Intracellular pH and membrane potassium conductance in rabbit distal colon

Eccrine sweat gland development and sweat secretion

Eccrine sweat glands help to maintain homoeostasis, primarily by stabilizing body temperature. Derived from embryonic ectoderm, millions of eccrine glands are distributed across human skin and secrete litres of sweat per day. Their easy accessibility has facilitated the start of analyses of their development and function. Mouse genetic models find sweat gland development regulated sequentially by Wnt, Eda and Shh pathways, although precise subpathways and additional regulators require further elucidation. Mature glands have two secretory cell types, clear and dark cells, whose comparative development and functional interactions remain largely unknown. Clear cells have long been known as the major secretory cells, but recent studies suggest that dark cells are also indispensable for sweat secretion. Dark cell-specific Foxa1 expression was shown to regulate a Ca(2+) -dependent Best2 anion channel that is the candidate driver for the required ion currents. Overall, it was shown that cholinergic impulses trigger sweat secretion in mature glands through second messengers - for example InsP3 and Ca(2+) - and downstream ion channels/transporters in the framework of a Na(+) -K(+) -Cl(-) cotransporter model. Notably, the microenvironment surrounding secretory cells, including acid-base balance, was implicated to be important for proper sweat secretion, which requires further clarification. Furthermore, multiple ion channels have been shown to be expressed in clear and dark cells, but the degree to which various ion channels function redundantly or indispensably also remains to be determined.

Native and recombinant Slc26a3 (downregulated in adenoma, Dra) do not exhibit properties of 2Cl-/1HCO3- exchange

The recent proposal that Dra/Slc26a3 mediates electrogenic 2Cl(-)/1HCO(3)(-) exchange suggests a required revision of classical concepts of electroneutral Cl(-) transport across epithelia such as the intestine. We investigated 1) the effect of endogenous Dra Cl(-)/HCO(3)(-) activity on apical membrane potential (V(a)) of the cecal surface epithelium using wild-type (WT) and knockout (KO) mice; and 2) the electrical properties of Cl(-)/(OH(-))HCO(3)(-) exchange by mouse and human orthologs of Dra expressed in Xenopus oocytes. Ex vivo (36)Cl(-) fluxes and microfluorometry revealed that cecal Cl(-)/HCO(3)(-) exchange was abolished in the Dra KO without concordant changes in short-circuit current. In microelectrode studies, baseline V(a) of Dra KO surface epithelium was slightly hyperpolarized relative to WT but depolarized to the same extent as WT during luminal Cl(-) substitution. Subsequent studies indicated that Cl(-)-dependent V(a) depolarization requires the anion channel Cftr. Oocyte studies demonstrated that Dra-mediated exchange of intracellular Cl(-) for extracellular HCO(3)(-) is accompanied by slow hyperpolarization and a modest outward current, but that the steady-state current-voltage relationship is unaffected by Cl(-) removal or pharmacological blockade. Further, Dra-dependent (36)Cl(-) efflux was voltage-insensitive in oocytes coexpressing the cation channels ENaC or ROMK. We conclude that 1) endogenous Dra and recombinant human/mouse Dra orthologs do not exhibit electrogenic 2Cl(-)/1HCO(3)(-) exchange; and 2) acute induction of Dra Cl(-)/HCO(3)(-) exchange is associated with secondary membrane potential changes representing homeostatic responses. Thus, participation of Dra in coupled NaCl absorption and in uncoupled HCO(3)(-) secretion remains compatible with electroneutrality of these processes, and with the utility of electroneutral transport models for predicting epithelial responses in health and disease.

Down-regulated in adenoma Cl/HCO3 exchanger couples with Na/H exchanger 3 for NaCl absorption in murine small intestine

BACKGROUND & AIMS

Electroneutral NaCl absorption across small intestine contributes importantly to systemic fluid balance. Disturbances in this process occur in both obstructive and diarrheal diseases, eg, cystic fibrosis, secretory diarrhea. NaCl absorption involves coupling of Cl(-)/HCO(3)(-) exchanger(s) primarily with Na(+)/H(+) exchanger 3 (Nhe3) at the apical membrane of intestinal epithelia. Identity of the coupling Cl(-)/HCO(3)(-) exchanger(s) was investigated using mice with gene-targeted knockout (KO) of Cl(-)/HCO(3)(-) exchangers: Slc26a3, down-regulated in adenoma (Dra) or Slc26a6, putative anion transporter-1 (Pat-1).

