Hormone-stimulated Ca2+ transport in rabbit kidney: multiple sites of inhibition by exogenous ATP

Regulation of renal function and blood pressure control by P2 purinoceptors in the kidney

Kidneys are important regulators of extracellular fluid volume (ECFV) homeostasis. ECFV is a key regulatory component of long-term blood pressure control influenced by controlling tubular sodium transport. In recent decades, renal P2 purinoceptors (P2 receptors) have come to the forefront as a mechanism for regulating ECFV. P2 receptors are broadly distributed in renal tubular and vascular elements where they confer segmental control of renal vascular resistance, autoregulation, and tubular reabsorption. Activation or impairment of renal P2 purinoceptors is implicated in the regulating blood pressure or causing renal pathologies including hypertension. In this brief review, we discuss the role of renal vascular and tubular P2 purinoceptors in the regulation of renal hemodynamics, maintenance of ECFV, regulation of sodium reabsorption and the control of blood pressure.

Influence of feeding and UVB exposition on the absorption mechanisms of calcium in the gastrointestinal tract of veiled chameleons (Chamaeleo calyptratus)

The purpose of this study was to investigate the influence of feeding and UVB exposition on the occurrence and distribution patterns of vitamin D receptors (VDR) and calbindin D28k (Cb-D28k) in the gastrointestinal tract of veiled chameleons. Thus, 56 veiled chameleon hatchlings were divided into six treatment groups: UV (with UVB exposure); No (no supplements, no UVB exposure); CaAUV (with calcium (Ca), vitamin A supplementation, UVB exposure); CaA (with Ca, vitamin A supplementation); CaADUV (with Ca, vitamin A, vitamin D supplementation, UVB exposure); and CaAD (with Ca, vitamin A, vitamin D supplementation). Animals were reared under the suspected conditions for 6 months on locust-based diets. Tissue samples of stomach, duodenum, ileum and colon were taken, and semi-quantitative immunohistochemical methods (IHC) were performed to detect Cb-D28k and VDR. VDR immunoreactions were higher in the luminal epithelium of the duodenum than in that of the ileum. VDR immunoreactions in the luminal epithelium were higher at the base of the villi of the duodenum as compared to the tip. Cb-D28k immunoreactions were mainly observed in the luminal epithelium of the duodenum. The two groups treated with all dietary supplements (CaADUV, CaAD) exhibited a higher Cb-D28k immunoreaction as those with no supplements and UVB exposure only. No immunoreaction for both proteins could be detected in the stomach. This study suggests that the duodenum plays an important role in the active transcellular absorption of Ca in veiled chameleons as shown by the immunohistochemical detection of VDR and Cb-D28k. Expression of Cb-D28k, in particular, appears to be regulated by dietary supplementation of vitamin D and vitamin A. VDRs, however, tended to be upregulated when animals were not supplemented with Ca, vitamin D and vitamin A. This may be due to the decreased Ca concentrations which caused vitamin D activation in the skin without any supplementation, but UVB exposure.

Angiotensin II enhances connecting tubule glomerular feedback

Increasing Na delivery to epithelial Na channels (ENaCs) in the connecting tubule (CNT) causes dilation of the afferent arteriole (Af-Art), a process we call CNT glomerular feedback (CTGF). Angiotensin II (Ang II) stimulates ENaC in the collecting duct via Ang II type 1 receptors. We hypothesized that Ang II in the CNT lumen enhances CTGF by activation of Ang II type 1 receptors, protein kinase C and ENaC. Rabbit afferent arterioles and their adherent CNT were microperfused and preconstricted with norepinephrine. Each experiment involved generating 2 consecutive concentration-response curves by increasing NaCl in the CNT lumen. During the control period, the maximum dilation of the afferent arteriole was 7.9±0.4 μm, and the concentration of NaCl in the CNT needed to achieve half maximal response (EC(50)) was 34.7±5.2 mmol/L. After adding Ang II (10(-9) mol/L) to the CNT lumen, the maximal response was 9.5±0.7 μm and the EC(50) was 11.6±1.3 mmol/L (P=0.01 versus control). Losartan, an Ang II type 1 antagonist (10(-6) mol/L) blocked the stimulatory effect of Ang II; PD123319, an Ang II type 2 antagonist (10(-6) mol/L), did not. The protein kinase C inhibitor staurosporine (10(-8) mol/L) added to the CNT inhibited the stimulatory effect of Ang II. The ENaC inhibitor benzamil (10(-6) mol/L) prevented both CTGF and its stimulation by Ang II. We concluded that Ang II in the CNT lumen enhances CTGF via activation of Ang II type 1 and that this effect requires activation of protein kinase C and ENaC. Potentiation of CTGF by Ang II could help preserve glomerular filtration rate in the presence of renal vasoconstriction.

