Effect of melittin on the apical membrane Na+ and Cl- conductances of frog cornea epithelium.
The transepithelial conductance increased with 10(-6) M melittin on the tear side of the frog cornea. This effect was attributed to the opening of an apical membrane Na+ conductance. However, the potency of the venom at this concentration (apical membrane fractional resistance (fRo) decreased to near zero) masked this and other effects. With 3 x 10(-7) M on the tear side, the effects were finite (fRo > 0) and reversible. With fRo > 0, the effects of melittin could be readily studied 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 (Isc) conditions, 20 min after melittin Isc increased by 3.1 from 8.2 microA/cm2., fRo decreased by 18 from 51%; the intracellular potential, Vo, depolarized by 19.4 from -56.5 mV; and the transepithelial conductance, gv increased by 0.57 from 0.29 mS/cm2. Tenfold decreases in tear Na+ or Cl- 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 supported by the minimal effect of melittin in Cl(-)-fre and, particularly, in Na(+)-free solutions. Changes in K+ concentration had no effect on transport 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.
Carrasquer
G
G
Department of Medicine, University of Louisville, KY 40292, USA.
Yang
S
S
Schwartz
M
M
Dinno
M A
MA
D000818:Animals; D002712:Chlorides / Q000378:metabolism* / Q000494:pharmacology; D004553:Electric Conductivity; D004728:Endothelium, Corneal / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology; D066298:In Vitro Techniques; D017136:Ion Transport; D008555:Melitten / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects; D008839:Microelectrodes; D011892:Rana catesbeiana; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D015222:Sodium Channels / Q000187:drug effects; D013666:Tears / Q000187:drug effects / Q000378:metabolism
D002712:Chlorides; D015222:Sodium Channels; D008555:Melitten; D012964:Sodium