, Volume 90, Issue 1, pp 89-96

Basolateral membrane potential and conductance in frog skin exposed to high serosal potassium

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In studies of apical membrane current-voltage relationships, in order to avoid laborious intracellular microelectrode techniques, tight epithelia are commonly exposed to high serosal K concentrations. This approach depends on the assumptions that high serosal K reduces the basolateral membrane resistance and potential to insignificantly low levels, so that transepithelial values can be attributed to the apical membrane. We have here examined the validity of these assumptions in frog skins (Rana pipiens pipiens). The skins were equilibrated in NaCl Ringer's solutions, with transepithelial voltageV t clamped (except for brief perturbations ΔV t) at zero. The skins were impaled from the outer surface with 1.5m KCl-filled microelectrodes (R el>30 MΩ). The transepithelial (short-circuit) currentl i and conductanceg t=−ΔI tV t, the outer membrane voltageV o (apical reference) and voltage-divider ratio (F oV oV t), and the microelectrode resistanceR el were recorded continuously. Intermittent brief apical exposure to 20 μm amiloride permitted estimation of cellular (c) and paracellular (p) currents and conductances. The basolateral (inner) membrane conductance was estimated by two independent means: either from values ofg i andF o before and after amiloride or as the ratio of changes (−ΔI cV i) induced by amiloride. On serosal substitution of Na by K, within about 10 min,I c declined andg t increased markedly, mainly as a consequence of increase ing p. The basolateral membrane voltage (V i(=−V o) was depolarized from 75±4 to 2±1 mV [mean±sem (n=6)], and was partially repolarized following amiloride to 5±2 mV. The basolateral conductance increased in high serosal K, as estimated by both methods. Essentially complete depolarization of the basolateral membrane and increase in its conductance in response to high [K] were obtained also when the main serosal anion was SO4 or NO3 instead of Cl. On clampingV t over the range 0 to +125 mV in K2SO4-depolarized skins, the quasi-steady-stateV o V t relationship was linear, with a mean slope of 0.88±0.03. The above results demonstrate that, in a variety of conditions, exposure to high serosal K results in essentially complete depolarization of the basolateral membrane and a large increase in its conductance.