Frog atrial natriuretic peptide and cGMP activate amiloride-sensitive Na⁺ channels in urinary bladder cells of Japanese tree frog, Hyla japonica

In our previous study, it was suggested that ANP and cGMP may increase Na⁺ absorption in the urinary bladder of the Japanese tree frog, Hyla japonica. Thus, Na⁺ transport activated by ANP was investigated electrophysiologically by using a cell-attached patch-clamp technique in freshly isolated cells from the urinary bladder. A predominant channel expressed was a low conductance Na⁺ channel in the epithelial cells. The channel exhibited conductance for inward currents of 4.9 ± 0.2 pS, long open and closed times (c.a. 190 ms), and positive reversal potential. The channel activity was decreased under the pipette solution including 10^-6 M amiloride. These characteristics were similar to those of amiloride-sensitive Na⁺ channels (ENaC). Addition of 10^-9 M ANP activated and significantly increased the ENaC activity from 0.58 ± 0.09 to 1.47 ± 0.34. On the other hand, mean amplitudes and conductance of single channel did not change significantly after the addition of ANP. Addition of 10^-5 M 8-Br-cGMP also activated the ENaC and significantly increased the channel activity from 0.56 ± 0.10 to 2.00 ± 0.33. The addition of ANP failed to activate the ENaC in the presence of 10^-6 M amiloride. These results suggested that ANP and cGMP activate Na⁺ transport via ENaC in the epithelial cells of frog urinary bladder.