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Hypertension. 2005;46:799-805
Published online before print September 19, 2005, doi: 10.1161/01.HYP.0000184251.01159.72
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(Hypertension. 2005;46:799.)
© 2005 American Heart Association, Inc.

Original Articles

Rat Strain Effects of AT1 Receptor Activation on D1 Dopamine Receptors in Immortalized Renal Proximal Tubule Cells

Chunyu Zeng; Zheng Wang; Ulrich Hopfer; Laureano D. Asico; Gilbert M. Eisner; Robin A. Felder; Pedro A. Jose

From the Department of Cardiology (C.Z.), Daping Hospital, Third Military Medical University, Chongqing, P.R. China; Departments of Pediatrics (C.Z., Z.W., L.D.A., G.M.E., P.A.J.) and Physiology and Biophysics (P.A.J.), and Internal Medicine (G.M.E.), Georgetown University Medical Center, Washington, DC; Department of Physiology and Biophysics (U.H.), Case Western Reserve School of Medicine, Cleveland, Ohio; Department of Pathology (R.A.F.), University of Virginia Health Sciences Center, Charlottesville, Va.

Correspondence to Chunyu Zeng, Department of Pediatrics, PHC-2, Georgetown University Medical Center, 3800 Reservoir Rd, NW, Washington, DC 20007. E-mail cyzeng1{at}hotmail.com


*    Abstract
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 *Abstract
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The dopaminergic and renin-angiotensin systems regulate blood pressure, in part, by affecting sodium transport in renal proximal tubules (RPTs). We have reported that activation of a D1-like receptor decreases AT1 receptor expression in the mouse kidney and in immortalized RPT cells from Wistar-Kyoto (WKY) rats. The current studies were designed to test the hypothesis that activation of the AT1 receptor can also regulate the D1 receptor in RPT cells, and this regulation is aberrant in spontaneously hypertensive rats (SHRs). Long-term (24 hours) stimulation of RPT cells with angiotensin II, via AT1 receptors increased total cellular D1 receptor protein in a time- and concentration-dependent manner in WKY but not in SHR cells. Short-term stimulation (15 minutes) with angiotensin II did not affect total cellular D1 receptor protein in either rat strain. However, in the short-term experiments, angiotensin II decreased cell surface membrane D1 receptor protein in WKY but not in SHR cells. D1 and AT1 receptors colocalized (confocal microscopy) and their coimmunoprecipitation was greater in WKY than in SHRs. However, AT1/D1 receptor coimmunoprecipitation was decreased by angiotensin II (10–8M/24 hours) to a similar extent in WKY (–22±8%) and SHRs (–22±12%). In summary, these studies show that AT1 and D1 receptors interact differently in RPT cells from WKY and SHRs. It is possible that an angiotensin II-mediated increase in D1 receptors and dissociation of AT1 from D1 receptors serve to counter regulate the long-term action of angiotensin II in WKY rats; different effects are seen in SHRs.


Key Words: dopamine • kidney • receptors, angiotensin II • rats, spontaneously hypertensive


*    Introduction
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 *Introduction
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Angiotensin II and dopamine are important regulators of sodium and water transport across the renal proximal tubule (RPT).1–8 Angiotensin II receptor subtypes (AT1, AT2, and AT4) are expressed in brush border and basolateral membranes of RPTs.1–5 The activation of AT1 receptors by low concentrations of angiotensin II (picomolar) causes an increase in sodium reabsorption in RPTs.1,3,6 All the dopamine receptor subtypes, D1-like (D1 and D5) and D2-like (D2, D3, and D4) are also expressed in brush border and basolateral membranes of RPTs.7–10 In contrast to the stimulatory effect of AT1 receptors on renal sodium transport, activation of D1 and D3 receptors decreases renal sodium reabsorption.7,8,11–13

Several reports have shown an interaction between the dopamine and renin-angiotensin system. Intrarenally produced angiotensin II opposes the natriuretic action of the D1- like dopamine receptor agonist, fenoldopam, in rats.14 D2-like receptor agonists also antagonize the stimulatory effect of angiotensin II, acting via AT1 receptors, on renal proximal tubular luminal sodium transport.15,16 When angiotensin II generation is inhibited or AT1 receptors are blocked, the natriuretic effect of dopaminergic drugs is enhanced.17

