Forskolin Stimulation of Water and Cation Permeability in Aquaporin 1 Water Channels

Aquaporin1, a six-transmembrane domain protein, is a water channel present in many fluid-secreting and -absorbing cells. In Xenopus oocytes injected with aquaporin1 complementary RNA, the application of forskolin or cyclic 8-bromo- adenosine 3′,5′-monophosphate increased membrane permeability to water and triggered a cationic conductance. The cationic conductance was also induced by direct injection of protein kinase A (PKA) catalytic subunit, reduced by the kinase inhibitor H7, and blocked by HgCl$_2$, an inhibitor of aquaporin1. The cationic permeability of the aquaporin1 channel is activated by a cyclic adenosine monophosphate-dependent mechanism that may involve direct or indirect phosphorylation by PKA.