Calcium-45 efflux was measured in squid axons whose internal solute concentration was controlled by internal dialysis. Most of the Ca efflux requires either external Na (Na-Ca exchange) or external Ca plus in alkali metal ion (Ca-Ca exchange; cf. Blaustein & Russell, 1975). Both Na-Ca and Ca-Ca exchange are apparently mediated by a single mechanism because both are inhibited by Sr and Mn, and because addition of Na to an external medium optimal for Ca-Ca exchange inhibits Ca efflux. The transport involves simultaneous (as opposed to sequential) ion counterflow because the fractional saturation by internal Ca (Cai) does not affect the external Na (Nao) activation kinetics; also, Nao promotes Ca efflux whether or not an alkali metal ion is present inside, whereas Ca-Ca exchange requires alkali metal ions both internally and externally (i.e., internal and external sites must be appropriately loaded simultaneously). ATP increases the affinity of the transport mechanism for both Cai and Nao, but it does not affect the maximal transport rate at saturating [Ca2+]i and [Na+]o; this suggest that ATP may be acting as a catalyst of modulator, and not as an energy source. Hill plots of the Nao activation data yield slopes congruent to 3 for both ATP-depleted and ATP-fueled axons, compatible with a 3 Na+-for-1 Ca2+ exchange. With this stoichiometry, the Na electrochemical gradient alone could provide sufficient energy to maintain ionized [Ca2+]i in the physiological range (about 10(-7) M).

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Coupled Na § exit/Ca 2+ entry (Na/Ca exchange operating in the Ca 2+ influx mode) was studied in giant barnacle muscle cells by measuring ~Na + efflux and 4~Ca2+ influx in internally perfused, ATP-fueled cells in which the Na + pump was poisoned by 0.1 mM ouabain. Internal free Ca ~+, [Ca ~+ ]i, was controlled with a Ca-EGTA buffering system containing 8 mM EGTA and varying amounts of Ca ~+. Ca ~+ sequestration in internal stores was inhibited with caffeine and a mitochondrial uncoupler (FCCP). To maximize conditions for Ca 2+ influx mode Na/Ca exchange, and to eliminate tracer Na/Na exchange, all of the external Na § in the standard Na+sea water (NaSW) was replaced by Tris or Li + (Tris-SW or LiSW, respectively). In both Na-free solutions an external Ca 2+ (Cao)-dependent Na + efflux was observed when A Nacdependent Ca ~+ influx was also observed in Tris-SW. This Ca ~+ influx also required [Ca2+]i > 10 -s M. Internal Ca 2+ activated a Na~-independent Ca 2+ influx from LiSW (tracer Ca/Ca exchange), but in Tris-SW virtually all of the Cai-actirated Ca ~+ influx was Nal-dependent (Na/Ca exchange). Half-maximal activation was observed with [Na+]i = 30 raM. The fact that internal Ca 2+ activates both a Cao-dependent Na + effiux and a Nai-dependent Ca 2+ influx in Tris-SW implies that these two fluxes are coupled; the activating (intracellular) Ca ~+ does not appear to be transported by the exchanger. The maximal (calculated) Nardependent Ca ~+ influx was -25 pmol/cm~.s. At various [Na+]i between 6 and 106 mM. the ratio of the Cao-dependent Na + effiux to the Nai-dependent Ca 2+ influx was 2.8-3.2:1 (mean = 3.1:1); this directly demonstrates that the stoichiometry (coupling ratio) of the Na/Ca exchange is 3:1. These observations on the coupling ratio and kinetics of the Na/Ca exchanger imply that in resting cells the exchanger turns over at a low rate because of the low [Ca~+]i; much of the Ca ~+ extrusion at rest Address reprint requests to Dr.