The Na+-HCO3- cotransporter of rat kidney (rkNBC) was expressed in Xenopus laevis oocytes to test whether cytosolic Ca2+ ([Ca2+]i) affects the cotransport stoichiometry. The current/voltage relationship of giant inside-out membrane patches of rkNBC-expressing oocytes was measured at near-physiological Na+ and HCO3- concentrations and the cotransport current, INBC, was defined as the current inhibited by 0.25 mmol/l tenidap. Essentially, we determined the reversal potential (VI=0) of INBC and the slope conductance (gNBC). The coupling ratio of (HCO3-) to Na+ (q) was calculated from VI=0. As reported in the preceding publication [Ducoudret et al., Pflügers Arch (2001) DOI 10.1007/s004240100594], in Ca2+-free solutions q was 2:1. This did not change when [Ca2+]i was increased to 0.1 micromol/l. At 0.5 micromol/l, however, only a few patches showed q=2:1, while most patches exhibited q=3:1. This indicates that [Ca2+]i affected the transport function of membrane-resident rkNBC molecules, and the bimodal distribution of VI=0 points to an indirect effect possibly mediated by differently expressed Ca2+-dependent protein kinases. The shift in q was associated with the predicted near twofold increase in gNBC and was confirmed by measurements of VI=0 at different Na+ and HCO3- concentrations. Because we previously observed that the cotransport in proximal tubule cells is susceptible to carbonic anhydrase (CA) inhibition, but only if it works at q=3:1, we propose that kNBC has three transport sites: when working at q=2:1 it binds 2 (HCO3-)+1 Na+, and while at q=3:1 it binds 1 CO3(2-)+1 HCO3- +1 Na+. The latter is equivalent to the transfer of 3 (HCO3-) +1 Na+, because in the presence of CA the generation of 1 CO3(2-) on one side of the membrane and its disintegration on the other transiently liberates 1 CO2, which follows by diffusion. This model explains the increase in (HCO3-) transport that is associated with the change in q from 2:1 to 3:1 by a selectivity change of a binding site from HCO3- to CO3(2-). This is more likely than the induction of a new transport pouch for a third (HCO3-) ion, which would require exceedingly large conformational changes of the transport protein.

In the present experiments we expressed the rat kidney Na+-HCO3- cotransporter (rkNBC) in Xenopus laevis oocytes to reinvestigate the flux coupling ratio under improved measuring conditions. Essentially the current/voltage (I/V) relationship of isolated inside-out giant membrane patches was measured and the stoichiometric ratio was calculated from the reversal potential (VI=0) of the cotransport current (INBC). INBC was defined as that part of the total current that was suppressed when rkNBC was inhibited. Previously we have used the disulfonic stilbene DIDS to inhibit rkNBC, but we now found that tenidap or benzamil are better suited as inhibitors. Tenidap blocked rkNBC rapidly and reversibly both from the intra- and extracellular surface with half maximal inhibition at 13 micromol/l and it did not cause the same potentially disturbing side effects as DIDS. In addition, we found that the endogenous depolarization-induced Na+ conductance of the oocyte, which may compromise the I/V analysis, can be suppressed by applying 1 mmol/l amiloride to the cytosolic surface of the patch. The new measuring conditions greatly increased the yield of successful experiments. The distribution of 27 measurements of VI=0 obtained at near physiological Na+ and HCO3- concentrations and in absence of Cl-, K+ and cytosolic Ca2+ showed that the calculated stoichiometric ratios closely approached the value of 2 HCO3-:1 Na+ if the expression density of rkNBC was high. This result fully confirms our previous observations. Further experiments showed that the difference between the stoichiometric ratio of 3:1 observed in rat proximal tubule in vivo and the present value is not due to the temperature difference. We conclude that, depending on local modulatory influences, rkNBC can operate with different stoichiometric ratios and the present data and those reported in an accompanying publication [Müller-Berger et al., Pflügers Arch (2001) DOI 10.1007/s004240100592] show that these ratios are integer numbers i.e. either 2:1 or 3:1.

