Hydrochlorothiazide action on the apical Cl-, Ca2+ and K+ conductances in rabbit gallbladder epithelium. Presence of an apamin-sensitive, Ca(2+)-activated K+ conductance

Electrogenic bicarbonate secretion by prairie dog gallbladder

Pathological rates of gallbladder salt and water transport may promote the formation of cholesterol gallstones. Because prairie dogs are widely used as a model of this event, we characterized gallbladder ion transport in animals fed control chow by using electrophysiology, ion substitution, pharmacology, isotopic fluxes, impedance analysis, and molecular biology. In contrast to the electroneutral properties of rabbit and Necturus gallbladders, prairie dog gallbladders generated significant short-circuit current (I(sc); 171 +/- 21 microA/cm(2)) and lumen-negative potential difference (-10.1 +/- 1.2 mV) under basal conditions. Unidirectional radioisotopic fluxes demonstrated electroneutral NaCl absorption, whereas the residual net ion flux corresponded to I(sc). In response to 2 microM forskolin, I(sc) exceeded 270 microA/cm(2), and impedance estimates of the apical membrane resistance decreased from 200 Omega.cm(2) to 13 Omega.cm(2). The forskolin-induced I(sc) was dependent on extracellular HCO(3)(-) and was blocked by serosal 4,4'-dinitrostilben-2,2'-disulfonic acid (DNDS) and acetazolamide, whereas serosal bumetanide and Cl(-) ion substitution had little effect. Serosal trans-6-cyano-4-(N-ethylsulfonyl-N-methylamino)-3-hydroxy-2,2-dimethyl-chroman and Ba(2+) reduced I(sc), consistent with the inhibition of cAMP-dependent K(+) channels. Immunoprecipitation and confocal microscopy localized cystic fibrosis transmembrane conductance regulator protein (CFTR) to the apical membrane and subapical vesicles. Consistent with serosal DNDS sensitivity, pancreatic sodium-bicarbonate cotransporter protein pNBC1 expression was localized to the basolateral membrane. We conclude that prairie dog gallbladders secrete bicarbonate through cAMP-dependent apical CFTR anion channels. Basolateral HCO(3)(-) entry is mediated by DNDS-sensitive pNBC1, and the driving force for apical anion secretion is provided by K(+) channel activation.

Thiazide-sensitive NaCl-cotransporter in the Intestine

Thiazides, such as hydrochlorothiazide (HCTZ), are used to control blood pressure and to reduce renal calcium excretion. These effects are a result of interactions with the NaCl-cotransporter (NCC). This is demonstrated by the fact that mutations within the NCC protein lead to salt-resistant hypotension and hypocalciuria, paralleled by an increase in bone mineral density. These symptoms are also known as Gitelman syndrome. It has become increasingly evident that the effect of HCTZ on blood pressure and calcium homeostasis cannot be attributed exclusively to kidney functions, where the primary action of HCTZ on NCC is postulated to occur. We demonstrated the presence of the NCC transporter in the rat small intestine (ileum and jejunum) and human HT-29 cells, by using reverse transcription-PCR, Northern blot, Western blot, and immunofluorescence. Furthermore, we show that HCTZ modulates Ca(2+) uptake by intestinal cells, while affecting the electrical parameters of the cellular membrane, thus suggesting a functional interaction between NCC and the epithelial voltage-dependent calcium channel. The experiments presented here support the hypothesis of a direct involvement of the intestinal cells in the interaction between HCTZ and NaCl, as well as calcium homeostasis.

K+ channel cAMP activated in guinea pig gallbladder epithelial cells

In guinea pig gallbladder epithelial cells, an increase in intracellular cAMP levels elicits the rise of anion channel activity. We investigated by patch-clamp techniques whether K(+) channels were also activated. In a cell-attached configuration and in the presence of theophylline and forskolin or 8-Br-cAMP in the cellular incubation bath, an increase of the open probability (P(o)) values for Ca(2+)-activated K(+) channels with a single-channel conductance of about 160 pS, for inward current, was observed. The increase in P(o) of these channels was also seen in an inside-out configuration and in the presence of PKA, ATP, and cAMP, but not with cAMP alone; phosphorylation did not influence single-channel conductance. In the inside-out configuration, the opioid loperamide (10(-5) M) was able to reduce P(o) when it was present either in the microelectrode filling solution or on the cytoplasmic side. Detection in the epithelial cells by RT-PCR of the mRNA corresponding to the alpha subunit of large-conductance Ca(2+)-activated K(+) channels (BK(Ca)) indicates that this gallbladder channel could belong to the BK family. Immunohistochemistry experiments confirm that these cells express the BK alpha subunit, which is located on the apical membrane. Other K(+) channels with lower conductance (40 pS) were not activated either by 8-Br-cAMP (cell-attached) or by PKA + ATP + cAMP (inside-out). These channels were insensitive to TEA(+) and loperamide. The data demonstrate that under conditions that induce secretion, phosphorylation activates anion channels as well as Ca(2+)-dependent, loperamide-sensitive K(+) channels present on the apical membrane.

Increase in intrinsic anion conductance upon inhibition of the electroneutral Cl(-)/HCO(3)(-) exchanger: effect of CO(2)/HCO(3)(-)

The electroneutral Cl(-)/HCO(3)(-) exchange, present at the apical membrane of rabbit gallbladder epithelium, apparently is converted into a stilbene- and dipyridamole-sensitive, nonrectifying, approximately 5-pS anion channel after the exchange is directly inhibited (inhibitors tested: hydrochlorothiazide (HCTZ), phlorizin, phenylglyoxal and diphenylamine-2-carboxylic acid (DPC)). In intact tissue, in the absence of CO(2)/HCO(3)(-) in the media, the opening of these channels causes an approximately 7-mV depolarization of the apical membrane. This has been shown to be a constant index of the total Cl(-) conductance (G(Cl)) activated. The effect of exogenous and endogenous CO(2)/HCO(3)(-) on the depolarization has now been investigated in the intact tissue by conventional microelectrodes. The anion exchange has been measured radiochemically. The presence of exogenous or endogenous CO(2)/HCO(3)(-) reduces the depolarization induced by HCTZ, phlorizin and DPC from approximately 7 to 3 mV, but 10(-4) mol/l acetazolamide restores the full depolarization. Response time, S(0.5) and Hill number are unchanged in each case. The way of bicarbonate replacement is irrelevant. The depolarization generated by phenylglyoxal, which covalently binds to the transport site of the exchanger and prevents HCO(3)(-) binding, is unaffected by CO(2)/HCO(3)(-) presence. HCO(3)(-) binding to the transport site is suggested to partially hinder the conversion of the exchanger into the channel.