1 Department of Molecular and Integrative Physiology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801; 2 Department of Biology, Indiana University-Purdue University at Indianapolis, Indianapolis 46202; and 3 Cardiovascular Research, Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, Indiana 46285
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ABSTRACT |
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Blocker-induced noise analysis of epithelial Na+ channels (ENaCs) was used to investigate how inhibition of an LY-294002-sensitive phosphatidylinositol 3-kinase (PI 3-kinase) alters Na+ transport in unstimulated and aldosterone-prestimulated A6 epithelia. From baseline Na+ transport rates (INa) of 4.0 ± 0.1 (unstimulated) and 9.1 ± 0.9 µA/cm2 (aldosterone), 10 µM LY-294002 caused, following a relatively small initial increase of transport, a completely reversible inhibition of transport within 90 min to 33 ± 6% and 38 ± 2% of respective baseline values. Initial increases of transport could be attributed to increases of channel open probability (Po) within 5 min to 143 ± 17% (unstimulated) and 142 ± 10% of control (aldosterone) from baseline Po averaging near 0.5. Inhibition of transport was due to much slower decreases of functional channel densities (NT) to 28 ± 4% (unstimulated) and 35 ± 3% (aldosterone) of control at 90 min. LY-294002 (50 µM) caused larger but completely reversible increases of Po (215 ± 38% of control at 5 min) and more rapid but only slightly larger decreases of NT. Basolateral exposure to LY-294002 induced no detectable effect on transport, Po or NT. We conclude that an LY-294002-sensitive PI 3-kinase plays an important role in regulation of transport by modulating NT and Po of ENaCs, but only when presented to apical surfaces of the cells.
epithelial sodium channels; noise analysis; electrophysiology; kidney; cortical collecting ducts; A6 cell line
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INTRODUCTION |
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PHOSPHATIDYLINOSITOL 3-KINASES (PI 3-kinases) catalyze the phosphorylation of the inositol head group of phosphoinositides at the D-3 position (7), generating lipids involved in receptor-mediated signal transduction pathways and membrane trafficking (39). The catalytic activity of PI 3-kinases can be inhibited both in vitro and in vivo by two structurally unrelated compounds, wortmannin and 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY-294002) (31, 40). Whereas wortmannin may also inhibit other kinases (11, 28), including some members of the PI 4-kinase family (27), LY-294002 was found to have no effect on other ATP-requiring enzymes including PI 4-kinase and protein kinases A and C (40). In contrast to wortmannin, LY-294002 is a highly specific and, importantly, reversible inhibitor for PI 3-kinases that acts as a competitive inhibitor for the ATP binding site of the enzyme with an IC50 at low micromolar concentrations (40).
There has been considerable recent interest in understanding the involvement of PI 3-kinases in a variety of cellular processes including constitutive and ligand-stimulated vesicular membrane traffic processes (8, 12, 16, 24, 33, 36-38). In particular, PI 3-kinase was shown to be involved in hormone-mediated transmembrane transport phenomena associated with insulin-stimulated insertion of the GLUT-4 glucose transporter into the cell membrane of adipocytes and rat skeletal muscle (9, 13, 42) and in the synthesis and insertion by vesicle trafficking of A-type K+ channels into the plasma membranes of hippocampal pyramidal neurons (41).
Previous work from our laboratories has shown that PI 3-kinase is required for insulin-mediated Na+ entry by way of highly selective apical membrane epithelial sodium channels (ENaCs) in A6 epithelia derived from the renal distal tubule of Xenopus laevis (31). Recently, we found also that PI 3-kinase activity is required for aldosterone-stimulated Na+ transport in cultured A6 epithelial cells that exhibit these highly Na+-selective apical membrane channels with the same characteristics as those of native tissues (5, 30, 32) and those expressed in oocytes (34). LY-294002 applied apically to A6 epithelia causes a reduction in the density of open ENaCs within the apical membrane of the cells, thus inhibiting Na+ transport in this model tissue (5). In this regard, it became of interest to know whether the effect of LY-294002 on open-channel density was due to modulation of functional channel densities and/or channel open probabilities of the ENaCs.
