Na+-Ca2+ exchange in squid optic nerve membrane vesicles is activated by internal calcium.
The role of intracellular Ca2+ as essential activator of the Na+-Ca2+ exchange carrier was explored in membrane vesicles containing 67% right-side-out and 10% inside-out vesicles, isolated from squid optic nerves. Vesicles containing 100 microM free calcium exhibited a 2-fold increase in the initial rate of Na+i-dependent Ca2+ uptake as compared with vesicles where intravesicular calcium was chelated by 2 mM EGTA or 10 mM HEDTA. The activatory effect exerted by intravesicular Ca2+ on the reverse mode of Na+-Ca2+ exchange (i.e. Na+i-Ca2+o exchange) is saturated at about 100 microM Ca2+i and displays an apparent K 1/2 of 12 microM. Intravesicular Ca2+ produced activation of Na+i-Ca2+i exchange activity rather than an increase in Ca2+ uptake due to Ca2+-Ca2+ exchange. The presence of Ca2+i was essential for the Na+i-dependent Na+ influx, a partial reaction of the Na+-Ca2+ exchanger. In fact, the Na+ influx levels in vesicles loaded with 2 mM EGTA were close to those expected from diffusional leak while in vesicles containing Ca2+i an additional Na+-Na+ exchange was measured. The results suggest that in nerve membrane vesicles Ca2+ at the inner aspect of the membrane acts as an activator of the Na+-Ca2+ exchange system.
Condrescu
M
M
Instituto Venezolano de Investigaciones Científicas, Centro de Biofísica y Bioquímica, Caracas, Venezuela.
Gerardi
A
A
DiPolo
R
R
D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D002474:Cell-Free System; D003593:Cytoplasm / Q000502:physiology; D049832:Decapodiformes; D066298:In Vitro Techniques; D007700:Kinetics; D009441:Neurilemma / Q000502:physiology*; D009900:Optic Nerve / Q000502:physiology*; D011188:Potassium / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism*
D012964:Sodium; D011188:Potassium; D002118:Calcium