Guanine nucleotide-binding protein, α(i-3)*, directly activates a cation channel in rat renal inner medullary collecting duct cells

Abstract

We examined whether GTP binding proteins (G proteins) regulate sodium conducting channels in the apical membrane of renal inner medullary collecting duct (IMCD) cells and thereby modulate sodium absorption. Patch clamp studies were conducted on inside-out patches of the apical membrane of IMCD cells grown in primary culture. Guanosine 5'-triphosphate (GTP) and the nonhydrolyzable GTP analogue, GTPγS, which activate G proteins, increased the open probability of the cation channel. In contrast, the nonhydrolyzable GDP analogue, GDPβS, which decreases G protein activity, inhibited the channel. Pertussis toxin also reduced the open probability of the channel. Addition of the α(i-3)* subunit of G(i) to the solution bathing the cytoplasmic surface of the membrane increased the open probability in a dose-dependent manner (2-200 pM). The threshold concentration for activation by α(i-3)* was 2 pM. Activation of the cation channel by α(i-3)* was not mediated via a protein kinase. The IMCD is the first polarized epithelium in which an ion channel has been shown to be directly regulated by a G protein. Thus, G proteins are important elements in regulating sodium absorption by the IMCD.