Cholinergic modulation of apical Na+ channels in turtle colon: analysis of CDPC-induced fluctuations.
Current fluctuation analysis was used to investigate the properties of apical Na+ channels during muscarinic inhibition of active Na+ absorption. A reversible Na+ channel blocker, 6-chloro-3,5-diaminopyrazine-2-carboxamide (CDPC), was used to induce fluctuations in the short-circuit current (I(sc)). Power density spectra of the CDPC-induced fluctuations exhibited a clearly discernible Lorentzian component, characterized by a corner frequency that was linearly related to CDPC concentration between 20 and 100 microM. The on (k'on) and off (k(off)) rate coefficients for the CDPC blocking reaction were k'on = 11.1 +/- 0.8 rad.s-1.microM-1 and k(off) = 744 +/- 53 rad/s, and the microscopic inhibition constant was 67 microM (n = 11). CDPC blocking kinetics were not significantly different after inhibition of Isc by 5 microM serosal carbachol. Single-channel Na+ current (iNa) and the density of open and blocked Na+ channels (N(ob)) were estimated from the fluctuations induced by 40 microM CDPC. Under control conditions, iNa was 0.43 +/- 0.05 pA and N(ob) was 251 +/- 42 X 10(6)/cm2 (n = 10). After exposure to serosal carbachol (2-10 microM) for 60 min, Na+ current and N(ob) were reduced by approximately 50%, but iNa was not changed significantly. These results indicate that muscarinic inhibition of electrogenic Na+ absorption was associated with a reduction in the number of open Na+ channels in the apical membrane. They also suggest that this downregulation of transport involved a coordinated decrease in both apical and basolateral membrane conductances.
Wilkinson
D J
DJ
Department of Physiology, University of Michigan Medical School, Ann Arbor 48109.
Dawson
D C
DC
D000584:Amiloride / Q000031:analogs & derivatives* / Q000494:pharmacology; D000818:Animals; D002217:Carbachol / Q000494:pharmacology; D003106:Colon / Q000502:physiology*; D004848:Epithelium / Q000187:drug effects / Q000502:physiology; D066298:In Vitro Techniques; D007700:Kinetics; D008433:Mathematics; D008564:Membrane Potentials / Q000187:drug effects; D008962:Models, Theoretical; D009130:Muscle, Smooth / Q000502:physiology; D015222:Sodium Channels / Q000187:drug effects / Q000502:physiology*; D014426:Turtles
D015222:Sodium Channels; C058206:6-chloro-3,5-diaminopyrazine-3-carboxamide; D000584:Amiloride; D002217:Carbachol