标题: Adenosine inhibits ENaC via cytochrome P -450 epoxygenase-dependent metabolites [打印本页] 作者: 轻羽 时间: 2009-4-22 08:44 标题: Adenosine inhibits ENaC via cytochrome P -450 epoxygenase-dependent metabolites
作者：Yuan Wei, Peng Sun, ZhiJian Wang, BaoFeng Yang, Mairead A. Carroll, and Wen-Hui Wang作者单位：1 Department of Pharmacology, New York Medical College, Valhalla, New York; and 2 Department of Pharmacology, Harbin Medical University, Harbin, China ) x4 i0 d0 f: |) T$ b
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- S7 `% P6 `3 A 【摘要】 ! V- E( i" H+ {7 A: y& f. U- {- {% l! L We used the patch-clamp technique to examine the effect of adenosine on epithelial sodium channel (ENaC) activity in rat cortical collecting duct (CCD). Application of adenosine inhibits ENaC activity, and the effect of adenosine was mimicked by cyclohexyladenosine (CHA), an A 1 adenosine-receptor agonist that reduced channel activity from 1.32 to 0.64. The inhibitory effect of CHA on ENaC was mimicked by cyclopentyladenosine (CPA), which reduced channel activity from 1.1 to 0.55. In contrast, application of CGS-21680, an A 2a adenosine-receptor agonist, had no effect on ENaC and increased channel activity from 0.96 to 1.22. This suggests that the inhibitory effect of adenosine analogs resulted from stimulation of the A 1 adenosine receptor. Inhibition of PLC with U-73122 failed to abolish the effect of CHA on ENaC. In contrast, the inhibitory effect of CHA on ENaC was absent in the presence of the PLA 2 inhibitor arachidonyl trifluoromethyl ketone (AACOCF 3 ). This suggests a role of arachidonic acid (AA) in mediating the effect of adenosine on ENaC. To determine the metabolic pathway of AA responsible for the effect of adenosine, we examined the effect of CHA in the presence of indomethacin or N -methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MS-PPOH). Inhibition of cytochrome P -450 (CYP) epoxygenase with MS-PPOH blocked the effect of CHA on ENaC. In contrast, CHA reduced ENaC activity in the presence of indomethacin. This suggests that CYP epoxygenase-dependent metabolites of AA mediate the effect of adenosine. Because 11,12-epoxyeicosatrienoic acid (11,12-EET) inhibits ENaC activity in the CCD (Wei Y, Lin DH, Kemp R, Yaddanapudi GSS, Nasjletti A, Falck JR, and Wang WH. J Gen Physiol 124: 719-727, 2004), we examined the role of 11,12-EET in mediating the effect of adenosine on ENaC. Addition of 11,12-EET inhibited ENaC channels in the CCD in which adenosine-induced inhibition was blocked by AACOCF3. We conclude that adenosine inhibits ENaC activity by stimulation of the A 1 adenosine receptor in the CCD and that the effect of adenosine is mediated by 11,12-EET. 3 L' |+ C& c% [' t9 O2 I0 u 【关键词】 epithelial sodium channel phospholipase A phospholipase C protein kinase C collecting duct adenosine receptor, `2 r" A% ~8 [: x& p9 A/ t
THE CORTICAL COLLECTING DUCT (CCD) and connecting tubule (CT) play an important role in the hormone-regulated Na absorption and K secretion ( 3, 4, 8, 19 ). Na transport in the CCD and CT takes place by a two-step process: Na enters the cell through epithelial Na channels (ENaC) across the apical membrane and is extruded via Na-K-ATPase in the basolateral membrane. It is generally accepted that apical Na permeability or ENaC activity is a rate-limiting step for Na absorption in the CCD and CT ( 3, 25, 27 ). ENaC activity in the CT and CCD is regulated by hormones such as aldosterone ( 11, 18, 27 ) and by a Na diet such that a high-Na diet has been shown to suppress apical Na conductance ( 16 ) and expression of the ENaC -subunit ( 12 ). Although decreased plasma aldosterone levels induced by high Na intake should play a key role in the downregulation of ENaC activity, it is possible that factors other than aldosterone also may be involved in inhibiting ENaC activity. In this regard, it has been shown that high Na intake increases adenosine concentrations in the kidney ( 32 ). However, the role of adenosine in the regulation of ENaC activity in the CCD is not completely understood.7 e0 y# Q$ {8 ^( b: k4 e) K