The AT1 receptor mediates the anti-natriuretic effect of angiotensin II, whereas the D1 dopamine receptor is responsible for {approx}80% of D1-like receptor activity in RPTs.18 We have reported that in RPT cells of Wistar-Kyoto (WKY) rats, the D1-like agonist, fenoldopam, increased D1 receptor but decreased AT1 receptor protein expression. In contrast, in RPT cells of spontaneously hypertensive rats (SHRs), fenoldopam also decreased AT1 receptor expression but no longer stimulated D1 receptor expression. We also reported that AT1 and D1 receptors coimmunoprecipitate in RPT cells; fenoldopam increased this coimmunoprecipitation in WKY RPT cells but decreased it in SHR RPT cells.19 We hypothesize that the AT1 receptor may also regulate the D1 receptor, including its expression. Therefore, we studied the effect of angiotensin II on D1 receptor expression and AT1 and D1 receptor interaction in immortalized rat RPT cells. We have previously shown that the D1 receptor effects on signal transduction and sodium transporters in immortalized RPT cells are similar to those obtained in freshly obtained renal proximal tubules.20–22 Parenti et al have also reported that the AT1 receptor behaves similarly in freshly obtained renal tubules and immortalized RPT cells.23

We now report that AT1 and D1 receptors colocalize in RPT cells. D1 and AT1 receptor coimmunoprecipitation was greater in WKY than in SHRs, in agreement with our previous report.19 In this earlier study, long-term fenoldopam stimulation (24 hours) increased the coimmunoprecipitation of D1 and AT1 receptors in WKY but decreased it in SHRs.19 We now show that angiotensin II decreases D1 and AT1 coimmunoprecipitation similarly in WKY and SHRs. However, D1 receptor expression is increased by angiotensin II in WKY but not in SHR. In contrast, short-term angiotensin II, via AT1 receptors (15 minutes), decreases cell surface membrane expression of D1 receptors in WKY but not in SHRs. These studies show that angiotensin II differentially affects total cellular and surface membrane D1 receptor expression in RPT cells from WKY and SHRs.


*    Methods
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Cell Culture
Immortalized RPT cells from 4- to 8-week-old WKY and SHRs were cultured at 37°C in 95% air/5% CO2 atmosphere in DMEM/F-12 culture media, as previously described.21–26 The cells (80% confluence) were extracted in ice-cold lysis buffer (phosphate-buffered saline with 1% NP40, 0.5% sodium deoxycholate, 0.1% SDS, 1 mmol/L EDTA, 1 mmol/L EGTA, 1 mmol/L PMSF, 10 µg/mL aprotinin, and 10 µg/mL leupeptin), sonicated, kept on ice for 1 hour, and centrifuged at 16 000g for 30 minutes. The supernatants were stored at –70°C until use for immunoblotting and/or immunoprecipitation.

Immunoblotting
The antibodies are polyclonal purified antipeptides. The amino acid sequence for the immunogenic peptide (rabbit anti-human AT1 receptor antibody) is QDDCPKAGRHC, amino acids 15 to 24 of the AT1 receptor. The specificity of this antibody to the AT1 receptor is indicated as previously reported.27,28 The amino acid sequence for rat D1 receptor antibody corresponds to positions 299 to 307 (GSEETQPFC) on D1 receptor (Research Genetics). The specificity of this D1 receptor antibody has also been reported.19,29 Rat RPT cells were treated with vehicle (dH2O), angiotensin II or an AT1 receptor antagonist (losartan) at the indicated concentrations and times. Immunoblotting was performed as previously reported,22,26 except that the transblots were probed with the D1 (1:800) or the AT1 receptor antibody (1:400). The amount of protein transferred onto the membranes was determined by Ponceau-S staining and immunoblotting for {alpha}-actin.19

Immunofluorescence Confocal Microscopy of Double-Stained RPT Cells
RPT cells, grown on coverslips, were fixed with 3% paraformaldehyde (30 minutes) and permeabilized with 0.1% Triton X-100 in phosphate-buffered saline (15 minutes). Reactions with antibodies were performed as described previously.22,30 The D1 and AT1 receptors were visualized by using a rabbit antipeptide polyclonal IgG affinity-purified rat D1 receptor antibody followed by fluorescein isothiocyanate-conjugated anti-rabbit secondary antibody (Molecular Probes) or by a mouse anti-human AT1 receptor monoclonal antibody followed by the secondary goat anti-mouse antibody labeled with Alexa 568 (Molecular Probes).22 Cells on coverslips were mounted with the ProLong Antifade Kit (Molecular Probes). The immunofluorescence densities and images were acquired (Olympus AX70) at an excitation wavelength of 488 nm and 568 nm; emission was detected at 535 and 645 nm.22,30

