Probing the water permeability of ROMK1 and amphotericin B channels using Xenopus oocytes

Roles of aquaporin-3 water channels in volume-regulatory water flow in a human epithelial cell line

Membrane water transport is an essential event not only in the osmotic cell volume change but also in the subsequent cell volume regulation. Here we investigated the route of water transport involved in the regulatory volume decrease (RVD) that occurs after osmotic swelling in human epithelial Intestine 407 cells. The diffusion water permeability coefficient (Pd) measured by NMR under isotonic conditions was much smaller than the osmotic water permeability coefficient (Pf) measured under an osmotic gradient. Temperature dependence of Pf showed the Arrhenius activation energy (Ea) of a low value (1.6 kcal/mol). These results indicate an involvement of a facilitated diffusion mechanism in osmotic water transport. A mercurial water channel blocker (HgCl(2)) diminished the Pf value. A non-mercurial sulfhydryl reagent (MMTS) was also effective. These blockers of water channels suppressed the RVD. RT-PCR and immunocytochemistry demonstrated predominant expression of AQP3 water channel in this cell line. Downregulation of AQP3 expression induced by treatment with antisense oligodeoxynucleotides was found to suppress the RVD response. Thus, it is concluded that AQP3 water channels serve as an essential pathway for volume-regulatory water transport in, human epithelial cells.

Isolation and characterization of theXenopusoocyte plasma membrane: a new method for studying activity of water and solute transporters

The intact Xenopus laevis oocyte is a useful model system for studying expressed water and solute transporters but suffers from a number of limitations, most notably large unstirred layers and other intracellular diffusion barriers. To overcome these, we have developed a method for isolating plasma membrane vesicles from oocytes. This approach facilitates more precise control of the intravesicular environment and virtually eliminates the problem of unstirred layers in kinetic experiments. The isolation procedure results in 50.6-fold enrichment of the plasma membrane marker alkaline phosphodiesterase compared with the homogenate. Markers of late endosomes/lysosomes and mitochondria were not enriched, and the endoplasmic reticulum was enriched only modestly. Permeabilities of native plasma membrane to water and urea were 8.1 × 10−4and 5.6 × 10−7cm/s, respectively, values that are sufficiently low to classify them as barrier membranes. Phospholipid analysis by mass spectrometry showed the membrane, not including cholesterol, to be rich in phosphatidylcholine (35.8 mole percent), sphingomyelin (25.8 mole percent), and phosphatidylinositol (6.8 mole percent). Cholesterol concentration was 20.7 mole percent. Membrane vesicles isolated from oocytes expressing aquaporin-1 exhibited fourfold higher water permeability in stopped-flow experiments. Oocytes expressing mouse urea transporter A3 (UT-A3) exhibited 7.5-fold faster phloretin-inhibitable urea transport compared with water-injected controls. There was no difference in water permeability between these membrane vesicles, suggesting that UT-A3 is not a water carrier. In conclusion, we describe an improved method for the isolation of the oocyte plasma membrane that will allow the study of water and solute transport kinetics as well as substrate selectivity in heterologously expressed proteins.

Combined transport of water and ions through membrane channels

The coupling of ion and water flow through membrane channels is under dispute. Among all human aquaporins only aquaporin-6 exhibits ion channel activity. Whether aquaporin-6 functions also as a water channel cannot yet be determined with confidence. Similarly, a comparison of single-channel water permeabilities from ion channels and aquaporins suggests that ion channels may play a secondary role as water channels. However, the fraction of absorbed fluid that crosses epithelial ion channels still remains to be determined.

Molecular diversity and regulation of renal potassium channels

K(+) channels are widely distributed in both plant and animal cells where they serve many distinct functions. K(+) channels set the membrane potential, generate electrical signals in excitable cells, and regulate cell volume and cell movement. In renal tubule epithelial cells, K(+) channels are not only involved in basic functions such as the generation of the cell-negative potential and the control of cell volume, but also play a uniquely important role in K(+) secretion. Moreover, K(+) channels participate in the regulation of vascular tone in the glomerular circulation, and they are involved in the mechanisms mediating tubuloglomerular feedback. Significant progress has been made in defining the properties of renal K(+) channels, including their location within tubule cells, their biophysical properties, regulation, and molecular structure. Such progress has been made possible by the application of single-channel analysis and the successful cloning of K(+) channels of renal origin.

Upregulation of swelling-activated Cl- channel sensitivity to cell volume by activation of EGF receptors in murine mammary cells

Whole-cell recordings showed that, in mouse mammary C127 cells transfected with the full genome of the bovine papilloma virus (BPV), a hypotonic challenge induced the activation of outwardly rectifying Cl- currents with a peak amplitude 2.7 times greater than that in control C127 cells. Cell-attached single-channel recordings showed that BPV-induced augmentation of the peak amplitude of the whole-cell current could not chiefly be explained by a small increase (1.2 times) in unitary conductance. There was no difference between control and BPV-transfected cells in the osmotic cell swelling rate, and hence, osmotic water permeability. However, a plot of the whole-cell current density as a function of cell volume, which was measured simultaneously, showed that the BPV-transfected cells had a strikingly greater volume sensitivity than control cells. Since the E5 protein of BPV has been reported to induce constitutive activation of the epidermal growth factor (EGF) receptor and platelet-derived growth factor (PDGF) receptor in a variety of cell lines including C127 cells, effects of the growth factors on volume-sensitive outwardly rectifying (VSOR) Cl- currents were examined in C127 cells. Application of PDGF peptides failed to affect the Cl- currents in control and BPV-transfected cells, although C127 cells are known to endogenously express PDGF receptors. In contrast, EGF peptides significantly increased the VSOR Cl- current in control cells. However, they failed to induce further augmentation of the current in BPV-transfected cells. VSOR Cl- currents were inhibited by tyrphostin B46, an inhibitor of the EGF receptor tyrosine kinase, in both control and BPV-transfected cells. The IC50 value in BPV-transfected cells (12 micro M) was lower than that in control cells (31 micro M). However, the VSOR Cl- currents in both cell types were insensitive to tyrphostin AG1296, an inhibitor of the PDGF receptor tyrosine kinase. The rate of regulatory volume decrease (RVD) was markedly diminished by tyrphostin B46 but not significantly affected by tyrphostin AG1296. We thus conclude that the EGF receptor tyrosine kinase upregulates the activity of the VSOR Cl- channel, mainly by enhancing the volume sensitivity.

Effects of a pulsing electric signal on the cross membrane potential and the cell division potentiality of a single cell of tobacco

A pulsing electric signal (pulse width 10 s) was applied to a single cell of cultured tobacco, line BY-2, by inserting a multifunctional microelectrode (MME) into the cell. The electric voltage (V(ET)) was loaded between the electrode terminals of the MME and the reference electrode situated in the extracellular medium. Since the electrical impedance of the MME was as large as that of the cell membrane, the effective potential acting across the cell membrane (V(CMP)) should be only some portion of V(ET). The MME enabled simultaneous measurement of V(ET) and V(CMP). When V(ET) was varied from 0 to -1 V, V(CMP) changed linearly in proportion to V(ET). When V(ET) variation range was enlarged (from 0 to -2 V), V(CMP) changing pattern became a declined curve. When V(ET) variation range was further enlarged (from 0 to -5 V), the V(CMP) changing pattern showed a saturation curve. Under this condition, the cell division potentiality decreased accordingly. Based on these results, the feasibility of V(CMP) as an indicator of the effective intensity of an electric stress signal is discussed. In the present case of a BY-2 cell, a proper intensity of V(CMP) that could cause an appreciable stress and not a lethal signal was estimated as -250 mV.