Different endothelin receptor subtypes are involved in phospholipid signalling in the proximal tubule of rat kidney

Regulation of sodium transport in the proximal tubule by endothelin

Human essential hypertension and rodent genetic hypertension are associated with increased sodium transport in the renal proximal tubule and medullary thick ascending limb of Henle. The proximal tubule, which secretes endothelin (ET), expresses the ET(B) receptor. Low (nM) concentrations of ET, via the ET(B) receptor, inhibit sodium and water transport and ATP-driven drug secretion in the proximal tubule. In contrast, very low (pM) and high nM concentrations of ET increase renal proximal sodium transport, but the receptor involved remains to be determined. The natriuretic effect of ET(B) receptor stimulation is impaired in spontaneously hypertensive rats, due in part to a defective interaction with D(3) dopamine and angiotensin II type 1 receptors. Impaired ET(B) receptor function in the renal proximal tubule may be important in the pathogenesis of genetic hypertension.

Impaired stimulatory effect of ETB receptor on D₃ receptor in immortalized renal proximal tubule cells of spontaneously hypertensive rats

BACKGROUND

Activation of renal D₃ receptor induces natriuresis and diuresis in Wistar-Kyoto (WKY) rats; in the presence of ETB receptor antagonist, the natriuretic effect of D₃ receptor in WKY rats is reduced. We hypothesize that ETB receptor activation may regulate D₃ receptor expression in renal proximal tubule (RPT) cells from WKY rats, which is impaired in RPT cells from spontaneously hypertensive rats (SHRs).

METHODS

D₃ receptor expression was determined by immunoblotting; the D₃/ETB receptor linkage was checked by coimmunoprecipitation; Na(+)-K(+)-ATPase activity was determined as the rate of inorganic phosphate released in the presence or absence of ouabain.

RESULTS

In RPT cells from WKY rats, the ETB receptor agonist BQ3020 increased D₃ receptor protein. In contrast, in RPT cells from SHRs, BQ3020 did not increase D₃ receptor. There was coimmunoprecipitation between D₃ and ETB receptors in RPT cells from WKY and SHRs. Activation of ETB receptor increased D₃/ETB coimmunoprecipitation in RPT cells from WKY rats, but not from SHRs. The basal levels of D₃/ETB receptor coimmunoprecipitation were greater in RPT cells from WKY rats than in those from SHRs. Stimulation of D₃ receptor inhibited Na(+)-K(+)-ATPase activity, which was augmented by the pretreatment with the ETB receptor agonist BQ3020 in WKY RPT cells, but not in SHR RPT cells.

CONCLUSION

ETB receptors regulate and physically interact with D₃ receptors differently in WKY rats and SHRs. The impaired natriuretic effect in SHRs may be, in part, related to impaired ETB and D₃ receptor interactions.

Altered AT 1 Receptor Regulation of ETB Receptors in Renal Proximal Tubule Cells of Spontaneously Hypertensive Rats

The renin-angiotensin and endothelin systems regulate blood pressure, in part, by affecting renal tubular sodium transport. In rodents, ETB receptors decrease proximal tubular reabsorption, whereas AT 1 receptors produce the opposite effect. We hypothesize that ETB and AT 1 receptors interact at the receptor level, and that the interaction is altered in spontaneously hypertensive rats (SHRs). In immortalized renal proximal tubule (RPT) cells from Wistar-Kyoto (WKY) rats, angiotensin II, via AT 1 receptors, increased ETB receptor protein in a time- and concentration-dependent manner. In contrast, in SHR RPT cells, angiotensin II (10 −8 M/24 hours) had no effect on ETB receptor protein. AT 1 /ETB receptors colocalized and co-immunoprecipitated in both rat strains but long-term angiotensin II (10 −8 M/24 hours) treatment increased AT 1 /ETB co-immunoprecipitation in WKY but not in SHR cells. Short-term angiotensin II (10 −8 M/15 minutes) treatment decreased ETB receptor phosphorylation in both WKY and SHR cells, and increased ETB receptors in RPT cell surface membranes of RPT cells in WKY but not SHRs. Basal cell surface membrane ETB receptor expression was also higher in WKY than in SHRs. We conclude that AT 1 receptors regulate ETB receptors by receptor interaction and modulation of receptor expression. The altered AT 1 receptor regulation of ETB receptors in SHRs may play a role in the pathogenesis of hypertension.