In the present experiments we exposed the intra- or extracellular surface of excised giant membrane patches of Xenopus laevis oocytes bathed in 140 mmol/l Na-aspartate solution to the anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS, 250 micromol/l). We observed that DIDS activated at least two cation conductances: (1) a non-selective cation (NSC) conductance that was mediated by channels of approximately 27 pS and resembled the stretch-activated cation conductance that has been observed in the oocyte cell membrane previously, and (2) a Na+-selective conductance, the single-channel events of which could not be resolved and which resembled the depolarization-induced Na+ conductance that has also been observed in the oocyte cell membrane previously. Both conductances were blocked by 1 mmol/l amiloride from the intra- and extracellular surfaces but inhibition of the NSC conductance by extracellular amiloride was less pronounced. Both conductances activated only slowly with a delay of 15-60 s after application of DIDS and remained active even after DIDS was washed off. This suggests that DIDS caused the exocytosis of preformed channels and this interpretation was supported by our additional observation that extracellular application of maitotoxin (MTX) mimicked the effects of DIDS. MTX is a marine toxin that has recently been reported to induce exocytosis in Xenopus laevis oocytes. The fact that DIDS and MTX each carry two sulfonyl groups suggests that they act on the same positively charged binding sites of an exocytosis-inducing protein. Our observations demonstrate that using DIDS to inhibit heterologously expressed anion transporters in the cell membrane of Xenopus laevis oocytes may compromise proper determination of the transporter currents. This effect can be prevented if the DIDS-activated endogenous cation conductances are suppressed by application of amiloride to the cytoplasmic surface of the cell membrane.

We have investigated volume-activated taurine transport and ultrastructural swelling response of sea bass gill cells in culture, assuming that euryhaline fish may have developed particularly efficient mechanisms of salinity adaptation. In vivo, when sea basses were progressively transferred from seawater to freshwater, we noticed a decrease in blood osmotic pressure. When gill cells in culture were subjected to 30% hypotonic shock, we observed a five-fold stimulation of [(3)H]taurine efflux. This transport was reduced by various anion channel inhibitors with the following efficiency: 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) > niflumic acid > DIDS = diphenylamine-2-carboxylic acid. With polarized gill cells in culture, the hypotonic shock produced a five-fold stimulation of apical taurine transport, whereas basolateral exit was 25 times higher. Experiments using ionomycin, thapsigargin, BAPTA-AM, or removal of extracellular calcium suggested that taurine transport was regulated by external calcium. The inhibitory effects of lanthanum and streptomycin support Ca(2+) entry through mechanosensitive Ca(2+) channels. Branchial cells also showed hypotonically activated anionic currents sensitive to DIDS and NPPB. Similar pharmacology and time course suggested the potential existence of a common pathway for osmosensitive taurine and Cl(-) efflux through volume-sensitive organic osmolyte and anion channels. A three-dimensional structure study revealed that respiratory gill cells began to swell only 15 s after hypoosmotic shock. Apical microridges showed membrane outfoldings: the cell surface became smoother with a progressive disappearance of ridges. Therefore, osmotic swelling may not actually induce membrane stretch per se, inasmuch as the microridges may provide a reserve of surface area. This work demonstrates mechanisms of functional and morphological plasticity of branchial cells during osmotic stress.

The aim of this study was to characterize the mechanism implicated in Zn(2+) transport in MDCK cells. Trace elements such as Zn(2+), Cd(2+) or Cu(2+) induced MDCK cell depolarization at the micromolar level as demonstrated by bis-oxonol fluorescence and whole-cell patch experiments. This depolarization was inhibited by La(3+) and Gd(3+) and was not related to the activation of Na(+) or Cl(-) channels. Uptake of 65Zn was assessed under initial rate conditions. The kinetic parameters obtained at 37 degrees C were a K(m) of 18.9 microM and a V(max) of 0.48 nmol min(-1) (mg protein(-1)). Intracellular pH measurements using BCECF probe demonstrated that Zn(2+) transport induced a cytoplasmic acidification. The cytoplasmic acidification resulting from Zn(2+) uptake activated Na(+)/H(+) antiporter, which allowed for the recycling of protons. These data suggest that Zn(2+) enters MDCK cells through a proton-coupled metal-ion transporter, the characteristics of which are slightly different from those described for the metal transporter DCT1. This mechanism could be in part responsible of the metal transport evidenced in the distal parts of the renal tubule.