METHODS

Intracellular pH (pH(i)) of intact jejunal villous epithelium was measured by ratiometric microfluoroscopy. Ussing chambers were used to measure transepithelial (22)Na(36)Cl flux across murine jejunum, a site of electroneutral NaCl absorption. Expression was estimated using immunofluorescence and quantitative polymerase chain reaction.

RESULTS

Basal pH(i) of DraKO epithelium, but not Pat-1KO epithelium, was alkaline, whereas pH(i) in the Nhe3KO was acidic relative to wild-type. Altered pH(i) was associated with robust Na(+)/H(+) and Cl(-)/HCO(3)(-) exchange activity in the DraKO and Nhe3KO villous epithelium, respectively. Contrary to genetic ablation, pharmacologic inhibition of Nhe3 in wild-type did not alter pH(i) but coordinately inhibited Dra. Flux studies revealed that Cl(-) absorption was essentially abolished (>80%) in the DraKO and little changed (<20%) in the Pat-1KO jejunum. Net Na(+) absorption was unaffected. Immunofluorescence demonstrated modest Dra expression in the jejunum relative to large intestine. Functional and expression studies did not indicate compensatory changes in relevant transporters.

CONCLUSIONS

These studies provide functional evidence that Dra is the major Cl(-)/HCO(3)(-) exchanger coupled with Nhe3 for electroneutral NaCl absorption across mammalian small intestine.

Effect of cytosolic pH on epithelial Na+ channel in normal and cystic fibrosis sweat ducts

The activities of cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel and the amiloride-sensitive epithelial Na(+) channel (ENaC) are acutely coordinated in the sweat duct. However, the mechanisms responsible for cross-talk between these ion channels are unknown. Previous studies indicated that luminal pH of sweat ducts varies over 3 pH units and that the cytoplasmic pH affects both CFTR and ENaC. Therefore, using basolaterally alpha-toxin-permeabilized apical membrane preparations of sweat ducts as an experimental system, we tested the hypothesis that the cytosolic pH may mediate the cross-talk between CFTR and ENaC. We showed that while luminal pH had no effect, cytosolic pH acutely affected ENaC activity. That is, acidic pH inhibited, while basic pH activated, ENaC. pH regulation of ENaC appears to be independent of CFTR or endogenous kinase activities because basic pH independently stimulated ENaC (1) in normal ducts even when CFTR was deactivated, (2) in CF ducts that lack CFTR in the plasma membranes and (3) after blocking endogenous kinase activity with staurosporine. Considering the evidence of Na(+)/H(+) exchange (NHE) activity as shown by the expression of mRNA and function of NHE in the basolateral membrane of the sweat duct, we postulate that changes in cytosolic Na(+) ([Na(+)]( i )) may alter cytosolic pH (pH( i )) as salt loads into the cell during electrolyte absorption. These changes may play a role in coordinating the activities of ENaC and CFTR during transepithelial salt transport.

Cytosolic pH regulates GCl through control of phosphorylation states of CFTR

Our objective in this study was to determine the effect of changes in luminal and cytoplasmic pH on cystic fibrosis transmembrane regulator (CFTR) Cl- conductance (GCl). We monitored CFTR GCl in the apical membranes of sweat ducts as reflected by Cl- diffusion potentials (VCl) and transepithelial conductance (GCl). We found that luminal pH (5.0-8.5) had little effect on the cAMP/ATP-activated CFTR GCl, showing that CFTR GCl is maintained over a broad range of extracellular pH in which it functions physiologically. However, we found that phosphorylation activation of CFTR GCl is sensitive to intracellular pH. That is, in the presence of cAMP and ATP [adenosine 5'-O-(3-thiotriphosphate)], CFTR could be phosphorylated at physiological pH (6.8) but not at low pH (approximately 5.5). On the other hand, basic pH prevented endogenous phosphatase(s) from dephosphorylating CFTR. After phosphorylation of CFTR with cAMP and ATP, CFTR GCl is normally deactivated within 1 min after cAMP is removed, even in the presence of 5 mM ATP. This deactivation was due to an increase in endogenous phosphatase activity relative to kinase activity, since it was reversed by the reapplication of ATP and cAMP. However, increasing cytoplasmic pH significantly delayed the deactivation of CFTR GCl in a dose-dependent manner, indicating inhibition of dephosphorylation. We conclude that CFTR GCl may be regulated via shifts in cytoplasmic pH that mediate reciprocal control of endogenous kinase and phosphatase activities. Luminal pH probably has little direct effect on these mechanisms. This regulation of CFTR may be important in shifting electrolyte transport in the duct from conductive to nonconductive modes.

