title: The asymmetric effect of lanthanides on Na+-gradient-dependent Ca2+ transport in synaptic plasma membrane vesicles.
abstract: Lanthanides (La3+, Pr3+ and Tb3+) inhibit Na+-gradient-dependent Ca2+ influx into synaptic plasma membrane vesicles. 50% inhibition is obtained by 7 microM lanthanide concentration. The inhibition of the Na+-gradient-dependent Ca2+ uptake exhibits competitive kinetic behaviour. The apparent Km of the Ca2+ influx is increased from 50 microM in the absence of lanthanides to 118 microM in the presence of La3+, 170 microM in the presence of Pr3+ and 130 microM in the presence of Tb3+. The maximal reaction velocity is not altered (8.35 nmol Ca2+ transported per mg protein per min in the absence of lanthanides and 8.16 nmol/mg per min in the presence of lanthanides). Lanthanides also inhibited Na+-gradient-dependent Ca2+ efflux from synaptic plasma membrane vesicles that were preloaded with Ca2+ in a Na+-gradient-dependent manner. Introduction of La3+ into the interior of the synaptic plasma membrane vesicles by rapid freezing of the vesicles in liquid N2 and slow thawing had no effect on either Na+-gradient-dependent Ca2+ influx or efflux. Synaptic plasma membrane vesicles can be preloaded with Ca2+ also in an ATP-dependent manner. This form of Ca2+ uptake is also inhibited by La3+ though at higher concentrations than the Na+-gradient-dependent Ca2+ uptake. Na+-gradient-dependent efflux from synaptic plasma membrane vesicles preloaded in an ATP-dependent fashion ('inside-out' vesicles) unlike efflux from synaptic plasma membrane vesicles preloaded in a Na+-gradient-dependent manner was not inhibited by La3+. These findings suggest that the inhibition by La3+ is manifested asymmetrically on both sides of the synaptic plasma membrane. Lanthanides are probably not transported via the Na+-Ca2+ exchanger since Tb3+ entry measured by fluorescence of Tb3+-dipicolinic acid complex formation occurred at high Tb3+ concentrations only (1.5 mM or above) and was not Na+-gradient dependent.
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D007700:Kinetics; D007811:Lanthanum / Q000494:pharmacology; D008674:Metals, Rare Earth / Q000494:pharmacology*; D011221:Praseodymium / Q000494:pharmacology; D051381:Rats; D012964:Sodium / Q000494:pharmacology*; D013570:Synaptic Membranes / Q000187:drug effects / Q000378:metabolism*; D013725:Terbium / Q000378:metabolism / Q000494:pharmacology
chemical_list: D008674:Metals, Rare Earth; D013725:Terbium; D007811:Lanthanum; D012964:Sodium; D000252:Calcium-Transporting ATPases; D011221:Praseodymium; D002118:Calcium