IN SQUID NERVE-FIBERS MONOVALENT ACTIVATING CATIONS ARE NOT COTRANSPORTED DURING NA+/CA-2+ EXCHANGE

Squid axons display a high activity of Na+/Ca2+ exchange which is largely increased by the presence of external K+, Li+, Rb+ and NH4+. In this work we have investigated whether this effect is associated with the cotransport of the monovalent cation along with Ca2+ ions. 86Rb+ influx and efflux have been measured in dialyzed squid axons during the activation (presence of Cai2+) of Cao2+/Nai+ and Cai2+/Cao2+ exchanges, while 86Rb+ uptake was determined in squid optic nerve membrane vesicles under equilibrium Ca2+/Ca2+ exchange conditions. Our results show that although Ko+ significantly increases Nai+-dependent Ca2+ influx (reverse Na+/Ca2+ exchange) and Rbi+ stimulates Cao2+-dependent Ca2+ efflux (Ca2+/Ca2+ exchange), no sizable transport of rubidium ions is coupled to calcium movement through the exchanger. Moreover, in the isolated membrane preparation no 86Rb+ uptake was associated with Ca2+/Ca2+ exchange. We conclude that in squid axons although monovalent cations activate the Na+/Ca2+ exchange they are not cotransported. © 1990.