We have used a noninvasive pulse method of weak blocker-induced noise analysis to monitor the time-dependent changes of single-channel currents (iNa), apical membrane functional channel densities (NT), and channel open probabilities (Po) that algebraically determine the net effect of LY-294002 on macroscopic rates of Na+ transport (INa). We observed and report that LY-294002 acts exclusively from the apical surface of the cells with entirely different Po and NT response times. Whereas Po is increased almost immediately (~1 min), the decreases of NT occurred relatively slowly over 60-90 min, resulting in a completely reversible inhibition of basal rates of Na+ transport in unstimulated and aldosterone-prestimulated A6 epithelia.
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MATERIALS AND METHODS |
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Tissues. A6 cells were used at passages 115 and 118 and grown in a humidified incubator at 28°C containing 1% CO2. The cells were transferred from their frozen state, seeded and grown on 75-cm2 plastic culture flasks (Costar, Cambridge, MA), and then subcultured on Transwell-Clear cluster inserts (Costar) for at least 10 days to achieve confluence and complete development of their transepithelial transport characteristics (19). The cells were fed twice a week with growth medium that was based in most cases on a mixture of equal parts of Ham's nutrient mixture F-12 with L-glutamine and without sodium bicarbonate (N-6760; Sigma Chemical, St. Louis, MO) and L-15 Leibovitz medium (L-4386, Sigma Chemical). This mixture was supplemented with 10% defined fetal bovine serum (FBS) (SH0070; Hyclone, Logan, UT), 2.57 mM sodium bicarbonate (E. K. Industries, Addison, IL), 3.84 mM L-glutamine (G-7513, Sigma Chemical), 96 U/ml penicillin, and 96 µg/ml streptomycin (17-710R; BioWhittaker, Walkersville, MD). A few tissues were fed with DMEM growth medium (91-5055EC; GIBCO BRL, Grand Island, NY) supplemented with 10% defined FBS, 25 U/ml penicillin, and 25 µg/ml streptomycin. Our laboratories have not observed differences in results between passages 76 and 123.
A6 tissues were short-circuited for the duration of the experiments in edge damage-free chambers (1) and continuously perfused with growth medium without FBS and glutamine at flow rates of ~7 ml/min through chamber volumes of ~0.5 ml. The short-circuit currents (Isc) were allowed to stabilize for 1.5 h before the start of 2-h control periods, during which time tissues were subjected to noise analysis to establish the baseline parameters of the tissues before challenge with LY-294002. To compare the effects of LY-294002 in tissues with different baseline rates of Na+ transport, A6 epithelia were studied in both their unstimulated and aldosterone-prestimulated states (0.27 µM, overnight). LY-294002 was used at concentrations of 10 and 50 µM. An IC50 of 1.4 µM was reported for both direct in vitro and in vivo inhibition of PI 3-kinase (40), and a half-maximal concentration near 6 µM was required for inhibition of basal and insulin-stimulated macroscopic rates of Na+ transport in A6 epithelia (31). The experiments were carried out at ambient room temperature.Electrical measurements. The methods of study of amiloride-sensitive ENaCs using blocker-induced noise analysis were identical to those of previous reports from our laboratory (4, 15, 20). The pulse method relies on the fact that weak Na+ channel blockers like 6-chloro-3,5-diamino-pyrazine-2-carboxamide (CDPC; 27,788-6; Aldrich Chemical, Milwaukee, WI) interact with open channels, causing fluctuations of the channels between open and blocked states and thereby giving rise to blocker-induced current noise characterized by Lorentzians in power density spectra (PDS) (18, 20). The apical chambers were perfused continuously with solution containing 10 µM CDPC except during pulse intervals, when the CDPC concentration was increased to 30 µM. In these experiments, the 100 µM amiloride-insensitive currents measured at the ends of the experiments averaged near 0.1 µA/cm2. After subtraction of the amiloride-insensitive currents from the Isc, the macroscopic blocker or amiloride-sensitive Na+ currents at 10 (INa10) and 30 µM CDPC (INa30) were used in all calculations.
