Effects of high potassium concentration on theophylline responses of toad bladder

It has been demonstrated previously that a high concentration of potassium in the serosal bathing medium (5-21.5 mM) potentiates the increase in short-circuit current caused by vasopressin or exogenous cyclic AMP. The same concentration of potassium in the bathing medium inhibited the increase in short-circuit current caused by theophylline. The increases in osmotic water permeability caused by vasopressin or cyclic AMP were unaffected by a serosal potassium concentration of 21.5 mM. The increase in osmotic water permeability caused by theophylline was inhibited by 21.5 mM potassium. The concentration of cyclic AMP in either intact total bladder or isolated toad bladder cells was increased two- or three-fold by theophylline. Increasing the concentration of potassium to 21.5 mM did not alter cyclic AMP concentration in either the absence of presence of theophylline. One interpretation of these results is that theophylline increases osmotic water flow and short-circuit current by a mechanism other than by inhibition of cyclic nucleotide phosphodiesterase.