Calcium transport systems in nerve terminals. Studies on membrane vesicles

A) Synaptosomal vesicles are closed membranous structures isolated by differential gradient centrifugation from lysed nerve terminals. 1. They are capable of transporting calcium in an ATP dependent process. 2. The calcium transport system is not inhibited by H+ ionophores, oligomycin, ruthenium red and atractyloside. 3. Calcium can be taken up into synaptosomal vesicles at very low calcium ion concentrations (below 0.5 microM Ca++) and the kinetics of calcium transport exhibits a sigmoidal dependency on calcium ion concentrations, with a cooperativity of about 2. 4. The divalent cation-activated ATPase associated with synaptosomal vesicle calcium transport exhibits Michaelis-Menten kinetics, with a Km for ATP of 19.13 +/- 5.8 microM ATP. 5. The synaptosomal vesicle ATP-dependent Ca carrier has been purified and reconstituted into phospholipid membranes. Two proteins associated with the ATP-dependent Ca transport were identified on SDS containing Laemmli gels with molecular weights of 94 000 and 140 000. B) Another type of membrane vesicle derived probably from synaptosomal plasma membrane has also been isolated. 1. This membrane vesicle takes up calcium when a sodium gradient (high inside) is set up across its membrane. 2. Magnesium is a competitive inhibitor of the Na dependent Ca uptake. 3. The Na dependent Ca uptake is not inhibited by tetrodotoxin or ouabain.