Effects of extracellular nucleotides on renal tubular solute transport

A range of P2 receptor subtypes has been identified along the renal tubule, in both apical and basolateral membranes. Furthermore, it has been shown that nucleotides are released from renal tubular cells, and that ectonucleotidases are present in several nephron segments. These findings suggest an autocrine/paracrine role for nucleotides in regulating tubular function. The present review catalogues the known actions of extracellular nucleotides on tubular solute transport. In the proximal tubule, there is firm evidence that stimulation of apical P2Y(1) receptors inhibits bicarbonate reabsorption, whilst basolaterally applied ATP has the opposite effect. Clearance studies suggest that systemic diadenosine polyphosphates profoundly reduce proximal tubular fluid transport, through as yet unidentified P2 receptors. To date, only circumstantial evidence is available for an action of nucleotides on transport in the loop of Henle; and no studies have been made on native distal tubules, though observations in cell lines suggest an inhibitory effect on sodium, calcium and magnesium transport. The nephron segment most studied is the collecting duct. Apically applied nucleotides inhibit the activity of small-conductance K(+) channels in mouse collecting duct, apparently through stimulation of P2Y(2) receptors. There is also evidence, from cell lines and native tissue, that apically (and in some cases basolaterally) applied nucleotides inhibit sodium reabsorption. In mice pharmacological profiling implicates P2Y(2) receptors; but in rats, the receptor subtype(s) responsible is/are unclear. Recent patch-clamp studies in rat collecting ducts implicate apical P2Y and P2X subtypes, with evidence for both inhibitory and stimulatory effects. Despite considerable progress, clarification of the physiological role of the tubular P2 receptor system remains some way off.

Recent advances in physiological calcium homeostasis

A constant extracellular Ca2+ concentration is required for numerous physiological functions at tissue and cellular levels. This suggests that minor changes in Ca2+ will be corrected by appropriate homeostatic systems. The system regulating Ca2+ homeostasis involves several organs and hormones. The former are mainly the kidneys, skeleton, intestine and the parathyroid glands. The latter comprise, amongst others, the parathyroid hormone, vitamin D and calcitonin. Progress has recently been made in the identification and characterisation of Ca2+ transport proteins CaT1 and ECaC and this has provided new insights into the molecular mechanisms of Ca2+ transport in cells. The G-protein coupled calcium-sensing receptor, responsible for the exquisite ability of the parathyroid gland to respond to small changes in serum Ca2+ concentration was discovered about a decade ago. Research has focussed on the molecular mechanisms determining the serum levels of 1,25(OH)2D3, and on the transcriptional activity of the vitamin D receptor. The aim of recent work has been to elucidate the mechanisms and the intracellular signalling pathways by which parathyroid hormone, vitamin D and calcitonin affect Ca2+ homeostasis. This article summarises recent advances in the understanding and the molecular basis of physiological Ca2+ homeostasis.

