cAMP-mediated effects of ouabain and theophylline on paracellular ion selectivity

We studied the effects of theophylline and ouabain on isolated intestinal mucosa of the goldfish. From continuous recordings of the intracellular potential difference, we concluded that theophylline has no considerable effect on the Cl- conductance of the apical membrane. Theophylline increased transepithelial conductance and bidirectional Cl- fluxes, while Na+ fluxes did not change appreciably. Both streaming potentials and NaCl dilution potentials were immediately reduced by theophylline. Moreover, we found a striking similarity between the effects of theophylline and ouabain. Ouabain initially increased transepithelial conductance, increased bidirectional Cl- fluxes, and instantly abolished NaCl dilution potentials. Determinations of cellular cAMP levels revealed that both theophylline and ouabain increase cAMP. The phosphodiesterase inhibitor theophylline amplified the ouabain-induced rise in cAMP, suggesting that ouabain stimulates adenylate cyclase. Since elevated extracellular K+ did not affect cAMP levels, it can be ruled out that depolarization of the cell membrane due to ouabain increases cAMP. We conclude that in this epithelium both theophylline and ouabain increase the Cl- permeability of the paracellular pathway by a cAMP-dependent mechanism.