Calcium Influx in Internally Dialyzed Squid Giant Axons

I N T R O D U C T I O N Since the f inding that a fraction o f the Ca efflux in squid axons might be involved in a N a < o u p l e d exchange mechanism (Blaustein and Hodgkin , 1969) a great deal o f work has been devoted to analyze this particular unidirectional Ca f u x . Al though not all the features o f the Ca efflux have been firmly established, most o f the quantitative dependencies on Nao, Nat, Cao, Cat, E m, and ATPi are well documen ted in the literature (Baker et al., 1967, Baker, 1972; Baker and McNaughton , 1976; DiPolo, 1973, 1974, 1977; Mullins and Brinley, 1975; Blaustein and Russell, 1975; Blaustein, 1977). This picture is in contrast with our knowledge concerning Ca entry th rough the Na/Ca exchange mechanism. Al though it is known that Ca influx depends on variables such as Nao and Na~, (Baker et al., 1969), at present very little is known about the quantitative relationship between Nat, Ca~, ATPt, and Ca influx. The main purpose o f this paper is to examine the detailed propert ies o f the Ca influx into squid axons J. GEN. PHYSIOL. @ The Rockefeller University Press 9 0022-1295/79/01/0091-2~1.00 91 Volume 73 January 1979 91-115 on July 7, 2017 jgp.rress.org D ow nladed fom 92 T H E J O U R N A L o r G E N E R A L P H Y S I O L O G Y 9 V O L U M E 7 3 9 1 9 7 9 u n d e r condit ions of internal solute control . T h e expe r imen t s r epo r t ed in this p a p e r fall into two groups . Those o f the first g r o u p were designed to investigate whe ther the magn i tude of the Ca influx in dialyzed axons resembles those of intact axons. T h e pu rpose o f the second g r o u p o f expe r imen t s was to analyze the activation of the Ca influx by Na~" and Ca~ -+ in axons with high and low A T P content . T h e results indicate that the Ca influx measu red in dialyzed axons agrees in magn i tude with those measu red in intact axons, and shows the same qualitative d e p e n d e n c e on Na~, Nao, and Era. Cation activation of the Ca influx indicates that a finite [Ca++]~ is necessary for the existence o f a N a r d e p e n d e n t Ca influx. T h e influx o f calcium in exchange for sod ium was found to be m e m b rane potential-sensitive and complete ly d e p e n d e n t on the presence of ATPi. T h e absence of a Ca~-activated Ca influx in axons dialyzed without internal sodium suggests that , u n d e r no rma l condit ions (i.e., with [Na]~ >0), [Ca+*]i activates the influx o f Ca ra the r than exchanges with Cao. T h e c o m p o n e n t o f the Ca influx that is insensitive to Na~-, Ca~ -+, and A T P was found to be sensitive to m e m b r a n e potent ial and partially blocked by external Co ++ ions.