title: Inhibition of plasma membrane Ca2+-ATPase by heparin is modulated by potassium.
abstract: Heparin is related to several protein receptors that control Ca2+ homeostasis. Here, we studied the effects of heparin on the plasma membrane Ca2+-ATPase from erythrocytes. Both ATP hydrolysis and Ca2+ uptake were inhibited by heparin without modification of the steady-state level of phosphoenzyme formed by ATP. Calmodulin did neither modify the inhibition nor the binding of heparin. Inhibition by heparin was counteracted by K+ but not by Li+. This effect was extended to other sulfated polysaccharides with high number of sulfate residues. Hydrolysis of p-nitrophenylphosphate was equally inhibited by heparin. No evidence for enzyme uncoupling was observed: Ca2+ uptake and ATP hydrolysis remained tightly associated at any level of heparin, and heparin did not increase the passive Ca2+ efflux of inside-out vesicles. Vanadate blocked this efflux, indicating that the main point of Ca2+ escape from these vesicles was linked to the Ca2+ pump. It is discussed that sulfated polysaccharides may physiologically increase the steady-state level of Ca2+ in the cytosol by inhibiting the Ca2+ pumps in a K+ (and tissue) regulated way. It is suggested that heparin regulates the plasma membrane Ca2+-ATPase by binding to the E2 conformer.
authors.affiliation: Instituto de Bioquímica Médica, Centro de Ciências da Saúde, Universidade Federal do Rio de Janeiro, Cidade Universitária, Ilha do Fundão, Rio de Janeiro, RJ 21941-590, Brazil.
authors.forename: Carla F
authors.initials: CF
authors.lastname: Felix
authors.forename: Vanessa H
authors.initials: VH
authors.lastname: Oliveira
authors.forename: Otacilio C
authors.initials: OC
authors.lastname: Moreira
authors.forename: Julio A
authors.initials: JA
authors.lastname: Mignaco
authors.forename: Hector
authors.initials: H
authors.lastname: Barrabin
authors.forename: Helena M
authors.initials: HM
authors.lastname: Scofano
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors* / Q000097:blood; D004347:Drug Interactions; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004910:Erythrocyte Membrane / Q000187:drug effects* / Q000201:enzymology*; D006493:Heparin / Q000494:pharmacology*; D006706:Homeostasis / Q000187:drug effects; D066298:In Vitro Techniques; D007700:Kinetics; D008094:Lithium / Q000494:pharmacology; D011134:Polysaccharides / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology*; D013552:Swine
chemical_list: D004791:Enzyme Inhibitors; D011134:Polysaccharides; D000255:Adenosine Triphosphate; D006493:Heparin; D008094:Lithium; D000252:Calcium-Transporting ATPases; D011188:Potassium