EFFECT OF MELITTIN ON THE APICAL MEMBRANE NA- CONDUCTANCES OF FROG CORNEA EPITHELIUM( AND CL)

The transepithelial conductance increased with 10(-6) M melittin on th e tear side of the frog cornea. This effect was attributed to the open ing of an apical membrane Na+ conductance. However, the potency of the venom at this concentration (apical membrane fractional resistance (f R(o)) decreased to near zero) masked this and other effects. With 3 x 10(-7) M on the tear side, the effects were finite (fR(o) > 0) and rev ersible. With fR(o) > 0, the effects of melittin could be readily stud ied on the transport parameters of Na+, Cl-, and K+. Epithelial cells of intact bullfrog corneas were impaled with microelectrodes using an in vitro preparation. Under short-circuit current (I-sc) conditions, 2 0 min after melittin I-sc increased by 3.1 from 8.2 mu A/cm(2); fR(o) decreased by 18 from 51%; the intracellular potential, V-o, depolarize d by 19.4 from -56.5 mV; and the transepithelial conductance, g(t), in creased by 0.57 from 0.29 mS/cm(2). Tenfold decreases in tear Na+ or C l- concentrations changed the transport parameters consistent with the formation of a Na+ conductance and an increase in the apical membrane Cl- conductance by the venom. These conclusions were further supporte d by the minimal effects of melittin in Cl--free and, particularly, in Na+-free solutions. Changes in K+ concentration had no effect on tran sport parameters. These findings indicate that the effect of melittin at this low concentration is upon a Na+ channel protein and not due to nonspecific conductances.