Guanosine nucleotide-dependent activation of the amiloride-blockable Na+ channel

Effects of guanosine nucleotides on the epithelial Na+ channel were studied in apical membrane vesicles derived from the toad bladder epithelium. Trapping 10 microM guanosine-5'-O-(thiotriphosphate) (GTP gamma S) in vesicles evoked two- to fourfold increase in the amiloride-sensitive (Na+ channel-mediated) 22Na+ uptake. The nucleotide had no significant effect on the amiloride-insensitive 22Na+ uptake or the valinomycin-mediated 86Rb+ uptake in the same membranes. The stimulatory action of GTP gamma S was mimicked by 5'-guanylylimidiodiphosphate (GppNHp) and could at least partly be reversed by guanosine-5'-O-(thiodiphosphate) (GDP beta S) (10-fold excess). GTP itself and adenosine-5'-O-(thiotriphosphate) (ATP gamma S) had no sustained effect on Na+ transport in vesicles. Thus it appears that the epithelial Na+ channel is directly or indirectly regulated by the occupancy of a guanosine-specific site, probably the alpha subunit of a G protein. The possibility that GTP gamma S acts indirectly by activating a membrane-bound, GTP-dependent enzyme the product of which modulates the channel conductance was assessed by measuring 22Na+ fluxes in membrane vesicles prepared to contain products of such enzymes. None of the reagents tested [adenosine 3',5' cyclic monophosphate (cAMP), guanosine 3',5' cyclic monophosphate (cGMP), inositol 1,4,5-trisphosphate (IP3), and diacylglycerol (DAG)] increased the tracer flux in vesicles or altered its response to GTP gamma S.