ETA receptor-mediated Ca2+ signaling in thin descending limbs of Henle's loop: impairment in genetic hypertension

BACKGROUND

Endothelins (ET) have diuretic and natriuretic actions via ETB receptors that are found in most renal tubular segments, although the thin limbs have not been studied. Data also suggest that dysfunction of the renal ET system may be important in the pathogenesis of hypertension. The present study was aimed at determining the presence and nature of ET receptors in the thin limbs of Henle's loop and their ability to activate a Ca2+-dependent signaling pathway, as well as whether ET-induced Ca2+ signals are altered in hypertension.

METHODS

Reverse transcription-polymerase chain reaction (RT-PCR) and Fura 2 fluoreselected strains of Lyon rats with low-normal (LL), normal (LN), and high (LH) blood pressure.

RESULTS

In SD rats, ET induced Ca2+ signals in DTL of long-looped nephrons, but not in DTL of short loops, or in ascending thin limbs. Ca2+ increases were abolished by BQ123, an antagonist of the ETA receptor, but not by BQ788, an antagonist of the ETB subtype. Endothelin-3 and sarafotoxin 6c, two ETB receptor agonists, were both inactive. RT-PCR showed the presence of both ETA and ETB receptor mRNA. Ca2+ signals measured scence measurements of [Ca2+]i were made to characterize ET receptors in descending thin limbs (DTL) of Sprague-Dawley rats, spontaneously hypertensive (SH) rats, and control Wistar-Kyoto (WKY) rats, and the three in DTL of WKY LL and LN rats were similar to those in Sprague-Dawley rats, but were significantly diminished (LH) or abolished (SH) in hypertensive rats.

CONCLUSION

A functional ETA receptor activating a Ca2+-dependent pathway is expressed in DTL. This ETA-induced calcium signaling is impaired in two strains of genetically hypertensive rats.

Xenobiotic export pumps, endothelin signaling, and tubular nephrotoxicants--a case of molecular hijacking

This article is a review on recent studies in intact renal proximal tubules that link tubular nephrotoxicants with endothelin (ET) regulation of xenobiotic export pump function. The data show that transport on p-glycoprotein and Mrp2 decreases rapidly when ET signals through an ET(B) receptor, NO synthase (NOS), and protein kinase C (PKC). Surprisingly, nephrotoxicants, such as radiocontrast agents, aminoglycoside antibiotics, and heavy metal salts, "hijack" this signaling pathway, causing ET release from the tubules, hormone binding to its receptor, activation of NOS and PKC, and reduced xenobiotic transport. These findings suggest a new common mechanism by which nephrotoxicants may act to disrupt renal tubular function.

Hepatocyte growth factor activates phosphoinositide 3-kinase C2 beta in renal brush-border plasma membranes

Upon stimulation of renal cortical slices with hepatocyte growth factor (HGF), inositol lipid metabolism was studied in basal-lateral plasma membranes (BLM) and brush-border plasma membranes (BBM). Whereas in BLM rapid increases in 1,2-diacylglycerol, PtdIns(3,4,5)P(3) and PtdIns(3,4)P(2) were observed, suggesting that in BLM HGF activates both phospholipase C (PLC) and phosphoinositide 3-kinase (PI3K), in BBM only HGF-induced transient accumulation of PtdIns3P was seen, which was temporarily delayed from signalling events in BLM and could be blocked by the PtdIns-specific-PLC inhibitor ET-18-OCH(3) and the calpain inhibitor calpeptin, suggesting that 3-kinase activation in BBM lies downstream of PLC activation in BLM and is a calpain-mediated event. Moreover, the increase in immunoprecipitable PI3K-C2 beta activity, which is sensitive to wortmannin (10 nM) and shows strong preference for PtdIns over PtdIns4P as a substrate, was observed only in BBM upon stimulation of renal cortical slices with HGF and could be mimicked by the Ca(2+) ionophore A23187 and blocked by the cell-penetrant Ca(2+) chelator BAPTA-AM [1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid tetrakis(acetoxymethyl ester)]. On Western blots PI3K-C2 beta revealed a single immunoreactive band of 180 kDa in BLM and BBM, while after stimulation with HGF a gel shift of 18 kDa was noticed only in BBM, suggesting that the observed enzyme activation is achieved by proteolysis. When BBM were subjected to short-term (15 min) exposure to mu-calpain, a similar gel shift together with an increase in PI3K-C2 beta activity was observed, when compared with the BBM harvested after HGF stimulation. The above-mentioned gel shift and increase in PI3K-C2 beta activity could be prevented by the calpain inhibitor calpeptin. The data presented in this report show that in renal cells there is a spatial separation of the inositol lipid signalling system between BLM and BBM, and that HGF causes activation of PLC and PI3K primarily in BLM, which leads to calpain-mediated activation of PI3K-C2 beta in BBM with a concomitant increase in PtdIns3P.