Current noise PDS were always measured in pairs at 10 and 30 µM CDPC, giving rise to blocker-induced Lorentzians and fractional inhibitions of the amiloride-sensitive Na+ currents that were used in calculation of iNa, Po, and NT at the time points of measurement. The PDS were fit by nonlinear regression (TableCurve 2D; Jandel Scientific, San Rafael, CA) to a mathematical model consisting of low-frequency 1/f noise characterized by the coefficient S1 and the exponentSingle-channel currents.
iNa were calculated at 10 µM CDPC with
Eq. 1
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(1) |
Open-channel densities.
Open-channel densities (No) at 10 µM CDPC
(No10) were calculated as
INa10/iNa10,
where the superscripts here and elsewhere indicate the blocker concentration. No, in the absence of blocker,
was calculated as described previously (18,
20) with Eq. 2
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(2) |
Channel open probabilities.
During control and experimental periods,
Po were calculated as described
previously (20) with Eq. 3.
NoB2/B1
represents the quotient of open-channel densities
(NoB2/NoB1)
determined at 10 (B1) and 30 (B2) µM CDPC
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(3) |
Functional channel densities. NT were calculated as the quotient No/Po. NT represents the total number of channels involved in apical membrane Na+ entry into the cells that fluctuate spontaneously between open and closed states of the channel. Channel densities are expressed in units of millions of channels per cm2 of planar area or per 100 µm2, approximating the planar area occupied by a single cell. It may be emphasized that channels or subunits of ENaCs that reside within the apical membranes in nonfunctional or quiescent states would not be detected.
Basolateral membrane resistance. The basolateral membrane resistance (Rb) was calculated at each time point with Eq. 5 (APPENDIX B).
Statistical analysis. Data are expressed as means ± SE. Statistical analyses were performed with SigmaStat (Jandel Scientific) using paired or unpaired t-tests where appropriate. A P value <0.05 was considered significant.
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RESULTS |
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LY-294002 inhibits Na+ transport in unstimulated and
aldosterone-prestimulated tissues.
Shown in Fig. 1 are typical strip-chart
recordings of the Isc response to 10 or 50 µM
LY-294002 added to the apical perfusion solution of either a control
unstimulated tissue or a tissue that had been pretreated with
aldosterone to stimulate its baseline rate of Na+
transport. After the tissues were short-circuited in chambers, the
Isc were allowed to stabilize for ~1.5 h
before onset of a 2-h control period that was followed by a 90-min
experimental period, during which time the tissues were exposed to
LY-294002. Subsequently, LY-294002 was removed from the apical solution
for 2.5 h, and the experiments were terminated after complete
inhibition of blocker-sensitive Na+ entry into the cells by
100 µM amiloride.
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Stimulation precedes inhibition of Isc caused by
LY-294002.
Summarized in Fig. 2 are the initial
increases of Isc caused by LY-294002. The
strip-chart digitized data (1 point/s) were normalized for the purpose
of summarization to their zero time values at intervals of 25 s.
LY-294002 (10 µM) caused relatively similar, small fractional
increases of Isc (mean of 4-5%) within 1-2 min, despite differences in baseline rates of Na+
transport in control and aldosterone-prestimulated tissues. With 50 µM LY-294002, Isc was increased absolutely and
fractionally more so than with 10 µM within 1-2 min (mean of
18%) before the Isc began to return toward zero
time values. Within 5 min, Isc returned to zero
time values and continued to decrease thereafter as indicated in Fig.
1.
