Passive H+/OH- permeability in epithelial brush border membranes

A molecular theory for nonohmicity of the ion leak across the lipid-bilayer membrane

The current-voltage relationship of ion leak (i.e., ion transport involving neither special channels nor carriers) across the lipid-bilayer membrane has been observed to be log-linear above the ohmic regime. The coefficient of the linear term has been found to be universal for membranes and penetrants examined. This universality has been explained in terms of diffusion in an external field, where the ion position is described as a Markovian process. Such a diffusion picture can be questioned, however. It is also probable that a leaking ion gets over the potential barrier before experiencing sufficient random collision in the membrane, considering that each ion is surrounded with long lipid molecules aligned almost unidirectionally. As an alternative, we discuss this ion leak in terms of velocity distribution of the ions entering the membrane and density fluctuation of the lipids. We conclude that we can explain the universality without resorting to the diffusion picture.

Cloning, functional analysis and cell localization of a kidney proximal tubule water transporter homologous to CHIP28

The localization and transporting properties of a kidney protein homologous to human erythrocyte protein CHIP28 was evaluated. The cDNA encoding rat kidney protein CHIP28k was isolated from a rat renal cortex cDNA library. A 2.8-kb cDNA was identified which contained an 807 bp open reading frame encoding a 28.8 kD protein with 94% amino acid identity to CHIP28. in vitro translation of CHIP28k cDNA in rabbit reticulocyte lysate generated a 28-kD protein; addition of ER-derived microsomes gave a 32-kD transmembrane glycoprotein. Translation of truncated RNA demonstrated glycosylation of residue Asn42 which is predicted to lie between the first and second transmembrane domains. Expression of in vitro transcribed mRNA encoding CHIP28k in Xenopus oocytes increased oocyte osmotic water permeability (Pf) from (4 +/- 1) x 10(-4) to (33 +/- 4) x 10(-4) cm/s at 10 degrees C; the increase in oocyte Pf was weakly temperature dependent and inhibited by HgCl2. Two-electrode voltage clamp measurements indicated that CHIP28k was not permeable to ions. Oocyte Pf also increased with expression of total mRNA from kidney cortex and papilla; the increase in Pf with mRNA from cortex, but not kidney papilla, was blocked by coinjection with excess antisense CHIP28k cRNA. In situ hybridization of a 150 base cRNA antisense probe to tissue sections from rat kidney showed selective CHIP28k localization to epithelial cells in proximal tubule and thin descending limb of Henle. Pf in purified apical membrane vesicles from rat and human proximal tubule, and in proteoliposomes reconstituted with purified protein, was very high and inhibited by HgCl2; stripping of apical vesicles with N-lauroylsarcosine enriched a 28-kD protein by 25-fold and yielded a vesicle population with high water, but low urea and proton permeabilities. CHIP28k identity was confirmed by NH2-terminus sequence analysis. These results indicate that CHIP28k is a major and highly selective water transporting protein in the kidney proximal tubule and thin descending limb of Henle, but not collecting duct.

Endocytic vesicles from renal papilla which retrieve the vasopressin-sensitive water channel do not contain a functional H+ ATPase

The water permeability of the kidney collecting duct epithelium is regulated by vasopressin (VP)-induced recycling of water channels between an intracellular vesicular compartment and the plasma membrane of principal cells. To test whether the water channels pass through an acidic endosomal compartment during the endocytic portion of this pathway, we measured ATP-dependent acidification of FITC-dextran-labeled endosomes in isolated microsomal fractions from different regions of Brattleboro rat kidneys. Both VP-deficient controls and rat treated with exogenous VP were examined. ATP-dependent acidification was not detectable in endosomes containing water channels from distal papilla (osmotic water permeability Pf = 0.038 +/- 0.004 cm/s). In contrast, the addition of ATP resulted in a strong acidification of renal cortical endosomes (pHmin = 5.8, initial rate = 0.18-0.25 pH U/s). Acidification of cortical endosomes was reversed with nigericin and strongly inhibited by N-ethyl-maleimide. Passive proton permeability was similar and low in both cortical and papillary endosomes from rats treated or not treated with VP. The fraction of labeled endosomes present in microsomal preparations was determined by fluorescence imaging microscopy of microsomes nonspecifically bound to poly-l-lysine-coated coverslips and was 25% in cortical preparations compared to 14% (+VP) and 9% (-VP) in papillary preparations. The fraction of cortical endosomes was enriched 1.5-fold by immunoabsorption to coverslips coated with mAbs against the bovine vacuolar proton pump. In contrast, the fraction of papillary endosomes was depleted more than twofold by immunoabsorption to identical coverslips. Finally, sections of distal papilla stained with antibodies against the lysosomal glycoprotein LGP120 showed that most of the entrapped FITC-dextran did not colocalize with this lysosomal protein. These results demonstrate that vesicles which internalize water channels in kidney collecting duct principal cells lack functional proton pumps, and do not deliver the bulk of their FITC-dextran content to lysosomes. The data suggest that the principal cell contains a specialized nonacidic apical endocytic compartment which functions primarily to recycle membrane components, including water channels, to the plasma membrane.

Water, proton, and urea transport in toad bladder endosomes that contain the vasopressin-sensitive water channel

Vasopressin (VP) increases the water permeability of the toad urinary bladder epithelium by inducing the cycling of vesicles containing water channels to and from the apical membrane of granular cells. In this study, we have measured several functional characteristics of the endosomal vesicles that participate in this biological response to hormonal stimulation. The water, proton, and urea permeabilities of endosomes labeled in the intact bladder with fluorescent fluid-phase markers were measured. The diameter of isolated endosomes labeled with horse-radish peroxidase was 90-120 nm. Osmotic water permeability (Pf) was measured by a stopped-flow fluorescence quenching assay (Shi, L.-B., and A. S. Verkman. 1989. J. Gen. Physiol. 94:1101-1115). The number of endosomes formed when bladders were labeled in the absence of a transepithelial osmotic gradient increased with serosal [VP] (0-50 mU/ml), and endosome Pf was very high and constant (0.08-0.10 cm/s, 18 degrees C). When bladders were labeled in the presence of serosal-to-mucosal osmotic gradient, the number of functional water channels per endosome decreased (at [VP] = 0.5 mU/ml, Pf = 0.09 cm/s, 0 osmotic gradient; Pf = 0.02 cm/s, 180 mosmol gradient). Passive proton permeability was measured from the rate of pH decrease in voltage-clamped endosomes in response to a 1 pH unit gradient (pHin = 7.5, pHout = 6.5). The proton permeability coefficient (PH) was 0.051 cm/s at 18 degrees C in endosomes containing the VP-sensitive water channel; PH was not different from that measured in vesicles not containing water channels. Measurement of urea transport by the fluorescence quenching assay gave a urea reflection coefficient of 0.97 and a permeability coefficient of less than 10(-6) cm/s. These results demonstrate: (a) VP-induced endosomes from toad urinary bladder have extremely high Pf. (b) In states of submaximal bladder Pf, the density of functional water channels in endosomes in constant in the absence of an osmotic gradient, but decreases in the presence of a serosal-to-mucosal gradient, suggesting that the gradient has a direct effect on the efficiency of packaging of water channels into endosomes. (c) The VP-sensitive water channel does not have a high proton permeability. (d) Endosomes that cycle the water channel do not contain urea transporters. These results establish a labeling procedure in which greater than 85% of labeled vesicles from toad urinary bladder are endosomes that contain the VP-sensitive water channel in a functional form.