FORSKOLIN STIMULATION OF WATER AND CATION PERMEABILITY IN AQUAPORIN1 WATER CHANNELS

Aquaporin1, a six-transmembrane domain protein, is a water channel pre sent in many fluid-secreting and -absorbing cells. In Xenopus oocytes injected with aquaporin1 complementary RNA, the application of forskol in or cyclic 8-bromo- adenosine 3',5'-monophosphate increased membrane permeability to water and triggered a cationic conductance. The catio nic conductance was also induced by direct injection of protein kinase A (PKA) catalytic subunit, reduced by the kinase inhibitor H7, and bl ocked by HgCl2, an inhibitor of aquaporin1. The cationic permeability of the aquaporin1 channel is activated by a cyclic adenosine monophosp hate-dependent mechanism that may involve direct or indirect phosphory lation by PKA.