Secretory apical Cl- channels in A6 cells: possible control by cell Ca2+ and cAMP

Distal kidney cells (A6) from Xenopus laevis were cultured to confluency on porous supports. Tissues were mounted in Using-type chambers to measure sh ort-circuit current (I-SC), transepithelial conductance and capacitance, an d to analyse the fluctuation in I-SC. In the absence of apical NaCl, but wi th normal basolateral NaCl Ringer's solution, extracellular addition of ATP , oxytocin, a membrane-permeant cAMP derivative, and forskolin produced a t ransient increase of the electrical parameters. Noise analysis revealed a s pontaneous Lorentzian component. All responses depend strictly on the prese nce of basolateral Cl- and are caused by the activation of an apical (CFTR type) Cl- permeability. Repetitive treatment with ATP (or oxytocin) resulte d in refractoriness. ATP and oxytocin acted antagonistically, whereas cAMP and ATP had additive effects. Incubation with the vesicular Ca2+ pump inhib itor thapsigargin or application of the Ca2+ channel blocker nifedipine eli cited finite but variable Cl- channel activity. After treatment with nifedi pine or thapsigargin, the response to oxytocin was severely impaired. We sp eculate that not only cAMP but also cell Ca2+ plays a crucial role in the a ctivation of CFTR in A6. Ca2+ may be multifunctional but the rise in capaci tance (apical area) observed with all stimulants strongly suggests its invo lvement in, and contribution to, exocytosis in the process of the CFTR-medi ated transcellular Cl- movements.