, ~- f; Y- x7 F0 x# JSeveral studies have shown that the A 1 adenosine receptor is expressed in the CCD ( 28, 30 ). Thus it is conceivable that increased adenosine levels induced by high Na intake may activate A 1 adenosine receptors in the CCD. Stimulation of A 1 adenosine receptor has been shown to decrease cAMP production, increase intracellular Ca 2 , and stimulate phospholipase A 2 (PLA 2 ) ( 1 ) ( 23 ). Because decreases in cAMP or increases in intracellular Ca 2 or arachidonic acid (AA) have been reported to inhibit ENaC activity ( 6, 20, 29 ), it has been suggested that stimulation of the adenosine receptor may affect ENaC activity. Therefore, the aims of the present study were to examine the effect of adenosine on ENaC activity and to illustrate the mechanism by which adenosine inhibits ENaC activity. 3 y& C9 f) |4 f6 Z: n 8 O c$ A% _9 M# w& ?METHODS- } i- K1 b( V
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Preparation of CCDs. Pathogen-free Sprague-Dawley rats of either sex (5-6 wk) were purchased from Taconic Farms (Germantown, NY). Rats were maintained on a Na -deficient diet for 3-5 days to increase the surface expression of ENaC. Rats were killed by cervical dislocation, and the kidneys were removed immediately. The animal use protocol was reviewed and approved by the institutional animal care and use committee of New York Medical College. We cut the kidneys into several thin slices ( " Z8 p( c A6 t4 `: F' @ I e) e) K( u! t, N7 H7 B
Patch-clamp technique. An Axon 200A patch-clamp amplifier was used to record channel current, which was low-pass filtered at 50 Hz by an eight-pole Bessel filter (902LPF; Frequency Devices, Haverhill, MA). The Na current was recorded and digitized by an Axon interface (Digidata 1200). Data were analyzed using pCLAMP software (system 7.0; Axon). Channel activity defined as NP o was calculated from data samples of 60-s duration in the steady state as follows:' X9 O. r+ A. n% `# ]
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( 1 )( ?$ b: B K. `5 ]$ @$ r
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where t i is the fractional open time spent at each of the observed current levels. The channel conductance was calculated by recording the current at three holding potentials. Because ENaC channel numbers varied in each patch from 1 to more than 10, it was very hard to determine the real channel-closure line if more than five channels were in the patch. Thus we selected the patches in which fewer than five channel current levels were identified. Also, we sometimes used a ruler to measure the channel closed and open duration if channel activity could not be analyzed with software. # `3 w* {$ _" G # y* [( [$ }1 @Solution and statistics. The bath solution contained (in mM) 140 NaCl, 5 KCl, 1.8 CaCl 2, 1.8 MgCl 2, and 10 HEPES (pH 7.4). The pipette solution was composed of (in mM) 140 NaCl, 1.8 MgCl 2, and 5 HEPES (pH 7.4). Indomethacin, adenosine, cyclohexyladenosine (CHA), cyclopentyladenosine (CPA), and CGS-21680 were purchased from Sigma (St. Louis, MO), and 11,12-epoxyeicosatrienoic acid (11,12-EET), the trifluoromethyl ketone analog of arachidonic acid (AACOCF 3 ), and U-73122 were obtained from Biomol (Plymouth Meeting, PA). N -methylsulfonyl-12,12-dibromododec-11-enamide (DDMS) and N -methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MS-PPOH) were synthesized by Dr. J. R. Falck's laboratory (Southwestern Medical Center, Dallas, TX). The data are presented as means ± SE. We used paired and unpaired Student's t- tests to determine the statistical significance. If the P value was ( K+ i8 w3 d0 V" t2 u: G" P# d2 [1 Y' U7 Y$ s3 @
RESULTS 7 ^5 O) N! d$ l7 s: _5 @: z, t: R! F% t- ?