Effect of endothelin 1 on proximal reabsorption and tubuloglomerular feedback

Endothelin 1 (ET-1) is a powerful constrictor of the afferent glomerular artery, implicated in the occurrence of both functional and acute renal failure. Besides being produced by the endothelium, ET-1 is also secreted by proximal tubular cells, suggesting that it may act as an endoluminal messenger. The present study is intended to verify whether ET-1 may play a role in the tubuloglomerular feedback system. The experiments were performed in rat superficial glomeruli. In 25 nephrons we measured by the total collection technique the single-nephron glomerular filtration rate (SNGFR; nl/min) and reabsorption rates before (control) and during microinjection (MIJ) of ET-1 10(-7) M into the first proximal convolution or Bowman's space. The SNGFR rose from 27+/-3 to 61+/-11 nl/min (p<0.01), the percent proximal reabsorption rose from 43 to 74%, and the absolute reabsorption rose from 13+/-2 to 46+/-11 nl/min (p<0.01). In additional 23 nephrons the collections were performed at the earliest distal convolution accessible on the renal surface, while MIJ was performed in the last proximal convolution of the same nephrons. The SNGFR rose during MIJ from 22+/-3 to 40+/-6 nl/min (p<0.01), the percent reabsorption rose from 61 to 66% (p>0. 77), and the absolute reabsorption rose from 12+/-2 to 26+/-4 nl/min, (p<0.003). Exposure of the macula densa to intraluminally injected ET-1 causes an abrupt increase in SNGFR of the experimental nephron, in the absence of changes in systemic and renal hemodynamics. During proximal MIJ, ET-1 may have reached the macula densa during the time preceding the beginning of collection and interruption of the delivery of fluid to the distal nephron. ET-1 directly stimulates fractional and absolute volume reabsorption along the proximal tubule. Proximal secretion and/or filtration of ET-1 could represent a physiological mechanism to activate the tubuloglomerular feedback, eliciting a response opposite to that triggered by systemic and intrarenal infusion.

Renal endothelin in hypertension

Due to the potent vasoconstrictor action of endothelin-1 and its synthesis throughout the vasculature and other tissues, most investigators believe that it is an active participant in the pathogenesis of hypertension. However, the autocrine and paracrine nature of the endothelin system has made its role difficult to define. In recent years, it has become apparent that endothelin-1 contributes to the regulation of renal salt and water excretion and that it is a major contributor to the hypertension associated with salt-dependency. Evidence suggests that endothelin-1 within the renal medulla is activated in conditions of salt loading and inhibits reabsorption of sodium in a nitric oxide-dependent manner. Blockade of endothelin A receptors lowers arterial pressure in animal models of salt-dependent hypertension. Furthermore, circulating levels of endothelin-1 are generally higher in African-Americans compared to white Americans as is the prevalence of salt-dependent hypertension. Therefore, it would appear that use of endothelin A-selective receptor antagonists should be targeted to those individuals at risk for salt-dependent hypertension. Blockade of endothelin B receptors would not be desirable because of their important role in eliminating a salt load.

