K+ current stimulation by Cl- in the midgut epithelium of tobacco hornworm (Manduca sexta). II. Analysis of Ba(2+)-induced K+ channel conduction noise.
Chloride-stimulated K+ secretion by Manduca sexta midgut (5th-instar larvae) was measured as K(+)-carried short-circuit current of the tissue mounted in an Ussing chamber. "Microscopic" parameters, such as single-channel current and channel density for the rate-determining passive transport step across the basolateral goblet cell membrane (i.e. K+ channels), were estimated by means of current-fluctuation analysis of the K+ channel blockade by haemolymph-side Ba2+ ions. Ba2+ was equally effective with Cl- or gluconate (Glu-) as the principal ambient anion. The Ba(2+)-induced K+ channel conduction noise is reflected by a Lorentzian, or relaxation, noise component in the power spectrum of the K+ current fluctuations. A reduced Lorentzian plateau value, but an unchanged corner frequency, were observed when Cl- was replaced by Glu-. The results from the analysis of a "two-state" model of K+ channel block by Ba2+, with respect to the anion-replacement effects, suggest that the observed changes in K+ current and Lorentzian plateau value mirror a complex change of the underlying parameters: Cl- omission reduces single channel current but increases channel density so that the product of single channel current and channel density is smaller in Glu- than in Cl-. It seems likely that basolateral K+ channels (1) are subject to anionic gating ligands, and (2) depend on anions with respect to the rate of K+ transfer through an open K+ channel.
Zeiske
W
W
Institut für Tierphysiologie und Angewandte Zoologie, Freien Universität Berlin, FRG.
Marin
H
H
D000818:Animals; D001464:Barium / Q000494:pharmacology; D001693:Biological Transport, Active / Q000187:drug effects; D002712:Chlorides / Q000378:metabolism* / Q000494:pharmacology; D004064:Digestive System / Q000378:metabolism; D004848:Epithelium / Q000378:metabolism; D005942:Gluconates / Q000378:metabolism / Q000494:pharmacology; D006458:Hemolymph / Q000378:metabolism; D009036:Moths; D011188:Potassium / Q000378:metabolism*; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism
D002712:Chlorides; D005942:Gluconates; D015221:Potassium Channels; D001464:Barium; C030691:gluconic acid; D011188:Potassium