The ENaC activity in the CCD from rats on a Na -deficient diet for 3-5 days varied from 0.1 to 3.21, and the mean NP o was 1.36 ± 0.18 ( n = 20). We first examined the effect of adenosine (10 µM) on ENaC in the CCD, and Fig. 1 shows a recording indicating that adenosine inhibited ENaC activity from 1.5 ± 0.2 to 0.6 ± 0.1 ( n = 5). Because the A 1 adenosine receptor has been shown to be expressed in the CCD, we studied the effect of adenosine analog on ENaC activity in cell-attached patches. Figure 2 shows a channel recording indicating the effect of CHA (10 µM) on ENaC. It is apparent from Fig. 2 that the addition of CHA inhibited ENaC activity. Typically, we were able to see the inhibitory effect of CHA within 10 min. Although we observed a complete recovery of ENaC activity after washout of CHA in only one of seven patches, the effect of CHA was reversible, because we observed the ENaC activity after washout when we patched the same cell again. The low success rate in observing the full recovery was due to the fact that it was very hard technically to hold the same patch for over 30 min. Data summarized in Fig. 3 show that CHA reduced channel activity from 1.32 ± 0.42 to 0.64 ± 0.27 ( n = 7). ! c4 Q N: v$ a" `9 m 7 ~" x5 i; }3 K" h! QFig. 1. A channel recording showing the effect of 10 µM adenosine on epithelial Na channels (ENaC) in a cell-attached patch. The channel closed level (C) is indicated by a dotted line. Holding potential was -60 mV (hyperpolarization). ' E9 Y/ {0 u- R$ t/ n$ N8 Y4 K0 v$ B: [1 W6 x* G# [2 s" q, Y
Fig. 2. A channel recording showing the effect of 10 µM cyclohexyladenosine (CHA) on ENaC in a cell-attached patch. The channel-closed level (C) is indicated by a dotted line. . U: q% i" `" O& x9 @ 0 d3 O F6 ?8 e+ f# w! OFig. 3. Effect of CHA (10 µM), cyclopentyladenosine (CPA; 1 µM), and CGS-21680 (CGS; 1 µM) on ENaC activity. The experiments were performed in cell-attached patches. NP o, channel activity. * P $ U" z+ _ x' ?6 ^1 a
; s/ @( ]' a* nAlthough CHA is an agonist of the A 1 adenosine receptor ( 5 ), CHA at high concentrations can also increase cAMP production ( 26 ). Thus we used CPA, another agonist of the A 1 adenosine receptor, to determine whether CPA could mimic the effect of CHA and inhibit ENaC. Data summarized in Fig. 3 demonstrate that addition of CPA (1 µM) decreased ENaC activity in cell-attached patches from 1.1 ± 0.13 to 0.55 ± 0.1 ( n = 8). The hypothesis that the inhibitory effects of CHA and CPA are the result of stimulating the A 1 adenosine receptor was further supported by the observation that addition of CGS-21680, an agonist of the A 2a adenosine receptor ( 17 ), had no significant effect on ENaC activity ( Fig. 3 ). Application of 1 µM CGS-21680 increased NP o from 0.96 ± 0. 20 to 1.22 ± 0. 22 ( n = 4). However, the difference was not significant.% v# Y0 u: H+ z2 i
$ J9 y' J* o: N- i2 }After demonstrating that the inhibitory effect of the adenosine analog was mediated by stimulation of the A 1 adenosine receptor, we examined the signaling pathway that mediates the effect of adenosine. Stimulation of the A 1 adenosine receptor has been shown to activate PLC and PLA 2 ( 23 ). Thus we first examined the effect of CHA on ENaC in the presence of the PLC inhibitor. Figure 4 summarizes results from eight patches in which the effect of CHA on ENaC was examined in the presence of U-73122 (1 µM). Inhibition of PLC did not significantly alter channel activity (control, 1.36 ± 0.20 and U-73122, 1.38 ± 0.26). However, in the presence of U-73122, addition of CHA significantly reduced ENaC activity to 0.9 ± 0.1 ( n = 8). We also examined the effect of inhibiting PLA 2 on ENaC activity and observed that addition of AACOCF 3 (1 µM), an inhibitor of PLA 2, did not change channel activity (control, 1.26 ± 0.2 and AACOCF 3, 1.28 ± 0.21) ( Fig. 4 ). We then tested the effect of CHA on ENaC in the continuous presence of AACOCF 3. Figure 5 shows a typical channel recording indicating that addition of CHA failed to inhibit ENaC activity in the presence of AACOCF 3. From six experiments, NP o after CHA was 1.18 ± 0.17 ( n = 6), a value that was not significantly different from the control value (1.28 ± 0.21). Thus blockade of PLA 2 abolished the inhibitory effect of CHA.% t7 H" L; G6 j* d
