Calcium transport mechanisms in membrane vesicles from guinea pig brain synaptosomes

Ca2+ transport mechanisms were investigated using membrane vesicles prepared from guinea pig brain synaptosomes by hypotonic lysis. Two major mechanisms of Ca2+ transport exist, Na+-Ca2+ exchange and ATP-dependent Ca2+ uptake. A third although minor component of Ca2+ uptake occurs under hyperpolarizing conditions (determined by increased uptake of [3H]tetraphenylphosphonium+). Na+-Ca2+ exchange results in a rapid increase of [Ca2+]i (up to 100-fold above [Ca2+]O), has a Km for Ca2+ of 40 microM, is fully reversed by added external Na+, is inhibited by agents dissipating Na+ gradients (monensin or veratridine), and is uninfluenced by mitochondrial inhibitors. ATP-dependent Ca2+ uptake has a higher affinity for CA2+ (Km = 12 microM), is dependent on Mg2+ or Mn2+, and is inhibited by beta, gamma-imidoadenosine 5'-triphosphate and VO43-, although only slightly (20%) inhibited by high concentrations of mitochondrial inhibitors. Both mechanisms are temperature-dependent, fully reversed by A23187, and higher in the presence of external K+. Ca2+ loaded in vesicles via ATP-dependent Ca2+ uptake is rapidly effluxed upon addition of external Na+ (as for Na+-Ca2+ exchange). Therefore a single population of vesicles exists containing both Ca2+ transport mechanisms. The two mechanisms are independent since they accumulate Ca2+ additively, are selectively inhibited by monensin and VO43-, and show distinct specificity toward other divalent cations and La3+. Although independent, Na+ (100 mM) inhibits ATP-dependent Ca2+ uptake (Km for ATP increased from 40 to 300 microM) in the absence of any net Na+ movement. Since Na+-Ca2+ exchange functions in the synaptosomal plasma membrane, the results suggest that both Ca2+ transport mechanisms originate from this membrane and function in the present experiments in inverted plasma membrane vesicles.