Artifactual expression of maxi-K+ channels in basolateral membrane of gallbladder epithelial cells

Stretch-activated single K+ channels account for whole-cell currents elicited by swelling

Functionally significant stretch-activated ion channels have been clearly identified in excitable cells. Although single-channel studies suggest their expression in other cell types, their activity in the whole-cell configuration has not been shown. This discrepancy makes their physiological significance doubtful and suggests that their mechanical activation is artifactual. Possible roles for these molecules in nonexcitable cells are acute cell-volume regulation and, in epithelial cells, the complex adjustment of ion fluxes across individual cell membranes when the rate of transepithelial transport changes. We report the results of experiments on isolated epithelial cells expressing in the basolateral membrane stretch-activated K+ channels demonstrable by the cell-attached patch-clamp technique. In these cells, reversible whole-cell currents were elicited by both isosmotic and hyposmotic cell swelling. Cation selectivity and block by inorganic agents were the same for single-channel and whole-cell currents, indicating that the same entity underlies single-channel and whole-cell currents and that the single-channel events are not artifactual. In these cells, when the rate of apical-membrane NaCl entry increases, the cell Na+ content and volume also increase, stimulating the Na+,K+-ATPase at the basolateral membrane, i.e., both Na+ extrusion and K+ uptake increase. We speculate that, under these conditions, the parallel activation of basolateral K+ channels (by the swelling) elevates conductive K+ loss, tending to maintain the cell K+ content constant ("pump-leak parallelism"). This study describes a physiologically relevant stretch-activated channel, at both the single-channel and whole-cell levels, in a nonneural cell type.

Reconstitution of single potassium channels from bovine gall-bladder epithelium

The study of molecular transport across gall-bladder epithelium may contribute to our understanding of the pathophysiology of gall-bladder disease. The aim of this study was to reconstitute and characterize single potassium ion channels in bovine gall-bladder epithelial mucosa - both apical and basolateral aspects. Standard subcellular fractionation techniques were used to form either apical or basolateral closed-membrane vesicles from the mucosal epithelium of fresh gall bladders from healthy young adult cattle. Vesicular ion channels were incorporated into voltage-clamped planar lipid bilayers under known ionic conditions and their conductances, reversal potentials, and voltages were characterized. Low-conductance voltage-insensitive apical membrane vesicle channels of at least four conductance levels were found (mean +/- SD): 12+/-4 pS, n = 10; 40+/-12 pS, n = 4; 273+/-31 pS, n = 3; and 151+/-24 pS, n = 5. Conductances of potassium ion channels in basolateral membrane vesicles were in the range 9 - 450 pS, and these channels included high-conductance calcium-activated potassium-ion channels 'K(Ca)' which were voltage- and calcium-dependent.

Mammalian retinal glial (Müller) cells express large-conductance Ca(2+)-activated K+ channels that are modulated by Mg2+ and pH and activated by protein kinase A

The cell-attached and excised patch configurations of the patch clamp technique were used to characterize Ca(2+)-activated maxi-K+ channels in freshly-isolated Müller glial cells. The cells were dissociated from postmortem adult human and porcine retinas. The maxi-K+ channels in Müller cells of both species display a single channel conductance of 175 pS in cell-attached and inside-out patches (125/110 mM K+). The channels are activated by membrane depolarization and by elevation of intracellular Ca2+. In the presence of 10(-5), 10(-4), and 10(-3) M intracellular free Ca2+, the half-activation voltages are +7.2, -26.6, and -47.5 mV, respectively. The half-activation-[Ca2+] at +10 mV is 8.1 microM, and the Hill coefficient of Ca2+ binding is 1.7, Ba2+ exerts a voltage-dependent block of the open-state probability. The maxi-K+ channels of Müller cells are activated by raising of the intracellular pH as well as by Mg2- ions at the cytosolic face of the channels. Phosphorylation of the channel after cytosolic addition of the catalytic subunit of a cAMP dependent protein kinase in the presence of Mg-ATP caused a shift of the activation curve to negative membrane potentials. Between -40 and -80 mV membrane potentials, the open-state probability rose to 190.3% of the control value (100%) after phosphorylation of the channel. Therefore, phosphorylation enhances sensitivity of the channels to Ca2+ and voltage. The maxi-K+ channels may provide a link between second messenger systems and membrane conductance of retinal Müller cells and may have an important function in repolarization of the Müller cell membrane and, therefore, in the maintainance of the retinal spatial K+ buffering mechanisms.