Secretagogues increase apical membrane conductance of isolated bullfrog corneal epithelium pretreated with loop diuretics.

Bullfrog (Rana catesbeiana) corneas were mounted in an Ussing type chamber and impaled with an intracellular microelectrode and the short circuit current was inhibited by pretreatment with the loop diuretics furosemide (0.3 to 1 mM) or bumetanide (10 to 100 microM). Subsequent addition of the secretagogues prostaglandin E2, forskolin, or 3-isobutyl-1-methylxanthine (IBMX) caused the fractional voltage drop of the apical barrier to decrease from 0.72 +/- 0.05 to 0.48 +/- 0.04 and the chloride-dependent conductance to increase by 0.15 +/- 0.03 mS/cm2, but caused only a small, transient increase in short circuit current. The loop diuretics by themselves always greatly reduced the short circuit current but did not consistently reduce conductance or fractional voltage drop of the apical membrane. Because the secretagogues were able to increase the apical membrane conductance of diuretic-inhibited corneas without large effects on the short circuit current, the loop diuretics must have a major effect at a site other than the apical membrane Cl- conductance, presumably at the basolateral membrane. An additional effect of the loop diuretics at the apical membrane is also possible.