title: Amiloride-sensitive sodium channels in confluent M-1 mouse cortical collecting duct cells.
abstract: Confluent M-1 cells show electrogenic Na+ absorption and possess an amiloride-sensitive Na(+)-conductance (Korbmacher et al., J. Gen. Physiol. 102:761-793, 1993). In the present study, we further characterized this conductance and identified the underlying single channels using conventional patch clamp technique. Moreover, we isolated poly(A)+ RNA from M-1 cells to express the channels in Xenopus laevis oocytes, and to check for the presence of transcripts related to the epithelial Na+ channel recently cloned from rat colon (Canessa et al., Nature 361:467-470, 1993). Patch clamp experiments were performed in 6-13-day-old confluent M-1 cells at 37 degrees C. In whole-cell experiments application of 10(-5) M amiloride caused a hyperpolarization of 24.9, SEM +/- 2.2 mV (n = 35) and a reduction of the inward current by 107 +/- 10 pA (n = 51) at a holding potential of -60 mV. Complete removal of bath Na+ had similar effects, indicating that the amiloride-sensitive component of the inward current is a Na+ current. The effect of amiloride was concentration-dependent with half-inhibition at 0.22 microM. The Na+ current saturated with increasing extracellular Na+ concentrations with an apparent Km of 24 mM. Na+ replacement for Li+ demonstrated a higher apical membrane conductance for Li+ than for Na+. In excised inside-out (i/o) or outside-out (o/o) patches from the apical membrane, we observed single-channels which showed slow kinetics and were reversibly inhibited by amiloride. Their average conductance for Na+ was 6.8 +/- 0.5 pS (n = 15) and for Li+ 11.2 +/- 1.0 pS (n = 14). They had no measurable conductance for K+. In o/o patches, channel activity was slightly voltage dependent with an open probability (NPo) of 0.46 +/- 0.14 and 0.16 +/- 0.05 at a holding potential of -100 and 0 mV, respectively (n = 8, P < 0.05). Using the two-microelectrode voltage-clamp technique, we assayed defolliculated stage V-VI Xenopus oocytes for an amiloride-sensitive inward current 1-6 days after injection with H2O or with 20-50 ng of M-1 poly(A)+ RNA. In poly(A)+ RNA-injected oocytes held at -60 or -100 mV application of amiloride (2 microM) reduced the Na-inward current by 25.5 +/- 4.6 nA (n = 25) while it had no effect in H2O-injected oocytes (n = 19). Northern blot analysis of M-1 poly(A+) RNA revealed the presence of transcripts related to the three known subunits of the rat colon Na+ channel (Canessa et al., Nature 367:463-467, 1994). We conclude that the channel in M-1 cells is closely related to the amiloride-sensitive epithelial Na+ channel in the rat colon and that the M-1 cell line provides a useful tool to investigate the biophysical and molecular properties of the corresponding channel in the cortical collecting duct.
authors.affiliation: Zentrum der Physiologie, Klinikum der Johann Wolfgang Goethe-Universität, Frankfurt am Main, Germany.
authors.forename: B
authors.initials: B
authors.lastname: Letz
authors.forename: A
authors.initials: A
authors.lastname: Ackermann
authors.forename: C M
authors.initials: CM
authors.lastname: Canessa
authors.forename: B C
authors.initials: BC
authors.lastname: Rossier
authors.forename: C
authors.initials: C
authors.lastname: Korbmacher
mesh_terms: D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D015152:Blotting, Northern; D002460:Cell Line; D004553:Electric Conductivity; D007672:Kidney Cortex / Q000166:cytology / Q000378:metabolism*; D007685:Kidney Tubules, Collecting / Q000166:cytology / Q000378:metabolism*; D051379:Mice; D009865:Oocytes; D018408:Patch-Clamp Techniques; D051381:Rats; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D015222:Sodium Channels / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D014981:Xenopus
chemical_list: D011994:Recombinant Proteins; D015222:Sodium Channels; D000584:Amiloride; D012964:Sodium