AJP - Cell Physiology, Vol 259, Issue 2 C215-C223, Copyright © 1990 by American Physiological Society
Forskolin-induced HCO3- current across apical membrane of the frog corneal epithelium
O. A. Candia
Department of Ophthalmology, Mount Sinai School of Medicine, New York, New York 10029.
Forskolin (and other Cl- secretagogues) does not affect the very small
Na(+)-originated short-circuit current (Isc) across frog corneal epithelium
bathed in Cl- free solutions. However, forskolin in combination with
increased PCO2 bubbling of the solutions (5-20% CO2) stimulated Isc
proportionally to PCO2 to a maximum of approximately 8 microA/cm2. This
current could be eliminated and reinstated by sequentially changing the gas
composition of the bubbling to 100% air and 20% CO2-80% air. The same
effects were observed when PCO2 changes were limited to the apical-side
solution. Stroma-to-tear HCO3- movement was deemed unlikely, since the
increase in Isc was observed with a HCO3(-)-free solution on the stromal
side and CO2 gassing limited to the tear side. From the effects of ouabain
and tryptamine, at least 80% of the Isc across the basolateral membrane can
be accounted for by the Na+ pump current plus K+ movement from cell to
bath. Methazolamide also inhibited Isc. Current across the apical membrane
cannot be attributed to an electronegative Na(+)-HCO3- symport given the
insensitivity of Isc to a disulfonic stilbene and the fact that
stroma-to-tear Na+ fluxes did not increase on stimulation of Isc. The
tear-to-stroma Na+ flux also remained unaltered, negating an increased
apical bath-to-cell Na+ flow. The forskolin-20% CO2 manipulation produced a
depolarization of the intracellular potential, a reduction in the
apical-to-basolateral resistance ratio, and a decrease in transepithelial
resistance.(ABSTRACT TRUNCATED AT 250 WORDS)
