We examined the mechanism of amiloride-sensitive Na(+) channels (ENaC) activated by fANP in epithelial cells of frog urinary bladder by using a cell-attached patch-clamp technique. ENaC activities in the epithelial cells were significantly increased following administration of both 10(-9)M fANP and 10(-5)M 8-Br-cGMP. Both fANP and 8-Br-cGMP, however, failed to activate the ENaC in the presence of 10(-6)M amiloride. In addition, 8-Br-cGMP failed to activate the ENaC in the presence of a PKA inhibitor, KT-5720. In the next experiment, we measured both cGMP and cAMP production levels after treatment of fANP on the frog urinary bladder cells. Frog ANP significantly increased cGMP production, but not the cAMP production. Taken together, these results suggest that fANP activates ENaC through increases in cGMP production and activation of PKA.