Modulation of chloride secretion in the rat ileum by intracellular bicarbonate

Increasing intracellular bicarbonate concentration ([HCO3-]i) inhibits calcium-mediated Cl- secretion in rat distal colon and T84 cells. We investigated the effect of [HCO3-]i on Cl- secretion in rat ileum. Segments of intact ileum from Sprague-Dawley rats were studied in Ussing chambers and villus and crypt intracellular pH and [HCO3-]i were determined using BCECF. A range of crypt and villus [HCO3-]i from 0 to 31 mM was obtained by varying Ringer's composition. Basal serosal-to-mucosal Cl- flux (JsmCl) averaged 8.5 +/- 0.2 mu eq.h-1.cm-2 and was unaffected by changing [HCO3-]i or serosal bumetanide. Carbachol increased JsmCl by 3.9 +/- 0.5 mu eq.h-1.cm-2 at [HCO3-]i = 0 mM but only by 1.0 +/- 0.3 mu eq.h-1.cm-2 at high crypt and villus [HCO3-]i. Dibutyryl-cAMP increased JsmCl by 2.5 +/- 0.2 mu eq.h-1.cm-2 at all [HCO3-]i. Carbachol and db-cAMP showed mutual antagonism at low [HCO3-]i and near-additivity at high [HCO3-]i. We conclude that like rat colon and T84 cells, calcium-mediated but not cAMP-mediated Cl- secretion in the ileum is inhibited by increasing [HCO3-]i. Mutual antagonism between carbachol and db-cAMP at low [HCO3-]i was present in ileum and distal colon but not in T84 cells.

The effect of short-chain fatty acids on Cl- and K+ conductance in rat colonic crypts

The effect of butyrate on membrane potential and membrane currents of colonic enterocytes was studied with the whole-cell patch-clamp method. Superfusion of crypts from the rat distal colon with butyrate-containing solutions induced a membrane depolarization of 16.5 +/- 2.3 mV. This response was only observed in the upper third of the crypt. The depolarization was dependent on the presence of Cl- and was accompanied by an increase in membrane inward current, indicating that it is caused by an increase in Cl- conductance. Membrane outward current, however, behaved inconsistently. Whereas in most cells an increase was observed, about 25% of the cells responded with a decrease. This unexpected inhibition of the outward current probably represents a decrease of K+ conductance caused by the cellular acidification in the presence of butyrate. Manoeuvres carried out to acidify the cell interior, like perfusion with acid buffer solutions or inhibition of the Na+/H+ exchanger by amiloride, mimicked this inhibition of the K+ conductance. Orientating cell-attached patch-clamp recordings performed in parallel revealed an activation of previously silent basolateral Cl- channels by butyrate. They had a linear current/voltage relationship and a single-channel conductance of 20-30 pS. The butyrate-induced depolarization was not dependent on intracellular adenosine 5'-triphosphate (ATP) and was also observed when the buffer capacity of the pipette for Ca2+ was increased. It was also not inhibited by guanosine-5'-O(2-thiodiphosphate) (GDP[beta S]).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of short-chain fatty acids on cell volume and intracellular pH in rat distal colon