Immunoprecipitation
RPT cells were incubated with vehicle or angiotensin II (10–8 M) for 24 hours, as described. The cells were lysed with ice-cold lysis buffer for 1 hour and centrifuged at 16 000g for 30 minutes. Equal amounts of lysates (500 µg protein/mL supernatant for RPT cells from both SHR and WKY rats) were incubated with affinity-purified anti-D1 receptor antibody (2 µL/mL) for 1 hour and protein-G agarose at 4°C for 12 hours. The immunoprecipitates were pelleted and washed 4 times with lysis buffer. The pellets were suspended in sample buffer, boiled for 10 minutes, and subjected to immunoblotting with the AT1 receptor antibody. To determine the specificity of the bands, pre-immune serum of D1 receptor antibody (negative control) and AT1 receptor antibody (positive control) were used as immunoprecipitants, instead of the D1 receptor antibody. The densities of the bands were quantified by densitometry using Quantiscan (Ferguson, Mo) as previously reported.19,22,26

Cell Surface D1 Receptor Expression
Cultured RPT cells were starved in serum-free medium for 2 hours, and then treated with angiotensin II (10–8 M) for 15 minutes. Surface membrane proteins were biotinylated by adding sulfo-NHS-LC-biotin (final concentration 250 µg/mL) into the medium 5 minutes before adding angiotensin II. The cells were washed 3 times with wash buffer. Then, the cells were lysed with lysis buffer, sonicated and placed on ice for 1 hour. The supernatant from the cell lysate was immunoprecipitated with the anti-rat D1 receptor antibody, followed by immunoblotting. The membrane sheets were blocked with 5% milk overnight and after washing (3 times), the sheets were incubated with peroxidase-conjugated streptavidin (Jackson ImmunoResearch Laboratory, Inc, in wash buffer at 1/5000 dilution for 30 minutes. The biotinylated protein bands were visualized by enhanced chemiluminescence (Western Blotting Detection Kit; Amersham, Arlington Heights, Ill).31

Materials
Rabbit anti-human AT1 receptor antibodies were purchased from Santa Cruz Biotechnology, Inc (Santa Cruz, Calif). Rabbit anti-rat D1 receptor antibody was produced against a synthetic oligopeptide from the amino acid sequence of rat D1 receptor (amino acids 299 to 307) (Research Genetics). Monoclonal mouse anti-human AT1 receptor was purchased from Abcam Limited (Cambridge, UK). Angiotensin II was purchased from Peninsula Laboratory, Inc (St Louis, Mo). Losartan was a gift from Merck & Co (Philadelphia, Pa). Peroxidase-conjugated streptavidin was purchased from Jackson ImmunoResearch Laboratory, Inc (West Grove, Pa). Sulfo-NHS-LC-biotin was purchased from PIERCE (Rockford, Ill). Other chemicals for various buffers were of the highest purity available and purchased either from Sigma or Gibco.

Statistical Analysis
The data are expressed as mean±SEM. Comparison within groups was made by ANOVA for repeated measures, and comparison among groups was made by factorial ANOVA with Duncan’s test. P<0.05 was considered significant.


*    Results
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 *Results
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Activation of AT1 Receptors Increases D1 Receptor Protein in RPT Cells From WKY Rats, But Not From SHRs
In RPT cells from WKY rats, angiotensin II increased D1 receptor expression in a concentration- and time-dependent manner. The stimulatory effect was evident at 10–9 M (Figure 1A) as early as 16 hours and maintained for at least 30 hours (Figure 1B). Consistent with the stimulatory effect of angiotensin II on D1 receptor expression (Figure 1A and 1B), a 24 hour-incubation with angiotensin II (10–8 M) increased D1 receptors in RPT cells from WKY rats. In contrast, this effect was not observed in RPT cells from SHRs (WKY: control=0.77±0.15, angiotensin II=1.33±0.19; SHR: control=0.87±0.13, angiotensin II=1.04±0.17; n=7) (Figure 1C).



