K+ current stimulation by Cl- in the midgut epithelium of tobacco hornworm (Manduca sexta). I. Kinetics and effect of Cl(-)-site-specific agents.
Goblet cells in the midgut epithelium of the tobacco hornworm (Manduca sexta larva, 5th instar) actively secrete K+. This can be measured as short-circuit current (Isc) when the tissue is mounted in an Ussing chamber and bathed in K(+)-rich standard saline containing 32 mmol K+.l-1. Isc depends strictly on basolateral (i.e. haemolymph side) K+ and is therefore termed K+ current, IK. Basolateral, but not apical, chloride, bromide and iodide stimulate IK when compared to the baseline current recorded with gluconate-, nitrate- or thiocyanate-containing salines. So-called "Cl(-)-specific" transport inhibitors (frusemide, 9-anthracene carboxylic acid, diphenylamine carboxylic acid and 4,4'-diisothiocyana-to-stilbene-2,2'-disulphonic acid) reduce IK when added to the basolateral bath, whether Cl- or gluconate is the principal ambient anion. Cl- stimulates IK according to saturation kinetics. The Michaelis-Menten-type, K+ concentration-dependent, saturation of IK is altered in a highly specific manner when gluconate is replaced by Cl-: maximal K+ current, as well as the apparent Michaelis constant, are increased by a factor of 4. Since IK develops in these conditions exclusively via basolateral, Ba(2+)-blockable K+ channels, these results can be understood if it is assumed that haemolymph Cl- interferes with the K+ channel by simultaneously lowering the binding affinity for K+ ions and increasing their subsequent transfer rate across the basolateral goblet cell membrane.
Zeiske
W
W
Institut für Tierphysiologie und Angewandte Zoologie, Freien Universität Berlin, FRG.
Schröder
H
H
Alpert
G
G
D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D018118:Chloride Channels; 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; D007700:Kinetics; D008565:Membrane Proteins / Q000378:metabolism; D009036:Moths / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism*
D018118:Chloride Channels; D002712:Chlorides; D005942:Gluconates; D008565:Membrane Proteins; C030691:gluconic acid; D011188:Potassium