Effect of the polyene antibiotic filipin and the calcium ionophore A23187 on sodium transport in isolated frog skin (Rana temporaria).

Addition of filipin (50 micrometer) to the inside bathing solution of the frog skin resulted in a transient increase in the active sodium transport [measured as short-circuit current (SCC)]. The filipin-induced increase in the SCC required the presence of calcium. The calcium ionophore A23187 (4 micrometer) also induced a transient increase in the SCC. After the activation of the SCC by A23187, the SCC could not be activated by filipin. This indicates that the polyene antibiotic filipin acts as a calcium ionophore. Higher concentrations (40 micrometer) of A23187 resulted in a shrinking of the cells in the transporting cell layer. A23187 also increased the potassium-42 exchange in the isolated epithelium. It is suggested that calcium ionophores enhanced the intracellular calcium concentration; this increase in the calcium concentration resulted in an increase in the potassium permeability of the inward-facing membrane. The increase in the potassium permeability might explain the observed increase in the SCC.