2 K$ X, O$ Q1 B# O/ M) `" }7 [5 BFig. 4. Effect of CHA on ENaC in the presence of arachidonyl trifluoromethyl ketone (AACOCF 3; 1 µM) and U-73122 (1 µM). The experiments were carried out in cell-attached patches. * P ! @( F4 a6 ^, E- Y
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Fig. 5. A channel recording showing the effect of CHA on ENaC in the presence of AACOCF 3. The channel-closed level (C) is indicated by a dotted line.) J( w$ _- w) ^ |
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We have previously shown that AA inhibited ENaC activity in the CCD and that the inhibitory effect of AA was mediated by 11,12-EET ( 29 ). After showing that PLA 2 was involved in mediating the effect of the adenosine analog on ENaC, we explored whether cytochrome P -450 (CYP) epoxygenase-dependent AA metabolites were responsible for the effect of stimulating the A 1 adenosine receptor. We examined the effect of CHA on ENaC in the presence of MS-PPOH, an inhibitor of CYP epoxygenase. We confirmed the previous finding that inhibition of CYP epoxygenase slightly increased ENaC activity (1.68 ± 0.29, n = 5) ( Fig. 6 ). Moreover, in the presence of MS-PPOH, application of CHA did not significantly alter ENaC activity ( Fig. 7 ), which was 1.50 ± 0.25 ( Fig. 6 ). In contrast, inhibition of cyclooxygenase did not abolish the inhibitory effect of CHA, because CHA reduced channel activity to 0.62 ± 0.2 in the presence of indomethacin ( Fig. 6 ). Thus inhibition of CYP epoxygenase abolished the effect of CHA on ENaC.$ H8 I' ^4 \$ b
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Fig. 6. Effect of CHA on ENaC in the presence of N -methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MS-PPOH; 5 µM) and indomethacin (5 µM). The experiments were carried out in cell-attached patches. * P : ~' j; o" D- d9 a! f5 C
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Fig. 7. A channel recording showing the effect of CHA on ENaC in the presence of MS-PPOH. The channel-closed level (C) is indicated by a dotted line. The experiment was performed in a cell-attached patch. & D; i$ C' B! w& O6 w7 V ! N' _" M; {" E2 M/ C, I- J0 r8 gAfter demonstrating that the inhibitory effect of CHA on ENaC can be abolished by blocking either PLA 2 or CYP epoxygenase, we tested whether 11,12-EET mediates the effect of CHA, because 11,12-EET has been shown to mediate the AA-induced inhibition of ENaC ( 29 ). If 11,12-EET is responsible for the adenosine-induced inhibition of ENaC, addition of 11,12-EET should block Na channels even in the presence of CHA and AACOCF 3 or MS-PPOH. Figure 8 shows a typical recording demonstrating that addition of 100 nM 11,12-EET blocked the ENaC activity from 1.37 ± 0.16 to 0.44 ± 0,08 ( n = 6) in the cells in which the inhibitory effect of CHA was blocked by PLA 2 inhibitor.# H+ X0 O( K3 y
' Z% c8 S }, T% B1 Z! fFig. 8. A channel recording showing the effect of CHA AACOCF 3 and 11,12-epoxyeicosatrienoic acid (11,12-EET) CHA AACOCF 3. The channel-closed level (C) is indicated by a dotted line. The experiment was performed in a cell-attached patch. $ L/ x* m7 k: Y7 z3 O) { _ ! P4 g6 t- z- ?5 Z1 v5 lDISCUSSION 9 g# a8 ?1 d& j7 J' }- d3 X: p/ V R7 h