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  		Figure 1. Effect of angiotensin II on D1 receptor protein in RPT cells from WKY and SHRs. A, Concentration-response of D1 receptor protein in RPT cells from WKY rats treated with angiotensin II. Immunoreactive D1 receptor protein was determined after 24 hour incubation with the indicated concentrations of angiotensin II. Results are expressed as relative density units (DU) (n=8, *P<0.05 vs control (C), ANOVA, Duncan’s test). B, Time-course of D1 receptor protein in RPT cells from WKY rats treated with angiotensin II. The cells were incubated for the indicated times with 10–8 M angiotensin II. Results are expressed as relative density units (DU) (n=8, *P<0.05 vs control [0 time], ANOVA, Duncan’s test). C, Effect of angiotensin (10–8 M/24 hours) on D1 receptor protein in RPT cells from WKY and SHRs. Results are expressed as the ratio of D1 receptor to {alpha}–actin densities (n=7, *P<0.05 vs control, ANOVA, Duncan’s test). D, Effect of angiotensin II (Ang) and an AT1 receptor antagonist (losartan) on D1 receptor protein in WKY rat RPT cells. The cells were incubated with the indicated reagents (angiotensin II, 10–8 M; losartan, 10–8 M) for 24 hour. Results are expressed as the ratio of D1 receptor to {alpha}–actin densities (n=8, *P<0.05 vs others, ANOVA, Duncan’s test). E, Concentration-response of D1 receptor protein in SHR RPT cells treated with angiotensin II. Immunoreactive D1 receptor protein was determined after a 24 hour-incubation with the indicated concentrations of angiotensin II. Results are expressed as relative density units (DU) (n=6, P=NS vs control (C), ANOVA, Duncan’s test).

To determine the specificity of angiotensin II action on AT1 receptors, we studied the effect of the AT1 receptor antagonist, losartan. Consistent with the results in WKY rats in Figure 1A, 1B, and 1C, angiotensin II (10–8 M/24 hours) increased D1 receptor expression (control=0.83±0.15, angiotensin II=1.48±0.15; n=8). The AT1 receptor antagonist, losartan (10–8 M), by itself, had no effect on D1 receptor expression (0.8±0.13) but reversed the stimulatory effect of angiotensin II on D1 receptor expression (0.7±0.08) (Figure 1D).

To investigate whether there is a right-shift of the concentration-response curve in SHRs, their RPT cells were incubated with varying concentrations of angiotensin II (10–9 M to 10–5 M) for 24 hours. Consistent with the results in Figure 1C, angiotensin II had no effect on D1 receptor expression in SHRs (Figure 1E).

AT1 Receptor Colocalizes With the D1 Receptor in Rat RPT Cells
Immunofluorescence laser confocal microscopy revealed a colocalization of D1 and AT1 receptors in WKY rat RPT cells (Figure 2). To determine whether there is a physical interaction between the D1 and the AT1 receptor, additional experiments were performed. As shown in Figure 3, in the basal state, the band of 45 kDa, representing the coimmunoprecipitated D1 and AT1 receptors, was greater in RPT cells from WKY rats (37±1 DU) than in those from SHRs (23±3 DU, n=7, P<0.05), similar to the findings in our previous report.19 A 24-hour incubation with angiotensin II (10–8 M) decreased D1/AT1 receptor coimmunoprecipitation to a similar degree in RPT cells from WKY rats and SHRs (WKY: control=37±1 density units (DU), angiotensin II=28±2 DU; SHRs: control=23±3 DU, angiotensin II=17±3 DU; n=7, P<0.05).



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  		Figure 2. D1 and AT1 receptor colocalization in rat RPT cells. Colocalization appears as yellow after merging the images of fluorescein isothiocyanate-conjugated anti-rabbit secondary antibody (green), directed against the anti-rat D1 receptor and Alexa 568-conjugated goal anti-mouse secondary antibody (red), directed against mouse anti-human AT1 receptor (red).



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  		Figure 3. Effect of angiotensin II on the coimmunoprecipitation of D1 and AT1 receptors in rat RPT cells. The cells were incubated with angiotensin II (10–8 M) for 24 hour. Results are expressed as relative density units (DU) (* P<0.05 vs control, #P<0.05, vs WKY, n=7, ANOVA, Duncan’s test). One immunoblot (45 kDa) is depicted in the inset: (lane 1=positive control, lane 2=negative control, lane 3=vehicle-treated RPT cells from WKY rats, lane 4=angiotensin II-treated RPT cells from WKY rats, lane 5=vehicle-treated RPT cells from SHRs, lane 6=angiotensin II-treated RPT cells from SHRs).

Activation of AT1 Receptors Decreases the D1 Receptor Expression in Surface Membranes of RPT Cells From WKY Rats But Not From SHRs
The effect of short-term stimulation with angiotensin II on cell surface D1 receptors is shown in Figure 4. Angiotensin II (10–8 M) decreased the quantity of D1 receptors on the cell surface membranes in WKY RPT cells, but not in SHR RPT cells (WKY: control=45±9 DU, angiotensin II=17±5 DU; SHR: control=19±3 DU, angiotensin II=19±5 DU, n=6).