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Time-dependent changes of iNa. Our previous analysis had indicated that inhibition of transport could not be ascribed to changes of iNa when control or aldosterone-prestimulated tissues were challenged with 10 µM LY-294002 (5) or, as indicated in Fig. 4B, with 50 µM LY-294002. At 5 min, iNa was unchanged (5) or essentially unchanged (Fig. 4B) from zero time values. At 15 min and thereafter, inhibition of transport was accompanied by relatively small increases of iNa that were reversed on withdrawal of LY-294002 and return of INa toward and above zero time values. Such behavior of iNa is expected and consistent with changes of fractional transcellular resistance [Ra/(Ra + Rb)], where apical membrane resistance (Ra) is increased (LY-294002 exposure) or decreased (LY-294002 withdrawn) relative to any change of basolateral membrane resistance (Rb) (see Changes of Rb caused by LY-294002). Clearly, inhibition of INa could not be due to changes of iNa, and so inhibition of transport at the apical membranes of the cells was due to time-dependent decrease of No.
Zero time single-channel currents averaged near 0.4 pA in all groups of experiments. With single-channel conductance (Time-dependent changes of Po. LY-294002 caused changes of both Po and NT. However, the time courses and directions of change were entirely different. Figures 3A and 4C summarize the time-dependent changes of Po. Within 5 min, the Po were increased acutely by 10 µM LY-294002 (Fig. 3A) and more so by 50 µM LY-294002 (Fig. 4C). Zero time Po averaged near 0.5 in unstimulated and aldosterone-prestimulated tissues challenged with 10 µM LY-294002. The increases of Po were essentially the same [to 143 ± 17% (unstimulated) and to 142 ± 10% of control (aldosterone)], despite differences in baseline rates of INa that averaged 4.00 ± 0.14 and 9.12 ± 0.94 µA/cm2, respectively, in unstimulated and aldosterone-prestimulated tissues (5). When tissues were challenged with 50 µM LY-294002, Po was increased from 0.42 ± 0.06 to 0.81 ± 0.07, or to 215 ± 38.3%, at 5 min (Fig. 4C). From maximum increases observed at 5 min, the Po returned slowly over 90 min toward zero time values. On withdrawal of LY-294002, Po was reversed toward or below zero time values and was particularly rapid in those tissues challenged with 50 µM LY-294002.
It should be noted that Po were calculated using the quotient INa30/10/iNa30/10 = No30/10. Zero time values of INa30/10 averaged near 0.82 in all groups of tissues. Zero time values of iNa30/10 averaged near 1.07 and remained essentially unchanged by LY-294002. Accordingly, the changes of Po reflected the time-dependent changes of INa30/10 because the KB were not changed by LY-294002, as indicated above.Time-dependent changes of NT. Compared with Po, the directional change, time course, and concentration dependence of NT on LY-294002 were completely different (Figs. 3B and 4D). At 10 µM LY-294002 in unstimulated and aldosterone-prestimulated tissues (Fig. 3B), NT decreased with quasi-exponential time constants of 16.9 and 19.6 min, and from zero time values of 24.0 ± 2.6 and 60.3 ± 9.0 channels/100 µm2, respectively. Parenthetically, aldosterone stimulation of transport in these groups of experiments is due solely to increase of NT and is the same as reported previously (20). At 90 min, NT was decreased to 6.9 ± 1.3 (unstimulated) and 19.5 ± 2.3 (aldosterone) channels/100 µm2, or to 28.4 ± 3.7% and 34.5 ± 3.1% of zero time values, respectively, irrespective of the 2.5-fold difference of zero time values. The limiting values of NT derived from the exponential fits of data indicated that at infinite time, NT would have stabilized at 28.1% (unstimulated) and 30.7% (aldosterone) of zero time values in tissues treated with 10 µM LY-294002.
Compared with tissues treated with 10 µM LY-294002, the NT of tissues treated with 50 µM LY-294002 decreased more rapidly with a time constant of 10.9 min, reaching stable values within ~60 min (Fig. 4D). From zero time values that averaged 29.4 ± 5.6 channels/100 µm2, NT was decreased to 4.3 ± 0.7 channels/100 µm2 at 90 min, or to 15.3 ± 1.7% of zero time values.LY-294002 does not act from the basolateral surface of the tissues.
