8-pCPT-cGMP stimulates αβγ-ENaC activity in oocytes as an external ligand requiring specific nucleotide moieties

Abstract

Epithelial sodium channels (ENaC) are regulated by protein kinase A, in addition to a broad spectrum of other protein kinases. It is not clear whether cGMP/PKG signaling might regulate ENaC activity. We examined the responses of αβγ-ENaC channels expressed in Xenopus oocytes to 8-(4-chlorophenylthio)-cGMP (8-pCPT-cGMP), a cell-permeable cGMP analog. This compound stimulated human αβγ-ENaC activity in a dose-dependent fashion, but cell-impermeable cGMP had no effect. Similar stimulatory effects of cGMP were observed in oocytes expressing either mouse or rat αβγ-ENaC channels. The identical ion selectivity and amiloride sensitivity of the 8-pCPT-cGMP-activated currents to those of αβγ-ENaC channels suggest that the cGMP-activated currents are associated with expressed ENaC. The PKGI activator Sp isomer of β-phenyl-1, N2-etheno-8-bromo-cGMP did not elicit a rise in ENaC current and that the 8-pCPT-cGMP-induced activation of ENaC channels was blocked by incubating oocytes with a PKG inhibitor, but not with other cGMP-sensitive kinase inactivators for PKA, MEK, MAP, and PKC. Surprisingly, both site-directed mutation of putative consensus PKG phosphorylation sites and truncation of entire cytosolic NH2- and COOH-terminal tails did not alter the response to 8-pCPT-cGMP. The ENaC activity was activated to the same extent by 8-pCPT-cGMP in cells in which PKGII expression was knocked down using small interfering RNA. Analog to 8-CPT-cAMP, 8-pCPT-cGMP was capable of activating ENaC in the identical manner in cell-free outside-out patches. We conclude that the rapid upregulation of human αβγ-ENaC activity in oocytes by external 8-pCPT-cGMP and 4-chlorothiolphenol-cAMP depends on the para-chlorophenylthiol and the hydroxy groups, and 8-pCPT-cGMP may serve as a novel ENaC ligand in addition to activating PKG signal.