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K+ current stimulation by Cl- in the midgut epithelium of tobacco hornworm (Manduca sexta)

I. Kinetics and effect of Cl--site-specific agents

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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 (I sc) when the tissue is mounted in an Ussing chamber and bathed in K+-rich standard saline containing 32 mmol K+ · l-1. I sc depends strictly on basolateral (i.e. haemolymph side) K+ and is therefore termed K+ current, I K. Basolateral, but not apical, chloride, bromide and iodide stimulate I K 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′-diisothiocyanato-stilbene-2,2′-disulphonic acid) reduce I K when added to the basolateral bath, whether Cl- or gluconate is the principal ambient anion. Cl- stimulates I K according to saturation kinetics. The Michaelis-Menten-type, K+ concentration-dependent, saturation of I K 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 I K develops in these conditions exclusively via basolateral, Ba2+-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.

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9-anthracene carboxylic acid


diphenylamine carboxylic acid


4,4′-diisothiocyanato-stilbene-2,2′-disulphonic acid



G :

conductance [cellular (c), shunt (sh)]

G t :

transepithelial conductance

G K :

K+ conductance

G Cl :

Cl- conductance

G Na :

Na+ conductance

x G :

conductance in absence of Cl-

G Na, KCl :

transepithelial conductance with Cl- saline

G Na, Kglu :

transepithelial conductance with gluconate saline

I K(max):

maximal K+ current

I sc :

short-circuit current

K :

Michaelis constant for saturating Cl- stimulation (index Cl) or K+ current saturation (index m)


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These results have already been communicated in part at the 1987 Copenhagen conference of the ESCPB (Zeiske and Schröder 1988) and the 1989 joint meeting of the German and Israel Physiological Societies at Jerusalem (Zeiske et al. 1990).

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Zeiske, W., Schröder, H. & Alpert, G. K+ current stimulation by Cl- in the midgut epithelium of tobacco hornworm (Manduca sexta). J Comp Physiol B 162, 331–339 (1992).

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Key words

  • K+ current
  • Cl- stimulation
  • Cl- blockers
  • Midgut
  • Manduca sexta larva