Calcium transport in human salivary glands: a proposed model of calcium secretion into saliva

Salivary calcium plays a vital role in bio-mineralization of dental enamel and exposed dentin. In order to elucidate the yet unknown cellular and molecular mechanisms of calcium secretion in human salivary glands the presence of various relevant plasma membrane transport systems for calcium were investigated. Using an RT-PCR approach, expression of the epithelial calcium channel (CaT-Like), the calcium binding protein (calbindin-2), the endoplasmic reticulum pumps (SERCA-2 and -3), and the plasma membrane calcium ATPases (PMCA-1, -2, and -4), were found in parotid and submandibular glands. Immunohistochemistry revealed that CaT-Like is located in the basolateral plasma membrane of acinar cells; while calbindin-2, SERCA-2 and SERCA-3 were found inside the acinar cells; and PMCA-2 was found in the apical membrane and in the secretory canaliculi between the cells. Based on these findings, we propose the following model of calcium secretion in human salivary glands: (1) calcium enters the acinar cell at the basolateral side via calcium channel CaT-Like (calcium influx); (2) intracellular calcium is taken up into the endoplasmic reticulum by SERCA-2 and possibly SERCA3 or bound to calbindin-2 (intracellular calcium pool); and (3) calcium is secreted by PMCAs at the apical plasma membrane (calcium efflux).

TRPV5 and TRPV6 in Ca(2+) (re)absorption: regulating Ca(2+) entry at the gate

Many physiological functions rely on the exact maintenance of body Ca(2+) balance. Therefore, the extracellular Ca(2+) concentration is tightly regulated by the concerted actions of intestinal Ca(2+) absorption, exchange of Ca(2+) to and from bone, and renal Ca(2+) reabsorption. Renal distal convoluted and connecting tubular cells as well as duodenal epithelial cells are unique in their ability to mediate transcellular (re)absorption of Ca(2+) at large and highly variable rates. Two members of the transient receptor potential (TRP) superfamily, TRP vanilloid (TRPV)5 and TRPV6, are specialized epithelial Ca(2+) channels responsible for the critical Ca(2+) entry step in transcellular Ca(2+) (re)absorption in intestine and kidney, respectively. Because transcellular Ca(2+) transport is fine-tuned to the body's specific requirements, regulation of the transmembrane Ca(2+) flux through TRPV5/6 is of particular importance and has, therefore, to be conspicuously controlled. We present an overview of the current knowledge and recent advances concerning the coordinated regulation of Ca(2+) influx through the epithelial Ca(2+) channels TRPV5 and TRPV6 in transcellular Ca(2+) (re)absorption.

TRPV5 and TRPV6 in Ca(2+) (re)absorption: regulating Ca(2+) entry at the gate

Many physiological functions rely on the exact maintenance of body Ca(2+) balance. Therefore, the extracellular Ca(2+) concentration is tightly regulated by the concerted actions of intestinal Ca(2+) absorption, exchange of Ca(2+) to and from bone, and renal Ca(2+) reabsorption. Renal distal convoluted and connecting tubular cells as well as duodenal epithelial cells are unique in their ability to mediate transcellular (re)absorption of Ca(2+) at large and highly variable rates. Two members of the transient receptor potential (TRP) superfamily, TRP vanilloid (TRPV)5 and TRPV6, are specialized epithelial Ca(2+) channels responsible for the critical Ca(2+) entry step in transcellular Ca(2+) (re)absorption in intestine and kidney, respectively. Because transcellular Ca(2+) transport is fine-tuned to the body's specific requirements, regulation of the transmembrane Ca(2+) flux through TRPV5/6 is of particular importance and has, therefore, to be conspicuously controlled. We present an overview of the current knowledge and recent advances concerning the coordinated regulation of Ca(2+) influx through the epithelial Ca(2+) channels TRPV5 and TRPV6 in transcellular Ca(2+) (re)absorption.

