Chloride and Sodium Influx" a Coupled Uptake Mechanism in the Squid Giant Axon

I N T R O D U C T I O N A majo r por t ion o f chor ide influx across the a x o l e m m a o f the squid giant axon depends u p o n cellular metabol i sm (Keynes, 1963) or , m o r e d i recdy, adenos ine 5 ' t r i phospha te (ATP; Russell, 1976). I na smuch as the intracel lular chlor ide concent ra t ion o f the axon is much h igher than expec ted f r o m passive t he rmodynamic considera t ions (Bear and Schmitt , 1939); Webb and Young, 1940; Mauro , 1954; Koechlin, 1955; Def ine r , 1961; Keynes, 1963; Brinley and Mullins, 1965 a and b; Russell, 1976), it seems likely that the A T P d e p e n d e n t chloride influx represen t s an active t r anspor t process. Active chlor ide t r anspor t has been p ropos ed for several excitable tissues (Keynes, 1963; Lux , 1971; Russell and Brown, 1972; Llinas et al., 1974; Russell, 1976), but in contras t to the case for epithelial tissues (Zadunaisky, 1972; Burg et al., 1973; Nellans et al., 1973; Karnaky et al., 1977), little a t tent ion has been paid to the possible involvement o f sod ium in such t r anspor t processes. In this r ega rd it is in teres t ing to note that a por t ion o f sod ium inf lux into the squid giant axon requi res A T P but does not seem to be the result o f sod ium-sod ium exchange (Brinley and Mullins, 1968; Caldwell et al., 1960). Some o f the ATP-s t imula ted sod ium inf lux may be via e i ther the Na-Ca exchange r (DiPolo, 1974; Blaustein and Santiago, 1977) or NaMg e x c h a n g e r (Mullins et al., 1977), but these mechan i sms cannot account for all the A T P d e p e n d e n t sod ium influx. It seemed possible that some o f the A T P J. GEN. PHYSIOL. ~) The Rockefeller University Press 9 0022-1295/79/06/0801/18 $1.00 801 Volume 73 June 1979 801-818 on Jne 5, 2017 D ow nladed fom Published June 1, 1979