Modulation of chloride, potassium and bicarbonate transport by muscarinic receptors in a human adenocarcinoma cell line

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 . In HCO 3 − 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 Ca 2+ ‐ and cyclic AMP‐mediated electrogenic responses were significantly inhibited. Basolateral Ba 2+ (1–10 m M ) and 293B (10 μ M ) selectively inhibited forskolin stimulation of I SC , without altering the effects of CCh. Under Ba 2+ ‐ or 293B‐treated conditions, CCh responses were potentiated by pretreatment with forskolin. Basolateral charybdotoxin (50 n M ) significantly increased the size of the 10 s spike of CCh responses in both KH and HCO 3 − free medium, without affecting the 2 min peak. The enhanced 10 s spike was inhibited by prior addition of 5 m M apical Ba 2+ . Charybdotoxin did not affect forskolin responses. In epithelial layers prestimulated with forskolin, the muscarinic antagonists atropine and 4‐diphenylacetoxy‐N‐methylpiperidine methiodide (4‐DAMP, both at 100 n M ) abolished subsequent 10 μ M CCh responses. Following addition of p ‐fluoro hexahydro‐sila‐difenidol ( p F‐HHSiD, 10 μ M ) or pirenzepine (1 μ M ), qualitative changes in the CCh response time‐profile also indicated a rightward shift of the agonist concentration‐response curve; however, 1 μ M gallamine had no effect. These results suggest that a single M 3 ‐like receptor subtype mediates the secretory response to CCh. It is concluded that CCh and forskolin activate discrete populations of basolateral K + channels gated by either Ca 2+ 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 Ba 2+ ‐sensitive apical K + conductance leading to electrogenic K + transport. Both agents may also modulate HCO 3 − secretion through a mechanism at least partially dependent on carbonic anhydrase.British Journal of Pharmacology (1999) 126 , 269–279; doi: 10.1038/sj.bjp.0702270