Activation of the basolateral membrane Cl super(-) conductance essential for electrogenic K super(+) secretion suppresses electrogenic Cl super(-) secretion

Adrenaline activates transient Cl super(-) secretion and sustained K super(+) secretion across isolated distal colonic mucosa of guinea-pigs. The Ca super(2+)-activated Cl super(-) channel inhibitor CaCCinh-A01 (30 mu m) significantly reduced electrogenic K super(+) secretion, detected as short-circuit current (I sub(sc)). This inhibition supported the cell model for K super(+) secretion in which basolateral membrane Cl super(-) channels provide an exit pathway for Cl super(-) entering the cell via Na super(+)-K super(+) -2Cl super(-) cotransporters. CaCCinh-A01 inhibited both I sub(sc) and transepithelial conductance in a concentration-dependent manner (IC sub(50)= 6.3 mu m). Another Cl super(-) channel inhibitor, GlyH-101, also reduced sustained adrenaline-activated I sub(sc) (IC sub(50)= 9.4 mu m). Adrenaline activated whole-cell Cl super(-) current in isolated intact colonic crypts, confirmed by ion substitution. This adrenaline-activated whole-cell Cl super(-) current was also inhibited by CaCCinh-A01 or GlyH-101. In contrast to K super(+) secretion, CaCCinh-A01 augmented the electrogenic Cl super(-) secretion activated by adrenaline as well as that activated by prostaglandin E sub(2). Synergistic Cl super(-) secretion activated by cholinergic/prostaglandin E sub(2) stimulation was insensitive to CaCCinh-A01. Colonic expression of the Ca super(2+)-activated Cl super(-) channel protein Tmem16A was supported by RT-PCR detection of Tmem16A mRNA, by immunoblot with a Tmem16A antibody, and by detection of immunofluorescence in lateral membranes of epithelial cells. Alternative splices of Tmem16A were detected for exons that are involved in channel activation. Inhibition of K super(+) secretion and augmentation of Cl super(-) secretion by CaCCinh-A01 support a common colonic cell model for these two ion secretory processes, such that activation of basolateral membrane Cl super(-) channels contributes to the production of electrogenic K super(+) secretion and limits the rate of Cl super(-) secretion. Maximal physiological Cl super(-) secretion occurs only for synergistic activation mechanisms that close these basolateral membrane Cl super(-) channels.