J Comp Physiol B 2002 Oct 01;1727:569-76. doi: 10.1007/s00360-002-0285-9.

Cloned bullfrog skin sodium (fENaC) and xENaC subunits hybridize to form functional sodium channels.

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For many years the adult frog skin has been used as a model system to study transepithelial sodium transport. In other sodium transporting tissues the three homologous subunits of the sodium channel have been cloned. The aim of this study was to clone and characterize the amiloride inhibitable sodium channel (ENaC) in adult bullfrog (Rana catesbeiana) skin. Three transcripts corresponding to the alpha, beta, and gamma subunits of ENaC were cloned and sequenced. Co-expression of all three in Xenopus oocytes yielded a functional frog sodium channel (fENaC). Amiloride sensitivity and current voltage relationships suggested that its characteristics were similar to other ENaCs. Subunits from the Xenopus sodium channel (xENaC) and fENaC were combined in all possible triplets. Although functional amiloride inhibitable sodium channels were formed in every case, the amiloride sensitivities were not identical. Subunit combination studies suggested that the alpha subunit made a major contribution to amiloride sensitivity but interactions of beta and gamma were also seen. When the amiloride sensitivities of intact skin from adult R. catesbeiana and Xenopus laevis were compared, Rana also had a consistently higher affinity. Comparison of fENaC and xENaC sequences may provide insight into which amino acids beyond those already identified are critical for amiloride binding.

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