Superfusion of isolated crypts from the rat colon with sodium-butyrate-containing solutions induced an increase in the crypt diameter indicating a swelling of the crypt cells. The response to butyrate (50 mmol l-1) was not uniform along the crypt axis, the most pronounced swelling being observed in the upper third of the crypt. The butyrate effect was concentration-dependent and was completely suppressed by amiloride, suggesting that it is caused by activation of the Na+/H+ exchanger. Acetate, propionate and isobutyrate had a similar action. In HEPES-buffered solution the butyrate-induced change in cell volume was monophasic, i. e. only a swelling took place, whereas in HCO3- buffer it was biphasic, i. e. swelling was followed by a regulatory volume decrease. This decrease was suppressed by K+ and Cl- channel blockers as well as inhibitors of leukotriene synthesis. Measurements of intracellular pH with the fluorescent dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) revealed that butyrate induced an acidification of the cell, which was stronger in HEPES than in HCO3- buffer. Estimation of Na+/H+ exchange activity, tested as recovery of intracellular pH from an acid load via an NH4Cl prepulse, revealed a much lower Na+/H+ exchange activity in the fundus region compared to the upper third of the crypt. The smaller volume response evoked by butyrate in the fundus region probably reflects the smaller Na+/H+ activity compared to the more differentiated cells near the opening of the crypt. It is concluded that cell swelling caused by short-chain fatty acids is a physiological stimulus for volume regulation. This response is restricted to the more differentiated cells.

Tetraethylammonium-sensitive apical K+ channels mediating K+ secretion by turtle colon

1. Apical membrane K+ channels in turtle colon were identified and characterized using current fluctuation analysis. 2. Under short-circuit conditions in NaCl-Ringer solution, the power density spectrum (PDS) of the short-circuit current (Isc) sometimes exhibited a clearly discernible Lorentzian component, indicating spontaneous fluctuations produced by a population of apical ion channels. The Lorentzian component had a characteristic corner frequency (fc) which averaged 10.2 +/- 0.9 Hz (mean +/- S.E.M., n = 20). 3. The power of the spontaneous fluctuations was enhanced (So increased) by manoeuvres that depolarize the apical membrane electrical potential (Va). Discernible fluctuations were enhanced or induced by raising the serosal K+ concentration ([K+]s = 50-115 mM, Na+ replacement), by clamping the transepithelial potential (Vt) to serosa-positive values, or by blocking basolateral K+ channels with Ba2+. 4. Mucosal amiloride (100 microM) attenuated the spontaneous fluctuations observed in NaCl-Ringer solution but had no effect in the presence of serosal high K+, indicating that amiloride did not block the K(+)-permeable channels but these channels resided in the same cells as the amiloride-sensitive Na+ channels. 5. Raising the mucosal K+ concentration attenuated spontaneous fluctuations. 6. In the presence of serosal high K+ and mucosal amiloride, the spontaneous fluctuations were often accompanied by a reversed Isc consistent with K+ secretion. These conditions were used to test the effects of putative channel blockers. 7. Mucosal Ba2+ and tetraethylammonium (TEA+) were effective inhibitors of the spontaneous fluctuations and the reversed Isc. At a concentration of 10 mM, TEA+ was maximally effective but the TEA+ analogues tetramethylammonium (TMA+) and tetrapropylammonium (TPrA+) were much less effective. Mucosal Rb+ or Cs+ did not inhibit at a concentration of 10 mM. 8. Mucosal lidocaine (200 microM), quinidine (200 microM), or diphenylamine-2-carboxylate (DPC, 1 mM) had little or no effect on the spontaneous fluctuations and reversed Isc. Quinine (100 microM), 4-aminopyridine (1 mM), and apamin (100 nM) were also without effect. 9. Mucosal TEA+ (10 mM) abolished the active secretory K+ flux measured in the presence of serosa-positive transepithelial potentials. 10. These experiments identified a population of TEA(+)-sensitive, apical K+ channels which mediate active K+ secretion in turtle colon. Sensitivity to external TEA+ distinguishes these channels from basolateral K+ channels in turtle colon and demonstrates similarity to apical K+ channels in mammalian colon.

Intracellular pH and its relationship to regulation of ion transport in normal and cystic fibrosis human nasal epithelia