In the present study, we demonstrated that adenosine inhibited ENaC activity in the CCD and that the effect of adenosine on ENaC was abolished by blocking PLA 2 or the CYP epoxygenase-dependent metabolic pathway of AA. These findings suggest that the effect of adenosine on ENaC is mediated by the CYP epoxygenase-dependent metabolites of AA. Our previous experiments showed that AA inhibited ENaC activity and that the effect of AA was specific, because other fatty acids failed to inhibit ENaC ( 29 ). Also, the observation that inhibition of CYP epoxygenase but not the blocking of cyclooxygenase abolished the effect of AA indicated that the effect of AA on ENaC was mediated by the CYP epoxygenase-dependent metabolic pathway of AA. Moreover, three lines of evidence suggest that 11,12-EET mediates the effect of AA on ENaC: 1 ) addition of 11,12-EET but not other EETs mimics the effect of AA and inhibits ENaC activity; 2 ) CYP2C23, a major isoform of CYP epoxygenase in the kidney that is able to convert AA to 11,12-EET is expressed in the CCD; and 3 ) 11,12-EET is detected in the isolated CCDs. Because CYP2C23 expression is upregulated by high Na intake (Capdevila JH, personal communication), we proposed that 11,12-EET may have a role in suppressing Na conductance in the CCD in response to high Na intake.2 a1 }# W3 F: F; W5 N+ U' c' ~% I
; Q' J7 m! Y5 oIn the present study, we have provided evidence that CYP epoxygenase-dependent metabolites of AA are responsible for the effect of adenosine on ENaC. First, inhibition of CYP epoxygenase activity abolished the CHA-induced inhibition of ENaC. Second, addition of 11,12-EET was able to inhibit ENaC activity in the presence of AACOCF 3, indicating that 11,12-EET is a downstream molecule that mediates the effect of adenosine. Three types of adenosine receptors, A 1, A 2a, and A 2b, are expressed in the kidney ( 10, 17 ). Stimulation of the A 1 receptor has been shown to inhibit adenylate cyclase, decrease cAMP levels, and stimulate PKC and PLA 2 ( 26 ). In contrast, stimulation of the A 2a or A 2b receptor has been reported to increase cAMP production and stimulate PKA ( 26 ). Although the classic effect of stimulating the A 1 adenosine receptor is to activate PLC, this possibility is not supported by the observation that inhibition of PLC failed to abolish the inhibitory effect of CHA on ENaC. However, the finding that inhibition of PLA 2 abolished the effect of CHA on ENaC activity suggests strongly that the adenosine-induced decreases in ENaC activity is the result of stimulation of the A 1 adenosine receptor, which increases the activity of PLA 2 pathway. ! L" w/ w# [. l [3 L % t- ?. D" \/ A, i+ c& ~2 c9 zIn the present study, we used CHA to stimulate the adenosine receptor. Although CHA at low concentrations (' z" I/ t% u6 k' E* t! }
+ ~8 e7 k# H5 M6 o- V7 u8 f- aThe A 1 adenosine receptor has been found to be expressed in the collecting duct ( 28, 30 ). Moreover, an increase in Na intake has been demonstrated to increase renal interstitial fluid adenosine levels more than 10-fold ( 24, 32 ). Although Western blot analysis has shown that the expression of A 1 receptors in renal cortex and medulla from rats on a 4% Na diet decreased compared with those on a 1% Na diet ( 32 ), the location where the expression of A 1 adenosine receptors decreased was not specifically identified in that study. Because adenosine receptors, including the A 1 type, are highly expressed in the vascular structure in the kidney ( 22 ), it is possible that decreases in A 1 adenosine receptors may mainly occur in the vascular structure rather than in renal tubules. Decreases in A 1 adenosine receptors in vascular structure would have