Localization of inositol 1,4,5-trisphosphate receptors in the rat kidney

Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) serve as intracellular calcium release channels involved in signal transduction of various hormones in the kidney. Molecular cloning studies have shown that there are three types of IP3R, designated type 1, type 2, and type 3. To characterize their localizations in the rat kidney, we employed immunohistochemical studies using type-specific monoclonal antibodies that were raised against the 15 C-terminal amino acids of each type of IP3R. Type 1 was detected in glomerular mesangial cells and vascular smooth muscle cells. Type 2 was expressed exclusively in intercalated cells of collecting ducts from the cortex to the inner medulla. Type 3 was expressed in vascular smooth muscle cells, glomerular mesangial cells, and some cells of cortical collecting ducts, probably principal cells. As to the subcellular distribution, type 1 and type 2 showed a homogenous distribution in the cytoplasm, whereas type 3 was present mainly in the basolateral portion of the cytoplasm. These results indicate that IP3R isoforms were expressed in a cell-specific manner. The heterogeneous subcellular localizations among the IP3R types suggests compartmentalization of distinct IP3-sensitive Ca2+ pools.

Luminal and basolateral endothelin inhibit chloride reabsorption in the mouse thick ascending limb via a Ca(2+)-independent pathway

1. The recent localization of endothelin synthesis and receptors in the thick ascending limb (TAL) prompted us to investigate a possible autocrine and/or paracrine effect of this agent. The net chloride flux (JCl) has been determined in isolated cortical and medullary TALs by the in vitro microperfusion technique. 2. In both segments, endothelin 1 (ET-1) at 10(-8) M in the bath significantly decreased JCl, an effect which was partially reversible and observed at concentrations equal to or greater than 10(-13) M. 3. This JCl inhibition (by 33.9 +/- 3.2%) was blocked by BQ788 and was also observed with sarafotoxin 6C and ET-3, indicating that endothelin receptor B (ETB) are present in TAL. 4. ET-1 did not affect cAMP content under basal or hormone-stimulated conditions. The presence of a prostaglandin synthesis inhibitor also did not prevent the ET-1 action on JCl. 5. The ET-1-induced inhibition of JCl was prevented by protein kinase C inhibitors (staurosporine or GF 109203) and was reproduced by diacylglycerol analogues (OAG and DiC8). However, ET-1 failed to increase intracellular Ca2+ concentration. 6. Addition of ET-1 or ET-3 to the apical surface induced a decrease of JCl throgh ETB receptors, an effect which was not additive with that induced by basolateral ET-1, and was not concomitant with an increase in intracellular Ca2+ concentration. 7. It is concluded that the basolateral and luminal inhibitions of JCl by ET-1 in TAL, through ETB receptors, is mediated by a protein kinase C activation which is independent of intracellular Ca2+ increase.

The effect of big endothelin-1 in the proximal tubule of the rat kidney

1. An obligatory step in the biosynthesis of endothelin-1 (ET-1) is the conversion of its inactive precursor, big ET-1, into the mature form by the action of specific, phosphoramidon-sensitive, endothelin converting enzyme(s) (ECE). Disparate effects of big ET-1 and ET-1 on renal tubule function suggest that big ET-1 might directly influence renal tubule function. Therefore, the role of the enzymatic conversion of big ET-1 into ET-1 in eliciting the functional response (generation of 1,2-diacylglycerol) to big ET-1 was studied in the rat proximal tubules. 2. In renal cortical slices incubated with big ET-1, pretreatment with phosphoramidon (an ECE inhibitor) reduced tissue immunoreactive ET-1 to a level similar to that of cortical tissue not exposed to big ET-1. This confirms the presence and effectiveness of ECE inhibition by phosphoramidon. 3. In freshly isolated proximal tubule cells, big ET-1 stimulated the generation of 1,2-diacylglycerol (DAG) in a time- and dose-dependent manner. Neither phosphoramidon nor chymostatin, a chymase inhibitor, influenced the generation of DAG evoked by big ET-1. 4. Big ET-1-dependent synthesis of DAG was found in the brush-border membrane. It was unaffected by BQ123, an ETA receptor antagonist, but was blocked by bosentan, an ETA.B-nonselective endothelin receptor antagonist. 5. These results suggest that the proximal tubule is a site for the direct effect of big ET-1 in the rat kidney. The effect of big ET-1 is confined to the brush-border membrane of the proximal tubule, which may be the site of big ET-1 sensitive receptors.