Resolution of the Potassium Ion Pump in Muscle Fibers Using Barium Ions

When frog sartorius muscles recover from Na enrichment in the presence of external K, net K entry into the fibers occurs by both passive movement and active inward transport via a K pump. Under normal conditions, it has not been possible to experimentally distinguish these processes. Fractionation into the flux components must be accomplished from inferences concerning the K conductance or permeability during a period of rapid Na extrusion. The best estimates indicate that 60-80 % of the K entry occurs via the K pump. In the presence of Ba ions, the membrane permeability to K is very much reduced. Under these conditions, Na-enriched muscles underwent a normal recovery in the presence of external K, and the amount of inward K movement due to the K pump rose to over 90% of the total K entry. The characteristics of the K pump studied by this means were: (a) essentially complete inhibition by 10 -4 M ouabain, (b) inhibition by [Na~o, (c) activation by [K]o according to a rectangular hyperbola in the absence of [Na]o, (d) linear activation by [Na~i over a wide range in concentration, (e) zero or undetectably low pumping rate as [Na]i ~ 0, (f) the number of Na ions actively transported per K ion actively transported is 1.4-1.7 normally and 1.1 in the presence of Ba. I N T R O D U C T I O N W h e n frog s t r ia ted muscles, in the presence of [K]o, recover f rom N a enr ichment , there occurs a 1 : 1 exchange of Na~ for Ko (Steinbach, 1940; Desmedt , 1953; Adr ian and S layman, 1966). Al though the proper t ies of Na efflux unde r these condi t ions have been studied extensively (Keynes , 1954, 1965; Edwards and Harr is , 1957; Mull ins and F rumen to , 1963; Horowicz et al., 1970; Sjodin, 1971 ; Beaug~ and Ort iz , 1970, 1972), on ly some of the proper ties of K influx u n d e r the same condi t ions have been investigated. Potass ium influx is r a t he r insensitive to card ioac t ive steroids in no rma l fresh muscle. I f [Nai l is e levated, an ex t ra K influx occurs tha t is abol ished by ouaba in or s t rophan th id in (Harris , 1957; Keynes , 1965; Sjodin and Beaug~, 1968). THE JOURNAL OF GENERAL PHYSIOLOGY • VOLUME 6 6 , I 9 7 5 • p a g e s 2 6 9 2 8 6 2 6 9 on Jne 0, 2017 D ow nladed fom Published September 1, 1975