Poorly selective cation channels in the skin of the larval frog (Stage≤XIX)

The abdominal skin of bullfrog larvae (Rana catesbeiana) was placed in an Ussing-type chamber, and its transepithelial electrical parameters were recorded with mucosal solutions of different ionic composition. With K⁺-like cations (K⁺, NH₄⁺, Rb⁺, Cs^–) the power spectra of the fluctuations in short-circuit current displayed a Lorentzian component (f_c=30–40 Hz). The relaxation noise could be suppressed by addition of the K⁺-channel blockers Ba²⁺ and TEA to the mucosal solution. Also, in presence of the ionophore antibiotic nystatin the Lorentzian noise was abolished. The Na⁺-channel probes amiloride and benzimidazolyl-2-guanidine (BIG) both enhanced the relaxation noise obtained with the K⁺-like cations but, with Na⁺, and Li⁺, also caused the rise of a relaxation component above the background noise. In presence of amiloride or BIG, the addition of Ba²⁺, TEA and nystatin still abolished the Lorentzian noise. It can be concluded that the relaxation-noise source is located in the apical cell membranes of the tadpole skin. These spontaneously fluctuating cation channels do not seem to strictly discriminate between K⁺-like ions (K⁺, NH₄⁺, Rb⁺, Cs⁺) and Na⁺-like ions (Na⁺, Li⁺). On the other hand, well-known specific probes for K⁺ channels (Ba²⁺, TEA) and for Na⁺ channels (amiloride, BIG) interact with this apical cation channel. It is possible that the poorly selective channel plays a role in the ontogenesis of the specific Na⁺ transport in the maturing frog skin.