Characterization of a Madin-Darby canine kidney cell line stably expressing TRPV5

To provide a cell model for studying specifically the regulation of Ca2+ entry by the epithelial calcium channel transient receptor potential-vanilloid-5 (TRPV5), green fluorescent protein (GFP)-tagged TRPV5 was expressed stably in Madin-Darby canine kidney type I (MDCK) cells. The localization of GFP-TRPV5 in this cell line showed an intracellular granular distribution. Ca2+ uptake in GFP-TRPV5-MDCK cells cultured on plastic supports was threefold higher than in non-transfected cells. Moreover, apical Ca2+ uptake in GFP-TRPV5-MDCK cells cultured on permeable supports was eightfold higher than basolateral Ca2+ uptake, indicating that GFP-TRPV5 is expressed predominantly in the apical membrane. Patch-clamp analysis showed the presence of typical electrophysiological features of GFP-TRPV5, such as inwardly rectifying currents, inhibition by divalent cations and Ca2+-dependent inactivation. Moreover, the TRPV5 inhibitor ruthenium red completely inhibited Ca2+ uptake in GFP-TRPV5-MDCK cells, whereas Ca2+ uptake in non-transfected cells was not inhibited. The characterized GFP-TRPV5-MDCK cell line was used to assess the regulation of TRPV5. The protein kinase C activator phorbol 12-myristate 13-acetate and the cAMP-elevating compounds forskolin/3-isobutyl-1-methylxanthine, 8-Br-cAMP and PGE2 stimulated TRPV5 activity in GFP-TRPV5-MDCK cells by 121+/-7, 79+/-5, 55+/-4 and 61+/-7%, respectively. These compounds did not affect Ca2+ uptake in non-transfected cells. In conclusion, the GFP-TRPV5-MDCK cell line provides a model to specifically study the regulation of TRPV5 activity.

Effects of luminal flow and nucleotides on [Ca(2+)](i) in rabbit cortical collecting duct

Nucleotide binding to purinergic P2 receptors contributes to the regulation of a variety of physiological functions in renal epithelial cells. Whereas P2 receptors have been functionally identified at the basolateral membrane of the cortical collecting duct (CCD), a final regulatory site of urinary Na(+), K(+), and acid-base excretion, controversy exists as to whether apical purinoceptors exist in this segment. Nor has the distribution of receptor subtypes present on the unique cell populations that constitute Ca(2+) the CCD been established. To examine this, we measured nucleotide-induced changes in intracellular Ca(2+) concentration ([Ca(2+)](i)) in fura 2-loaded rabbit CCDs microperfused in vitro. Resting [Ca(2+)](i) did not differ between principal and intercalated cells, averaging approximately 120 nM. An acute increase in tubular fluid flow rate, associated with a 20% increase in tubular diameter, led to increases in [Ca(2+)](i) in both cell types. Luminal perfusion of 100 microM UTP or ATP-gamma-S, in the absence of change in flow rate, caused a rapid and transient approximately fourfold increase in [Ca(2+)](i) in both cell types (P < 0.05). Luminal suramin, a nonspecific P2 receptor antagonist, blocked the nucleotide- but not flow-induced [Ca(2+)](i) transients. Luminal perfusion with a P2X (alpha,beta-methylene-ATP), P2X(7) (benzoyl-benzoyl-ATP), P2Y(1) (2-methylthio-ATP), or P2Y(4)/P2Y(6) (UDP) receptor agonist had no effect on [Ca(2+)](i). The nucleotide-induced [Ca(2+)](i) transients were inhibited by the inositol-1,4,5-triphosphate receptor blocker 2-aminoethoxydiphenyl borate, thapsigargin, which depletes internal Ca(2+) stores, luminal perfusion with a Ca(2+)-free perfusate, or the L-type Ca(2+) channel blocker nifedipine. These results suggest that luminal nucleotides activate apical P2Y(2) receptors in the CCD via pathways that require both internal Ca(2+) mobilization and extracellular Ca(2+) entry. The flow-induced rise in [Ca(2+)](i) is apparently not mediated by apical P2 purinergic receptor signaling.

