OPENING OF FROG-SKIN SODIUM-CHANNELS BY PHENOBARBITAL

This paper presents a study of the action of phenobarbital (anticonvul sant barbituric acid with pK(1) = 7.3 and pK(2) = 11.8) on the short-c ircuit current and on the transepithelial conductance of the frog isol ated skin. An increase in these parameters was found after addition of the drug to the outside medium (apical). This effect is significantly higher at pHs lower than pK(1), suggesting that the neutral form of t his substance is the active form. All studies were performed at pH 7. Phenobarbital induces a depolarization of the apical barrier and an in crease in Na-22(+) radioactive tracer fluxes, which parallel the incre ase in the observed short-circuit current. This suggests an opening of the Na+ channels in this barrier. A dose-response curve shows that ph enobarbital possibly acts on two different sites, with different affin ity constants and stoichiometries. To further characterize these chann els, comparative studies with oxytocin (a hormone that opens amiloride -sensitive Na+ channels in the apical side) were performed. The result s show that phenobarbital causes the same percentage of increase in I- sc in control and oxytocin-pretreated skins. Amiloride was also found to inhibit the phenobarbital-sensitive channels with an affinity const ant [K-AMIL=(60+/-1.78)x10(-8) M] insensitive to the phenobarbital con centration.