Recent work with cultured mammalian cells indicates that the capability for active Na/K transport increases in response to partial inhibition of this transport system. I propose to combine the approaches of ion transport physiology and somatic cell genetics to investigate the changes in Na/K transport in cultured Chinese hamster ovary cells (CHO) exposed to low (K) medium. I will study the transport properties of cell lineswhich differ in their ability to recover Na/K transport capacity during a 72 h low (K) treatment. Tracer flux measurements will be used to investigate the Na:K coupling ratio and the kinetic parameters of K influx. Also, changes in the synthesis of Na/K pumps will be estimated by measuring the amount of metabolically labelled Na, K-ATPase in the cell membranes. Additional low (K) resistant variants will be isolated and utilized to explore the variety of mechanisms which can produce this phonotype. I will attempt to isolate a variant which is temperature sensitive for ability to recover transport capacity during low (K) treatment. This type of variant will be used to investigate further into the metabolic mechanisms involved in this compensatory response.