Opening of frog skin sodium channels by phenobarbital

This paper presents a study of the action of phenobarbital (anticonvulsant barbituric acid with pK1 = 7.3 and pK2 = 11.8) on the short-circuit current and on the transepithelial conductance of the frog isolated 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 pK1, suggesting that the neutral form of this substance is the active form. All studies were performed at pH 7. Phenobarbital induces a depolarization of the apical barrier and an increase in 22Na+ radioactive tracer fluxes, which parallel the increase in the observed short-circuit current. This suggests an opening of the Na+ channels in this barrier. A dose-response curve shows that phenobarbital possibly acts on two different sites, with different affinity constants and stoichiometries. To further characterize these channels, comparative studies with oxytocin (a hormone that opens amiloride-sensitive Na+ channels in the apical side) were performed. The results show that phenobarbital causes the same percentage of increase in Isc in control and oxytocin-pretreated skins. Amiloride was also found to inhibit the phenobarbital-sensitive channels with an affinity constant [KAMIL = (60 +/- 1.78) x 10(-8) M] insensitive to the phenobarbital concentration.