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  		Figure 4. Effect of angiotensin II on cell surface D1 receptors in rat RPT cells. The cells were incubated with angiotensin II (10–8 M) for 15 minutes. Results are expressed as relative density (DU) (*P<0.05 vs control, #P<0.05 vs WKY, n=6, ANOVA, Duncan’s test). One immunoblot is depicted on top of the bar graphs.


*    Discussion
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 *Discussion
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Polygenic essential hypertension is associated with increased sodium transport in renal proximal tubules.32,33 As mentioned, dopamine and angiotensin II are important regulators of sodium and water reabsorption in the kidney, serving contrasting functions in the proximal tubule.1,5,17,34,35 Both dopamine and renin-angiotensin systems exist in the RPT, and the components of the renin-angiotensin system, including angiotensinogen mRNA, renin, and angiotensin-converting enzyme, have been localized to the proximal tubule.1,6,36,37 The proximal tubule is also the site of local dopamine production.38–40 Urinary dopamine and angiotensin II concentrations exceed circulating levels, suggesting both systems may have autocrine and/or paracrine effects in RPTs.1,6,38,41 Angiotensin II stimulates sodium reabsorption via activation of sodium transporting proteins that are inhibited by dopamine (NHE3, Cl/HCO3 exchanger, Na/HCO3 cotransporter, and Na+/K+ ATPase).34,35 Dopamine and D1-like and D2-like receptor agonists have been reported to antagonize the actions of angiotensin II on NHE3 and Na+K+-ATPase activities in brush border and basolateral membranes of renal proximal tubules.15,16 Dopamine, via D1-like receptors, has also been reported to decrease the AT1 receptor mRNA and protein, as well as angiotensin II binding sites in renal proximal tubules.19,42 The effect of angiotensin II on D1-like receptors has not been reported.

In WKY RPT cells, stimulation of the AT1 receptor with angiotensin II for 2 different periods (short-term or long-term) yields opposite results; short-term stimulation decreases cell surface D1 receptor expression, whereas long-term stimulation increases whole cell D1 receptor expression. It is unlikely that the change in the short-term could account for the long-term result. A decrease of cell surface D1 receptor expression would remove a restraint allowing AT1 receptor activation to increase sodium reabsorption to a greater extent. However, the increase in D1 receptor expression would tend to counterbalance the prolonged angiotensin II effect.

Cell surface membrane localization of G protein-coupled receptors is decreased by phosphorylation of the receptor.8,43 We have reported that G protein-coupled receptor kinases (GRKs), GRK2 and GRK4, are involved in D1 receptor phosphorylation.44 Among the GRKs, GRK2 is most important in promoting AT1 receptor phosphorylation.45 It is possible that the GRK2 activation subsequent to AT1 receptor stimulation also results in the alteration of the phosphorylation and membrane localization of the D1 receptor. Such a scenario can explain the ability of AT1 receptors to decrease the membrane localization of the D1 receptor in WKY. This does not occur in SHRs because the D1 receptor is already hyperphosphorylated and fewer D1 receptors are found on renal cell surface membranes in SHR.46 Although GRK4 is the major GRK regulating D1 receptor function,43,44 GRK4 is not involved in the desensitization of the AT1 receptor.47 The possibility exists that the AT1 receptor can cause heterologous desensitization of the D1 receptor via stimulation of effector enzymes, but it is not clear which one.48

The dephosphorylation of D1 receptors in renal proximal tubules is also different in WKY and SHRs. We have reported that short-term D1-like receptor stimulation increases protein phosphatase 2A activity in renal proximal tubules in WKY but not in SHRs49. It is possible that in the WKY, the AT1 receptor inhibits protein phosphatase 2A activity and thus prevents dephosphorylation and recycling of the D1 receptor to the cell surface membrane. Although there is no report to support this mechanism, AT2 receptor has been reported to stimulate protein phosphatase 2A activity.50,51

