Poorly selective cation channels in the skin of the larval frog (stage less than or equal to 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+, NH+4, RB+, Cs+) the power spectra of the fluctuations in short-circuit current displayed a Lorentzian component (fc = 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 Lorentizian 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 Lorentizian 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+, NH+4, 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.