title: Two modes of inhibition of the Ca2+ pump in red cells by Ca2+.
abstract: Two different and independent modes of inhibition of the Ca2+ pump by Ca2+ can be detected measuring active Ca2+ extrusion from resealed ghosts of human red cells: one requires extracellular and the other requires intracellular Ca2+. Ki for inhibition by extracellular Ca2+ is about 10 mM. Extracellular Mg2+ replaces Ca2+ in inhibiting Ca2+ transport but with an apparent affinity for inhibition about 3-times less than that for Ca2+. Inhibition by external Ca2+ is not affected by Na+ or K+ at both surfaces of the cell membrane, external EGTA, internal Ca2+ or ATP. The apparent affinity for external Ca2+ progressively raises as pH increases. The effects of extracellular Ca2+ and Mg2+ are consistent with the idea that for Ca2+ pumping to proceed, external sites in the pump must be protonated and not occupied by extracellular Ca2+ or Mg2+. Inhibition by intracellular Ca2+ takes place with a Ki of about 1 mM and is independent of external Ca2+. The inhibitory effects of intracellular Ca2+ can be accounted for if Ca2+ and CaATP were competitive inhibitors of the activation of the pump by Mg2+ and MgATP, respectively.
authors.forename: R B
authors.initials: RB
authors.lastname: Kratje
authors.forename: P J
authors.initials: PJ
authors.lastname: Garrahan
authors.forename: A F
authors.initials: AF
authors.lastname: Rega
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors*; D004533:Egtazic Acid / Q000494:pharmacology; D004912:Erythrocytes / Q000201:enzymology*; D005110:Extracellular Space / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007424:Intracellular Fluid / Q000378:metabolism; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008433:Mathematics; D011188:Potassium / Q000494:pharmacology; D012964:Sodium / Q000494:pharmacology
chemical_list: D004533:Egtazic Acid; D000255:Adenosine Triphosphate; D012964:Sodium; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D011188:Potassium; D002118:Calcium