LY-294002 presented to the tissues at their basolateral surface was,
surprisingly, completely without effect on Na+ transport
(Fig. 5). The typical patterns of
inhibition of INa elicited by apically applied
LY-294002 at the same concentration (10 µM) were completely absent,
as were the changes of iNa,
Po, and NT. To the extent
that apical and basolateral membrane permeabilities to LY-294002 are
the same (per unit area) and to the extent that basolateral membrane
surface area is far greater than apical membrane surface area, the
actual concentration of intracellular LY-294002 should be far greater
when LY-294002 is presented to the tissues from the basolateral
solution. Consequently, the absence of detectable changes of transport
is most surprising if it is assumed that the LY-294002 inhibitable PI
3-kinase is accessible equally well from the extracellular fluids
bathing the apical and basolateral surfaces of the cells. Clearly, a
simple cytoplasmic site(s) of action is precluded among schemes that
may ultimately explain how LY-294002 causes both increase of
Po and decrease of NT
with entirely different time courses, but only when cells are presented with LY-294002 from the apical face of the cells.
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Changes of Rb caused by LY-294002.
Although not a major aim of these studies, it was of interest to know
if apical LY-294002 caused changes of the basolateral membrane
resistance of the cells. Calculations were carried out with Eq. 5 (APPENDIX B) and the results summarized as indicated
in Fig. 6. Zero time
Rb of unstimulated tissues averaged 9,028 ± 783 · cm2 (n = 15), whereas
those of aldosterone-prestimulated tissues averaged 3,864 ± 879
· cm2 (n = 7). Thus steroid
stimulation of transport by aldosterone was mediated not only by
increases of apical membrane NT but also by
increases of basolateral membrane conductance, due most likely to
activation of a basolateral membrane K+ conductance as
suggested originally by Schultz (35). Whereas stimulation
of transport by aldosterone was accompanied by decrease of
Rb, inhibition of transport by LY-294002 was
accompanied by a reversible increase of Rb as
indicated in Fig. 6A. When normalized to zero time values as
shown in Fig. 6B, the fractional increases of
Rb were essentially the same in unstimulated and
aldosterone-prestimulated tissues at 10 and 50 µM LY-294002. At 90 min, Rb was increased about two- to threefold
above zero time values. Notably, however, LY-294002-related increases
of Rb were delayed by at least 15 min from onset
of the increases of Po reported above and the
decreases of NT that were observed during this
same time period. In this regard, it also remains curious that
LY-294002 applied to the basolateral membrane is without effect on
Rb, suggesting that changes of
Rb do not occur by direct interaction of
LY-294002 with the basolateral membrane of the cells or by access to
the cytosol from the basolateral surface of the cells.
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DISCUSSION |
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We have examined the role of an LY-294002-sensitive PI 3-kinase in modulating basal (unstimulated) and aldosterone-prestimulated Na+ transport in cell-cultured A6 epithelia derived from the renal distal tubule of Xenopus laevis. A noninvasive pulse method of weak blocker-induced noise analysis was used to determine whether inhibition of transport in response to LY-294002 was due to changes of iNa, NT, and/or Po that together determine the overall effect of the inhibitor on Na+ transport.
Our analysis has revealed that apical exposure of A6 epithelia to LY-294002 causes, after a relatively small initial stimulation, a marked and completely reversible inhibition of both basal and aldosterone-prestimulated Na+ transport due to slow time-dependent decreases of functional ENaC densities within the apical membranes of the cells. The initial stimulation of transport was due to considerably more rapid but quantitatively smaller fractional increases of ENaC Po. LY-294002 applied apically not only increased apical membrane resistance (decrease of ENaC NT) but also caused, after a substantial delay, a reversible increase of basolateral membrane resistance. Similar qualitative changes were seen at 10 and 50 µM LY-294002. However, significant differences became apparent not only in the time course of change of the Po and NT but also in the sensitivity of the Po and NT to these concentrations of LY-294002.
