The presence and regulation by hyperosmotic medium of the ouabain-sensitive (Na++K+)ATPase of the Malpighian tubule cells of Rhodnius prolixus was investigated. The ouabain-sensitive (Na++K+)ATPase activity was 5.4 ± 0.5 nmol Pi x m g-1 x m in-1. Vanadate 100 piM completely abolished this ATPase activity. In hyperosmotic medium, obtained by addi tion of 180 mM mannitol, the (Na++K+)ATPase activity was inhibited by 60%. When the cell lysates were preincubated in hyperosmotic medium for 30 minutes and the ATPase activity was assayed in isosmotic medium, the (Na++K+)ATPase activity was not modified. Addition of 50 ng/ml sphingosine, a protein kinase C inhibitor, abolished the inhibition of (N a++K+)ATPase activity in hyperosmotic medium. Furtherm ore, phorbol ester (TPA), an activator of protein kinase C, mimicked the effect of hyperosmotic shock on (N a++K+)ATPase activity. The increase in Ca2+ concentration decreased the (Na++K+)ATPase activity by 60% in isosmotic medium, with maximal effect obtained in 10-6 m Ca2+. No effect was observed in hyperosmotic medium. The inhibitory effect of Ca2+ on the (Na++K+)ATPase was not reversed by sphingo sine. These results indicate that the ouabain-sensitive (Na++K +) ATPase activity of the Malpigh ian tubule is regulated by both increasing Ca2+ concentration and by the osmolality of the me dium by different and integrative ways.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.