of PT cells originating from the S1-S2 segment of the nephron. Isolated HPTC also maintained levels of reduced glutathione (GSH) (11.9 +/- 3.2 nmol mg-1 protein) and exhibited cytochrome P450-dependent activity, levels of spectrally determined P450 being 0.22 +/- 0.07 nmol mg-1 protein. 3. These results demonstrate the isolation of a viable and functioning homogeneous preparation of HPTC from cortical tissue, with potential for use in short term pharmacological, physiological and toxicological studies.

Phosphate excretion during 24 h of hypoxia in conscious rats

The objective of this study was to investigate renal phosphate excretion during 24 h of hypoxia in conscious rats fed by total parenteral nutrition. Wistar rats weighing 190 g were exposed to hypoxia (inspired oxygen fraction = 0.10) or normoxia (inspired oxygen fraction = 0.21) for 24 h in a normobaric chamber. Renal clearance and hormonal studies were performed. The results showed a greater fractional excretion of phosphate (5.37 +/- 0.07%, P < 0.05) and hypophosphataemia (7.40 +/- 0.12 mg dL-1, P < 0.01) in hypoxic rats (n = 10) than in normoxic rats (n = 13; 3.50 +/- 0.37% and 8.02 +/- 0.16 mg dL-1, respectively). In addition, during hypoxia there was a significant decrease in the excretion of urinary adenosine 3',5'-cyclic monophosphate per glomerular filtrate (2.97 +/- 1.27 nmol dL-1, P < 0.005), a parameter of the renal action of parathyroid hormone, and a stable level of serum parathyroid hormone (10.2 +/- 2.6 ng mL-1) (cf. normoxia: 8.57 +/- 0.70 nmol dL-1 and 8.0 +/- 1.7 ng mL-1, respectively). However, creatinine clearance and the renal adenosine triphosphate level, both of which affect adenosine 3',5'-cyclic monophosphate excretion, were not different between the two groups. These data suggest that exposure of conscious rats to 24 h of hypoxia causes renal hyporesponsiveness to physiological levels of parathyroid hormone, which is manifested as a decrease in adenosine 3',5'-cyclic monophosphate excretion. Phosphaturia is not a direct net effect of hypoxia and secondary hypocapnia on renal phosphate transport, which is known to be regulated by parathyroid hormone through adenosine 3',5'-cyclic monophosphate.

Aquaretic effects of the nonpeptide V2 antagonist OPC-31260 in hydropenic humans

The antidiuretic action of arginine vasopressin (AVP) is mediated by interaction with renal V2 receptors. A nonpeptide V2 receptor antagonist, OPC-31260, has recently been developed. In this study, the effects of OPC-31260 on the excretion of water and electrolytes were investigated in normal subjects who were under water restriction. Since the clinical circumstances in which a V2 antagonist would be useful generally would be in patients with concentrated urine, a hydropenic state with nearly maximally concentrated urine was selected as the experimental condition. Intravenous injection of OPC-31260 caused an increase in urine volume and a decrease in urine osmolality in a dose dependent manner without any significant changes in the excretion of sodium and other electrolytes. A high dose of OPC-31260 (1 mg/kg body wt) caused free water excretion equivalent to that obtained during maximum diuresis after water loading, followed by an increase in plasma sodium and AVP concentration. Excretion of urea increased transiently during diuresis. Thus, OPC-31260 is demonstrated to be the first V2 antagonist which exhibits water diuresis (aquaresis) in hydropenic humans with endogenous AVP secretion and maximally concentrated urine. The drug will be useful as an aquaretic in the treatment of some types of hyponatremia where there is excess AVP and water retention.

Role of adenosine on glucagon-induced cAMP in a human cortical collecting duct cell line

The hormonal responsiveness profile of the cortical collecting duct varies from one species to another. To identify the hormones and agonists that modulate the functions of this tubule segment in the human species, we generated a cell line (HCD) immortalized by SV40 virus. The tubular origin of this cell line was assessed by the expression of collecting duct-specific antigens and the ability of vasopressin to increase by nine-fold cAMP synthesis. Glucagon and adenosine stimulated cAMP synthesis, and atrial natriuretic peptide stimulated cGMP synthesis in a concentration-dependent manner. Bradykinin, adenosine and angiotensin increased intracellular calcium concentration ([Ca2+]i). Because adenosine can regulate tubular functions, we examined its role on glucagon-induced cAMP synthesis. Using adenosine analogs, we demonstrated that HCT cells both expressed adenosine type-2 (A2) receptors which stimulated cAMP production, and adenosine type-1 (A1) receptors linked to [Ca2+]i increase which inhibited glucagon-stimulated cAMP synthesis. The inhibitory effect was abolished by pertussis toxin, and was neither due to [Ca2+]i increase nor to protein kinase C activation, which indicated that some A1 adenosine receptors were directly negatively coupled to adenylyl cyclase. These results suggest that adenosine can modify human cortical collecting duct functions in opposite ways according to the adenosine receptor activated.

Signal transduction by calcitonin Multiple ligands, receptors, and signaling pathways

Calcitonin (CT) is a peptide hormone that is secreted by the parafollicular cells of the thyroid in response to elevated serum calcium levels. It acts to reduce serum calcium by inhibiting bone resorption and promoting renal calcium excretion. In addition to this hypocalcemie effect, calcitonin modulates the renal transport of water and several ions other than calcium and acts on the central nervous system to induce analgesia, anorexia, and gastric secretion. The CT receptor, a member of a newly described family of serpentine G protein-coupled receptors, has recently been shown to couple to multiple trimeric G proteins, thereby activating several signaling proteins, including protein kinase C, cAMP-dependent protein kinase and calcium/calmodulin-dependent protein kinase. In kidney proximal tubule cells (LLC-PK1), the CT-activated signaling mechanisms vary in a cell cycle-dependent manner, with the receptor coupling through a G(s) protein during G(2) phase and through a G(i) protein and possibly a G(q) protein during S phase. These signaling mechanisms differentially modulate the activities of Na(+)/K(+)-ATPase and the apical Na(+)/H(+) exchanger, effector molecules that play important roles in transepithelial Na(+) transport. Cloning of CT receptors has revealed the presence of alternatively spliced cassettes, resulting in the expression of different isoforms of the receptor. The availability of these recombinant CT receptors has allowed preliminary characterization of the effects of changes in the receptor's structure on its ligand binding and signal transduction properties. Thus, the cellular and molecular biology of CT is complex, with several structurally related peptide ligands and multiple isoforms of the CT receptor that can independently activate diverse signaling pathways. As the recent exciting results in this field are extended, we can expect rapid progress in understanding the molecular basis of the diverse effects of CT and, possibly, of the CT-related peptides CGRP and amylin.