The difference in sensitivity of the Po and the
NT to LY-294002 is emphasized in Fig.
7. Near-maximal increases of
Po were measured at 5 min. The response at 5 min
expressed in absolute terms (Figs. 3 and 4) or normalized to zero time
values (Fig. 7) indicated clearly that Po was
concentration dependent in the range of 10-50 µM LY-294002. With
Po averaging in the range of 0.4-0.5, the
maximal increases of Po would not exceed a
normalized increase of ~2- to 2.5-fold, so the increases of
Po at 50 µM LY-294002 are close to the maximum
that can be elicited by LY-294002. Accordingly, regardless of
mechanism, a half-maximal concentration of LY-294002 in the range of
~15-20 µM LY-294002 would be required to cause a 50% increase
of Po.
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Maximal or near-maximal decreases of NT could be elicited by LY-294002 within 60 min at 50 µM and within 90 min at 10 µM (Figs. 3 and 4). Examination of the normalized decreases of NT (Fig. 7) indicated that NT was most sensitive to LY-294002 at concentrations <10 µM. The dependency of NT on LY-294002 could as a first approximation be reasonably characterized by an IC50 in the range of 1-2 µM LY-294002, which would be remarkably close to the IC50 of 1.4 µM of the LY-294002 inhibitable PI 3-kinase reported by Vlahos et al. (40).
It is thus interesting to note that the response of NT to LY-294002 is most likely attributable to an LY-294002 inhibitable PI 3-kinase. This PI 3-kinase is involved directly at least in one of the steps or processes mediating regulation of apical membrane ENaC densities. Stimulation of transport by insulin and aldosterone is mediated by increases of NT (4, 20), and LY-294002 inhibits both the insulin- and aldosterone-mediated increases of transport (5, 31). It remains to be determined whether LY-294002 inhibits hormone-stimulated transport by preventing increases of NT. If LY-294002 causes substantial increases of Po as reported here, then the LY-294002-mediated prevention of a threefold or greater stimulation of transport by aldosterone and/or insulin must be due, on quantitative grounds, to loss of hormone-mediated increases of NT.
It may be useful to note that we are not aware, at least from our own studies, of how the Po of ENaC is regulated in intact epithelial cells. It is thus, at this time, impossible to know how and why LY-294002 increases Po in reversible fashion. The concentrations of LY-294002 required to increase Po are substantially greater than those required to decrease NT. Whether this action of LY-294002 on Po is a nonspecific effect or an effect mediated by yet another PI 3-kinase or a PI 3-kinase with a different IC50 remains to be determined, among other possibilities. Notably, whatever the mechanism, it was readily apparent that the effect of LY-294002 on Po occurred rapidly (minutes) and reversibly.
Understanding the complex interactions that regulate the channel kinetics of ENaCs will require a detailed analysis of all the potential effectors in the regulatory pathway(s). Activation of PI 3-kinase is an early step in the phosphoinositide pathway, which typically includes downstream kinase cascades. Which of the effector pathways forms the physical link to ENaC via PI 3-kinase activation is unknown.
Interestingly, one of the kinases that is activated downstream of PI 3-kinase, the serum glucocorticoid-induced kinase (sgk), has recently been shown to be an aldosterone-induced protein (10, 29). Coexpression of sgk and ENaC in oocytes leads to increased Na+ flux, suggesting that sgk can regulate channel activity. However, we have previously shown that aldosterone treatment has a direct effect on phosphatidylinositol phosphorylation, indicating the existence of another aldosterone-induced protein at or before the PI 3-kinase step, which is well before the sgk protein (5). Thus multiple components of the phosphoinositide pathway may be induced by aldosterone, whereas a constitutive level of activation of the pathway appears to be necessary for maintaining basal transport. The task of identifying each of the proteins, their functions as well as the number of steps in the complete pathway(s), remains a challenge for future investigations.
