title: ATP-dependent calcium pump and Na+-Ca2+ exchange in plasma membrane vesicles from squid optic nerve.
abstract: Purified plasma membrane vesicles from the optic nerve of the squid Sepiotheutis sepioidea accumulate calcium in the presence of Mg2+ and ATP. Addition of the Ca2+ ionophore A23187 to vesicles which have reached a steady state of calcium-active uptake induces complete discharge of the accumulated cation. Kinetic analysis of the data indicates that the apparent Km for free Ca2+ and ATP are 0.2 muM and 21 muM, respectively. The average Vmax is 1 nmol Ca2+/min per mg protein at 25 degrees C. This active transport is inhibited by orthovanadate in the micromolar range. An Na+-Ca2+ exchange mechanism is also present in the squid optic nerve membrane. When an outwardly directed Na+ gradient is imposed on the vesicles, they accumulate calcium in the absence of Mg2+ and/or ATP. This ability to accumulate Ca2+ is absolutely dependent on the Na+ gradient: replacement of Na+ by K+, or passive dissipation of the Na+ gradient, abolishes transport activity. The apparent Km for Ca2+ of the Na+-Ca2+ exchange is more than 10-fold higher than that of the ATP-driven pump (app. Km=7.5 muM). While the apparent Km for Na+ is 74 mM, the Vmax of the exchanger is 27 nmol Ca2+/min per mg protein at 25 degrees C. These characteristics are comparable to those displayed by the uncoupled Ca pump and Na+-Ca2+ exchange previously described in dialyzed squid axons.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism*; D049832:Decapodiformes; D009900:Optic Nerve / Q000378:metabolism / Q000648:ultrastructure; D012964:Sodium / Q000378:metabolism*; D014638:Vanadates; D014639:Vanadium / Q000494:pharmacology
chemical_list: D014639:Vanadium; D014638:Vanadates; D000255:Adenosine Triphosphate; D012964:Sodium; D002118:Calcium