Volume 126, Issue 1, Page 269

British Journal of Pharmacology

January 1999, Volume 126, Issue 1, Pages 269 - 279

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Original Article

Modulation of chloride, potassium and bicarbonate transport by muscarinic receptors in a human adenocarcinoma cell line
Nicholas D. Holliday & Helen M. Cox¹
Division of Pharmacology & Therapeutics, GKT, St. Thomas's Medical School, Lambeth Palace Road, London SE1 7EH

¹Author for correspondence: E-mail: h.cox@umds.ac.uk

Keywords

Epithelial transport;   carbachol;   potassium channel

Abstract

1   Short-circuit current (I_SC) responses to carbachol (CCh) were investigated in Colony 1 epithelia, a subpopulation of the HCA-7 adenocarcinoma cell line. In Krebs-Henseleit (KH) buffer, CCh responses consisted of three I_SC components: an unusual rapid decrease (the 10 s spike) followed by an upward spike at 30 s and a slower transient increase (the 2 min peak). This response was not potentiated by forskolin; rather, CCh inhibited cyclic AMP-stimulated I_SC.
2   In HCO₃^- free buffer, the decrease in forskolin-elevated I_SC after CCh was reduced, although the interactions between CCh and forskolin remained at best additive rather than synergistic. When Cl^- anions were replaced by gluconate, both Ca²⁺- and cyclic AMP-mediated electrogenic responses were significantly inhibited.
3   Basolateral Ba²⁺ (1 - 10 mM) and 293B (10 µM) selectively inhibited forskolin stimulation of I_SC, without altering the effects of CCh. Under Ba²⁺- or 293B-treated conditions, CCh responses were potentiated by pretreatment with forskolin.
4   Basolateral charybdotoxin (50 nM) significantly increased the size of the 10 s spike of CCh responses in both KH and HCO₃^- free medium, without affecting the 2 min peak. The enhanced 10 s spike was inhibited by prior addition of 5 mM apical Ba²⁺. Charybdotoxin did not affect forskolin responses.
5   In epithelial layers prestimulated with forskolin, the muscarinic antagonists atropine and 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, both at 100 nM) abolished subsequent 10 µM CCh responses. Following addition of p-fluoro hexahydro-sila-difenidol (pF-HHSiD, 10 µM) or pirenzepine (1 µM), qualitative changes in the CCh response time-profile also indicated a rightward shift of the agonist concentration-response curve; however, 1 µM gallamine had no effect. These results suggest that a single M₃-like receptor subtype mediates the secretory response to CCh.
6   It is concluded that CCh and forskolin activate discrete populations of basolateral K⁺ channels gated by either Ca²⁺ or cyclic AMP, but that the Cl^- permeability of the apical membrane may limit their combined effects on electrogenic Cl^- secretion. In addition, CCh activates a Ba²⁺-sensitive apical K⁺ conductance leading to electrogenic K⁺ transport. Both agents may also modulate HCO₃^- secretion through a mechanism at least partially dependent on carbonic anhydrase.

Received 22 May 1998; Revised 21 September 1998; Accepted 6 October 1998