Sided action of LY-294002. Our results with regard to the ineffectiveness of LY-294002 when applied to the basolateral surface were completely unexpected. LY-294002 acts exclusively from the apical surface of the epithelium, with no significant effects on INa, NT, Po, and Rb when presented at the same concentration to the basolateral surface of the tissues. To our knowledge, this is a novel finding because this inhibitor is known to exhibit equal efficacy in whole cell assays using both adherent and suspended cells and in in vitro enzyme assays, suggesting that it is membrane permeable (40) (C. J. Vlahos, personal communication). The striking absence of response to basolateral LY-294002 is clearly not consistent with the action of a membrane-permeable inhibitor blocking the activity of a cytosolic enzyme.
Our finding suggests that the current view of PI 3-kinase as a cytosolic enzyme (at least of the LY-294002 inhibitable PI 3-kinase) that is translocated to the membrane on activation may be too simplistic. In many experiments, "cytosolic" simply means that the enzyme is found in the soluble fraction during subcellular fractionation procedures after cellular disruption. Likewise, "membrane bound" simply means associated with a membrane fraction during the fractionation procedure. Neither of these terms allows one to assess exactly where in the cell the enzyme may be found. It is difficult to imagine that a single event such as the binding of the catalytic subunit of PI 3-kinase to a phosphorylated intermediate (e.g., insulin receptor substrate) will cause the enzyme to migrate through the cytoplasm of the cell to a very specific area of the cell membrane. Rather, it is easier to imagine that complexes of intermediates may reside in close proximity to the final effectors. To our knowledge, there have been no detailed studies attempting to localize PI 3-kinase within the cytosolic or membrane subcompartments. This task will be complicated by the number of isoforms of PI 3-kinase that may be localized in various compartments, respond to different stimuli, and have different substrates. Our results suggest that the PI 3-kinases that are responsible for basal as well as hormone-stimulated Na+ transport are localized and compartmentalized in such a way that only apically applied LY-294002 is effective. Further investigations are required to resolve unequivocally whether LY-294002 can traverse the plasma membrane and to determine precisely the localization of the specific PI 3-kinase that is involved in the modulation of membrane ion transport. Our results do not prove, but are consistent with the idea, that LY-294002 prevents insertion and/or stimulates withdrawal of ENaCs from the apical plasma membrane of A6 cells. A similar effect was demonstrated for A-type potassium channels in hippocampal pyramidal neurons (41). In these cells, LY-294002 and wortmannin inhibition of PI 3-kinase was shown to cause decreases of membrane area and three different types of K+ currents. Both wortmannin and LY-294002 induced a decrease of A-type K+ channel density within the plasma membrane. There are, however, a large number of studies that describe how PI 3-kinase plays a crucial role in endocytotic and/or exocytotic phenomena (2, 6, 8, 9, 16, 23, 24, 33, 38) [see also reviews by Shepherd et al. (37) and De Camilli et al. (12) and references therein]. Our own data do not rule out the possibility that PI 3-kinase may be involved in withdrawal of channels from the apical membrane because, on balance, the steady-state apical membrane density of functional ENaCs will be determined by the rates of insertion and withdrawal. Analogously, PI 3-kinase inhibition was shown to induce a decrease in the endosomal recycling of intracellular NHE3 isoforms of the Na+/H+ exchanger to the cell membrane in AP-1 Chinese hamster ovary cells (23). On the weight of the available evidence cited above, our bias favors the view that PI 3-kinase is involved in shuttling of channels to and/or from the apical membrane of the cells. However, we cannot rule out the possibility that PI 3-kinase is involved in recruiting channels and/or subunits from quiescent or nonfunctional states that are resident within the apical plasma membranes of the cells. Thus, in summary, it will be most interesting to resolve where and how LY-294002 inhibitable PI 3-kinase is involved in regulation of basal ENaC channel densities and to learn at what step or steps PI 3-kinase is required for the hormonal response to insulin and aldosterone in regulation of transepithelial absorption of sodium. Our findings suggest that LY-294002 inhibits a novel member of the PI 3-kinase family involved in regulation of epithelial sodium channels. ![]() |
APPENDIX A |
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Correction of the Lorentzian So for Power Gain Loss
Shown in Fig. 8 are the usual direct current (DC) and alternating current (AC) electrical equivalent circuits of short-circuited epithelia with apical (Ra and Ca) and basolateral (Rb and Cb) membrane slope resistances and capacitances. Eb is the Thévenin electromotive force (EMF) of the basolateral membrane with values that exceed the potassium equilibrium potential difference due to the contribution of the current generated by the Na+-K+-ATPase pump current flowing through Rb (17, 21). Because the electrical operating point of apical membrane ENaCs is far removed from equilibrium, the Thévenin Ea is at or very near zero, so the apical membrane behaves electrically as a resistor paralleled by its capacitance.