Long-term stimulation of AT1 receptors increases total cellular D1 receptor expression in WKY. This finding in vitro was also found in vivo; chronic angiotensin II infusion in mice also caused an increase in D1 receptor expression (unpublished data). Presumably, cell surface D1 receptor expression is also increased. Although the mechanism by which AT1 receptor stimulation increases D1 receptor expression remains to be determined, such an effect could serve as a physiological negative feedback under normal circumstances (in WKY). However, in SHR cells, this upregulation of D1 receptors by stimulation of AT1 receptors is lost because of decreased responsiveness of the hyperphosphorylated D1 receptor.46,52,53 We have reported that the renal D1 receptor is hyperphosphorylated in SHRs as a consequence of increased activity of GRK454 and decreased protein phosphatase 2A activity.49

Long-term incubation with angiotensin II decreases D1/AT1 receptor coimmunoprecipitation to the same degree in cells from SHR and WKY rats. Our previous study showed that long-term exposure to angiotensin II decreases AT1 receptor protein expression in RPT cells from WKY rats but increases it in SHRs.26 The decreased interaction between D1 and AT1 receptors in renal proximal tubules in WKY rats is, therefore, not the result of decreased AT1 receptor expression, per se, because a similar response is seen in SHRs where AT1 expression is increased by angiotensin II. Thus, angiotensin II, via the AT1 receptor, probably causes a decrease in the physical interaction of AT1 and D1 receptors in both WKY and SHR cells. Further studies are needed to determine whether the decreased interaction between these 2 receptors is direct or indirect, possibly by the alteration of an adaptor gene or adaptor proteins.

In both SHR and Dahl salt-sensitive rats, dopamine and D1-like receptor agonist-mediated natriuretic and diuretic responses are impaired.8,43 The impaired D1-like receptor function in hypertension is not caused by abnormalities in G proteins, effectors, or ion transporting proteins, such as adenylyl cyclase, NHE3 or Na+-K+ATPase. Rather, the renal D1-like receptor is uncoupled from G protein subunits, leading to decreased D1-like receptor interaction with G protein subunits, and resulting in decreased production of second messengers and decreased interaction between G protein subunits, effector enzymes, and ion transporters.8,43,53 To determine whether the impaired D1 receptor function occurs at the receptor level, we investigated the expression of D1 receptor expression in cell surface membranes and whole cells. We found that cell surface membrane expression of D1 receptors is lower and D1 receptor phosphorylation is higher in SHR cells than in WKY cells, although total and D1 receptor expression are not different in the 2 cell lines.46,54 Renal D1 receptor phosphorylation and GRK4 activity impact on the regulation of blood pressure. Inhibition of renal GRK4 activity by antisense oligonucleotides attenuates the increase in blood pressure with age in SHRs without affecting blood pressure in WKY rats.55

In summary, we have demonstrated that long-term stimulation of AT1 receptors positively regulates the expression of D1 receptors in RPT cells from WKY but not from SHRs. AT1 and D1 receptors colocalize in RPT cells from both rat strains, but the basal level of cell surface membrane D1 receptor expression is higher in WKY than in SHR RPT cells. It appears that a greater proportion of D1 receptors are located on the cell surface membrane in WKY cells than in SHR cells. Short-term AT1 receptor stimulation decreases cell surface membrane D1 receptor expression in WKY rats but not in SHRs. It is possible that differences in AT1 receptor regulation of D1 receptor expression and cell surface expression may participate in the abnormal regulation of renal proximal sodium transport in genetic hypertension.

Perspectives
The dopaminergic and renin-angiotensin system are 2 important systems that regulate blood pressure.1–8 Dopamine promotes natriuresis, whereas angiotensin II decreases sodium excretion.1–8 The major D1-like receptor subtype mediating the increase in sodium excretion is probably the D1 receptor, whereas the major angiotensin II receptor mediating the decrease in renal sodium excretion is the AT1 receptor.1–8 In SHRs, renal proximal tubular D1 receptor function is impaired.7,8 Regardless of the mechanism involved in the interaction between D1 and AT1 receptors, the dissociation of D1 from AT1 receptors after angiotensin II stimulation may allow the AT1 and D1 receptor to exert their functions separately. Renal proximal tubular D1 receptors are functional in WKY but not in SHRs. In contrast, AT1 receptor function is enhanced in SHRs.32 The inability of AT1 receptors to increase D1 receptor expression in renal proximal tubules in SHRs and a decreased D1 and AT1 receptor interaction could lead to enhanced AT1 receptor function.


*    Acknowledgments
 
These studies were supported in part by grants from the National Institutes of Health, HL23081, DK39308, HL68686, DK52612, HL074940, HL41618, and National Natural Science Foundation of China, 30470728.

Received June 23, 2005; first decision July 11, 2005; accepted August 18, 2005.


*    References
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 *References
 

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