In squid nerve fibers monovalent activating cations are not cotransported during Na+/Ca2+ exchange.
Squid axons display a high activity of Na+/Ca2+ exchange which is largely increased by the presence of external K+, Li+, Rb+ and NH+4. 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 Ca2+i) of Ca2+o/Na+i and Ca2+i/Ca2+o exchanges, while 86Rb+ uptake was determined in squid optic nerve membrane vesicles under equilibrium Ca2+/Ca2+ exchange conditions. Our results show that although K+o significantly increases Na+i-dependent Ca2+ influx (reverse Na+/Ca2+ exchange) and Rb+i stimulates Ca2+o-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.
Condrescu
M
M
Centro de Biofisica y Bioquímica, IVIC, Caracas, Venezuela.
Rojas
H
H
Gerardi
A
A
DiPolo
R
R
Beaugé
L
L
D000818:Animals; D001369:Axons / Q000502:physiology*; D001692:Biological Transport; D002118:Calcium / Q000502:physiology*; D002352:Carrier Proteins / Q000502:physiology*; D002414:Cations, Monovalent / Q000378:metabolism*; D049832:Decapodiformes; D066298:In Vitro Techniques; D008564:Membrane Potentials; D009441:Neurilemma / Q000502:physiology; D012964:Sodium / Q000502:physiology*; D019831:Sodium-Calcium Exchanger
D002352:Carrier Proteins; D002414:Cations, Monovalent; D019831:Sodium-Calcium Exchanger; D012964:Sodium; D002118:Calcium