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In the presence of a Na+ channel blocker, the conductance
fluctuations of ENaCs between open and blocked states give rise to ac current noise (INan).
With Ca, the basolateral membrane conductance
(Gb), and Cb acting as a
current divider, the short-circuit AC noise
(Iscn) is less than
INan. Defining the noise current gain
(Igain) as
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(4) |
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It may be noted in particular for CDPC-induced noise that the corner frequencies of the Lorentzians range upward from ~40 Hz, so current noise power levels at these and higher frequencies are attenuated by the limiting value of |Pgain|, as indicated above. At much lower frequencies, ~10 Hz and less, and in the range of the 1/f noise that far exceeds in value the power levels of the plateau values of the Lorentzians, Lorentzian noise contributes negligibly to the total power measured. Accordingly, the So values of the Lorentzians measured were corrected as indicated in Eq. 1 for attenuation of current noise attributable to recirculation of Na+ current noise through the apical membrane capacitance.
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APPENDIX B |
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Calculation of Rb
While tissues are short-circuited, the absolute values of apical and basolateral membrane voltages (Va and Vb, respectively) must be identical. If single-channel conductance of ENaCs (
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(5) |
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APPENDIX C |
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Blocker Concentration-Related Changes of Single-Channel Currents
Inhibition of apical membrane Na+ entry by amiloride and other ENaC blockers causes hyperpolarization of apical membrane voltage as expected according to the DC electrical equivalent circuit shown in Fig. 8A. Changes in voltage are determined by the changes of fractional transcellular resistance fRa = Ra/(Ra + Rb), where Va = fRa Eb with reference to a grounded apical solution (19). Because changes of Va are directly proportional to changes of iNa
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(6) |
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(7) |
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ACKNOWLEDGEMENTS |
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We gratefully acknowledge the meticulous work of A. L. Helman in the growth and preparation of the A6 epithelia and in the care of our tissue culture facility.
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FOOTNOTES |
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This work was supported by National Institute of Diabetes and Digestive and Kidney Diseases Grant DK-30824 to S. I. Helman and with American Heart Association (Indiana Affiliate) support to B. L. Blazer-Yost.
This work was carried out while T. G. Punescu was a
doctoral candidate in the Center for Biophysics and Computational
Biology at the University of Illinois at Urbana-Champaign.
Present address of T. G. Punescu: Dept. of Molecular and
Integrative Physiology, 524 Burrill Hall, University of Illinois at
Urbana-Champaign, Urbana, IL 61801.
Address for reprint requests and other correspondence: S. I. Helman, Dept. of Molecular and Integrative Physiology, 524 Burrill Hall, 407 South Goodwin Ave., Univ. of Illinois at Urbana-Champaign, Urbana, Illinois 61801(E-mail: s-helman{at}uiuc.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Received 31 August 1999; accepted in final form 23 February 2000.
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