a physiological significance, because decreased expression of A 1 adenosine receptors should favor a vasodilation in the afferent arteriole of glomerulus and increase the glomerular filtration rate, leading to increases in renal Na excretion during high Na intake. On the other hand, we speculate that increases in adenosine levels induced by high Na intake should stimulate the A 1 adenosine receptor and suppress the Na absorption in the CCD. However, our experiments were performed in the CCD from rats on a Na -deficient diet rather than a high-Na diet, so it is not possible to know whether the effect of adenosine on ENaC would be the same in animals on high Na . Because ENaC activity is suppressed in the CCD from rats on a high-Na diet, it is difficult to conduct such a study in rats on a high-Na diet. Therefore, we can only speculate that adenosine may inhibit ENaC in the CCD from rats on a high-Na diet, too. In addition to inhibiting ENaC, adenosine can decrease Na excretion by constriction of the afferent arteriole. If adenosine-induced vasoconstriction of the afferent arteriole is predominant, the net effect of adenosine on renal Na transport is to cause a severe Na retention. This may explain the clinical finding that increased adenosine levels in liver during hepatorenal reflex is closely related to a significant decrease in Na excretion. 9 u/ p7 h) O+ c3 r% e+ k2 M' q* _- M! a
The physiological role of adenosine in the regulation of renal function has been well explored ( 15 ). Adenosine has been shown to regulate the glomerular filtration rate, renin release, and epithelial transport in the kidney ( 10 ). Also, adenosine has been demonstrated to play an important role in mediating tubuloglomerular feedback. Now, we have demonstrated that adenosine inhibits ENaC activity. Because adenosine levels in the interstitial fluids increase in response to high Na intake, it is possible that A 1 adenosine receptors are involved in stimulation of renal Na excretion during high Na intake. In this regard, it has been shown that the expression of CYP2C23/Cyp2C44, which is a major isoform of CYP epoxygenase in the kidney and is responsible for making 11,12-EET ( 21 ), is regulated by Na intake: a high Na intake increases, whereas a low Na intake decreases the expression of the enzyme ( 2, 9, 14, 31 ). Figure 9 shows a scheme illustrating a possible mechanism by which adenosine regulates ENaC activity. We propose that high Na intake increases the adenosine levels and stimulates A 1 adenosine receptors in the CCD. Because CYP epoxygenase expression also is upregulated by high Na intake, adenosine should increase 11,12-EET release and inhibit ENaC activity. We conclude that adenosine inhibits ENaC activity in the CCD by stimulation of A 1 adenosine receptors and that the effect of adenosine is mediated by a CYP epoxygenase-dependent pathway of AA.7 V5 v$ |# [/ B7 d
. m1 `( [7 b4 zFig. 9. A cell scheme illustrating a possible mechanism by which adenosine inhibits ENaC activity in the cortical collecting duct via 11,12-EET. The location of A 1 -adenosine receptor (A1) is only a speculation. AA, arachidonic acid; PLA 2, phospholipase A 2; CYP2C23, a major isoform of cytochrome P -450 epoxygenase in the kidney that is able to convert AA to 11,12-EET. 0 f, F& M# m* z6 g9 A/ M , y, B0 ? M) K- N7 fGRANTS $ K8 o2 U6 O0 O. K; C" [8 j8 s' g1 m! U
This work was supported by National Heart, Lung, and Blood Institute Grant HL-34300. 1 w; g. b' f- H) t t+ t3 @. l$ Q- n8 b) R
ACKNOWLEDGMENTS0 t2 e6 S6 k F. K
8 Y( G( S- {4 _/ s! Q# q% pWe thank Dr. A. Nasjletti for stimulating discussions. " @3 A5 q1 Y B% {3 G* E7 e 【参考文献】, [4 u& [0 p: g1 w" O
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