Resistance properties of the diluting segment ofAmphiuma kidney: Influence of potassium adaptation

Abstract

Chronic exposure to high potassium stimulates K+-secretory mechanisms in the diluting segment of the amphibian kidney (K+ adaptation). Since K+ net flux depends critically on the passive cell membrane permeabilities for K+ ions, cable analysis and K+-concentration step changes were applied in this nephron segment to assess the individual resistances of the epithelium and the K+ conductance of the luminal cell membrane. Experiments were performed in the isolated, doubly-perfused kidney of both control and K+-adapted Amphiuma. In control animals transepithelial resistance was 290 +/- 27 omega cm2, which decreased significantly to 199 +/- 17 omega cm2 after K+ adaptation. The resistance in parallel of the luminal and peritubular cell membrane decreased from a control value of 157 +/- 14 to 108 +/- 6 omega cm2 after chronic K+ treatment. This was paralleled by a decrease of the ratio of the luminal to peritubular cell membrane resistance from 2.5 +/- 0.1 to 1.9 +/- 0.1, respectively. Estimation of the individual cell membrane resistances reveals that the combined resistance of the luminal and peritubular cell membrane is in the same order of magnitude as the paracellular shunt resistance in diluting segments of both control and K+-adapted animals. The luminal cell membrane is K+ selective under both conditions, but the absolute luminal K+ conductance increases by some 60% with K+ adaptation. This leads to an increased back-leak of K+ from cell to lumen and may explain stimulated K+ net secretion found after chronic K+ loading.

High-resistance MDCK-C7 monolayers used for measuring invasive potency of tumour cells

“…The two cell clones are stable and their transepithelial electrical resistances and other cell properties are virtually independent of passage number. For obvious reasons (based on the assumption that a high transepithelial electrical resistance primarily reflects a low passive ion permeability of the paracellular pathway; [11]) the highresistance C7 cell clone is the most suitable for identifying resistance breakdown due to invasion of carcinoma cells. Experiments can be started at electrical resistances of above 4,000 Ω cm 2 down to 25 Ω cm 2 , the background resistance of the filter set-up.…”

Regulation of tight junction permeability with switch-like speed

“…The Malpighian tubule has a transepithelial resistance of 58 O×cm 2 , of which the resistance of the transcellular active transport pathway is 180 O×cm 2 and the paracellular resistance 85 O×cm 2 [13][14][15]. The paracellular resistance relative to the transcellular resistance is thus approximately 0.5 as in amphibian distal tubules or rabbit colon which are considered moderately tight epithelia [16,17]. The transepithelial voltage is 59 mV, lumen positive, again consistent with a moderately tight epithelium.…”

Apical potassium channels in Amphiuma diluting segment: effect of barium.

“…For example, the inward conductance is almost the same as that of heart muscle (Sakmann & Trube, 1984a;Kurachi, 1985), yet bigger than that of egg cell membranes (Ohmori, 1978;Fukushima, 1981). However, in these other channel types it was virtually impossible to detect any outward currents in the steady state since the channels rapidly closed upon depolarization (Kurachi, 1985 The K+ conductance of the apical membrane of the Amphiuma diluting segment is known to be sensitive to Ba2+ (Oberleithner et al 1985). From the present study, the mechanism of block has been shown to be dependent upon the concentration of barium and the transmembrane voltage.…”