1. Intracellular pH (pHi) of cultured human airway epithelial cells from normal and cystic fibrosis (CF) subjects were measured with double-barrelled pH-sensitive liquid exchanger microelectrodes. The cells, which were grown to confluence on a permeable collagen matrix support, were mounted in a modified miniature Ussing chamber. All studies were conducted under open circuit conditions. Values are given as means +/- S.E.M. and n refers to the number of preparations. 2. Normal preparations (n = 15) were characterized by a transepithelial potential difference (Vt) of -18 +/- 2 mV, an apical membrane potential (Va) of -19 +/- 2 mV, a basolateral membrane potential (Vb) of -37 +/- 2 mV, a transepithelial resistance (Rt) of 253 +/- 15 omega cm2, a fractional apical membrane resistance (fRa) of 0.40 +/- 0.04 and an equivalent short circuit current (Ieq) of -73 +/- 7 microA cm-2. 3. CF preparations (n = 13) were characterized by a Vt of -46 +/- 7 mV, a Va of 3 +/- 5 mV, a Vb of -43 +/- 3 mV, Rt of 373 +/- 47 omega cm2, fRa of 0.44 +/- 0.04 and an Ieq of -130 +/- 16 microA cm-2. All parameters except Vb and fRa were significantly different (P < 0.025) from those of normal preparations. 4. Despite large differences in electrochemical driving force for proton flow across the apical cell membranes between normal and CF preparations (-4 +/- 3 mV and 20 +/- 7 mV, respectively), pHi was similar (7.15 +/- 0.02 and 7.11 +/- 0.05, respectively). The driving force across the basolateral membrane was similar in normal and CF preparations (22 +/- 3 and 26 +/- 3 mV, respectively). 5. Intracellular alkalinization achieved by removal of CO2 from the luminal Ringer solution or by luminal ammonium prepulse led to stimulation of Ieq in both normal (from -58 to -70 microA cm-2, n = 4; P < 0.05) and CF (from -144 to -163 microA cm-2, n = 4; P < 0.005) preparations. The increase in Ieq was associated with a reduction of Rt, increase in fRa, and hyperpolarization of Vb. All changes in bioelectric properties in response to intracellular alkalinization were fully reversible. 6. Intracellular acidification achieved by serosal ammonium prepulse led to marked reductions of Ieq in both normal (from -95 to -31 microA cm-2, n = 6; P < 0.05) and CF (from -111 to -67 microA cm-2, n = 7; P < 0.005) preparations.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of chloride secretion in the rat colon by intracellular bicarbonate

Extracellular HCO3- stimulates colonic net Cl- absorption in part by inhibiting basal Cl- secretion. This inhibition was investigated by measuring serosal-to-mucosal Cl- flux across short-circuited colonic segments from Sprague-Dawley rats. Mucosal intracellular pH and bicarbonate were estimated using the pH-sensitive dye BCECF. When extracellular [HCO3-] ([HCO3-]e) was increased from 0 to 39 mmol/L at PCO2 33 mm Hg, mucosal intracellular [HCO3-] ([HCO3-]i) increased to 25.3 mmol/L and serosal-to-mucosal Cl- flux decreased from 13.0 to 7.1 microEq.cm-2.h-1. When PCO2 was increased to 72 mm Hg at [HCO3-]e 39 mmol/L, [HCO3-]i increased to 29.8 mmol/L and serosal-to-mucosal Cl- flux decreased to 5.9 microEq.cm-2.h-1. In Ringer's solution containing 21 mmol/L HCO3- and 20 mmol/L Cl- (but not 100 mmol/L Cl-), increasing PCO2 from 21 to 70 mm Hg increased [HCO3-]i to 22.6 mmol/L and decreased serosal-to-mucosal Cl- flux from 3.0 to 1.7 microEq.cm-2.h-1. Overall, serosal-to-mucosal Cl- flux was inversely related to [HCO3-]i on either side of an [HCO3-]i plateau of 9-18 mmol/L at which flux was stable. These data suggest that [HCO3-]i is an important modulator of basal Cl- secretion in rat distal colon.

Coupling between the intracellular pH and the active transport of sodium in an epithelial cell line from Xenopus laevis

1. The relationship between the rate of active Na+ transport and intracellular pH (pHi, measured as the BCECF fluorescence) was studied in A6 monolayers in the presence of CO2. The total buffering power (beta total) and its components (beta i and beta CO2) were assessed at various pHi. 2. The activity of Na+/H+ and Cl-/HCO3- exchangers were expressed in A6 cells; both antiports were found to participate in pHi homeostasis under standard conditions. 3. Alterations in the rate of Na+ transport induced variations in pHi. 4. Na+ transport rate was a hyperbolic function of external Na+ concentration. The curve was shifted by changing pHi: a mixed inhibition of Na+ transport by pHi was found. 5. The pHi appears as a possible mediator coupling the rate of Na+ transport across the apical and the basolateral membrane in tight epithelia.