ATP inhibits Mg(2+) uptake in MDCT cells via P2X purinoceptors

Nucleotides have diverse effects on water and electrolyte reabsorption within the distal tubule of the nephron. As the distal tubule is important in control of renal Mg(2+) balance, we determined the effects of ATP on cellular Mg(2+) uptake in this segment. The effects of ATP on immortalized mouse distal convoluted tubule (MDCT) cells were studied by measuring Mg(2+) uptake with fluorescence techniques. The mean basal Mg(2+) uptake rate was 165 +/- 6 nM/s. ATP inhibited basal Mg(2+) uptake and hormone-stimulated Mg(2+) entry by 40%. Both P2X (P2X1-P2X5 subtypes) and P2Y2 receptor subtypes were identified in MDCT cells using differential RT-PCR. Activation of both receptor subtypes with selective agonists increased intracellular Ca(2+) concentration, P2X purinoceptors by ionotropic-gated channels, and P2Y receptors via G protein-mediated intracellular Ca(2+) release. The more relatively selective P2X agonists [beta,gamma-methylene ATP (beta,gamma-Me-ATP) and 2'- and -3'-O-(4-benzoyl-benzoyl)-ATP] inhibited arginine vasopressin (AVP)- and parathyroid hormone (PTH)-mediated Mg(2+) uptake whereas agonists more selective for P2Y purinoceptors (UTP, ADP, and 2-methylthio-ATP) were without effect. Removal of extracellular Ca(2+) diminished beta,gamma-Me-ATP-mediated increase in intracellular Ca(2+) and inhibition of AVP-stimulated Mg(2+) entry. We conclude from this information that ATP inhibited Mg(2+) uptake in MDCT cells through P2X purinoceptors expressed in this distal convoluted tubule cell line.

Magnesium transport in the renal distal convoluted tubule

The distal tubule reabsorbs approximately 10% of the filtered Mg(2+), but this is 70-80% of that delivered from the loop of Henle. Because there is little Mg(2+) reabsorption beyond the distal tubule, this segment plays an important role in determining the final urinary excretion. The distal convoluted segment (DCT) is characterized by a negative luminal voltage and high intercellular resistance so that Mg(2+) reabsorption is transcellular and active. This review discusses recent evidence for selective and sensitive control of Mg(2+) transport in the DCT and emphasizes the importance of this control in normal and abnormal renal Mg(2+) conservation. Normally, Mg(2+) absorption is load dependent in the distal tubule, whether delivery is altered by increasing luminal Mg(2+) concentration or increasing the flow rate into the DCT. With the use of microfluorescent studies with an established mouse distal convoluted tubule (MDCT) cell line, it was shown that Mg(2+) uptake was concentration and voltage dependent. Peptide hormones such as parathyroid hormone, calcitonin, glucagon, and arginine vasopressin enhance Mg(2+) absorption in the distal tubule and stimulate Mg(2+) uptake into MDCT cells. Prostaglandin E(2) and isoproterenol increase Mg(2+) entry into MDCT cells. The current evidence indicates that cAMP-dependent protein kinase A, phospholipase C, and protein kinase C signaling pathways are involved in these responses. Steroid hormones have significant effects on distal Mg(2+) transport. Aldosterone does not alter basal Mg(2+) uptake but potentiates hormone-stimulated Mg(2+) entry in MDCT cells by increasing hormone-mediated cAMP formation. 1,25-Dihydroxyvitamin D(3), on the other hand, stimulates basal Mg(2+) uptake. Elevation of plasma Mg(2+) or Ca(2+) inhibits hormone-stimulated cAMP accumulation and Mg(2+) uptake in MDCT cells through activation of extracellular Ca(2+)/Mg(2+)-sensing mechanisms. Mg(2+) restriction selectively increases Mg(2+) uptake with no effect on Ca(2+) absorption. This intrinsic cellular adaptation provides the sensitive and selective control of distal Mg(2+) transport. The distally acting diuretics amiloride and chlorothiazide stimulate Mg(2+) uptake in MDCT cells acting through changes in membrane voltage. A number of familial and acquired disorders have been described that emphasize the diversity of cellular controls affecting renal Mg(2+) balance. Although it is clear that many influences affect Mg(2+) transport within the DCT, the transport processes have not been identified.