title: Effects of endogenous calcium transport inhibitor from heart muscle on the active calcium uptake and passive calcium release properties of sarcoplasmic reticulum.
abstract: In the present study, the effects of the cytosolic Ca2+ transport inhibitor on ATP-dependent Ca2+ uptake by, and unidirectional passive Ca2+ release from, sarcoplasmic reticulum enriched membrane vesicles were examined in parallel experiments to determine whether inhibitor-mediated enhancement in Ca2+ efflux contributes to inhibition of net Ca2+ uptake. When assays were performed at pH 6.8 in the presence of oxalate, low concentrations (less than 100 micrograms/mL) of the inhibitor caused substantial inhibition of Ca2+ uptake by SR (28-50%). At this pH, low concentrations of the inhibitor did not cause enhancement of passive Ca2+ release from actively Ca2+-loaded sarcoplasmic reticulum. Under these conditions, high concentrations (greater than 100 micrograms/mL) of the inhibitor caused stimulation of passive Ca2+ release but to a much lesser extent when compared with the extent of inhibition of active Ca2+ uptake (i.e., twofold greater inhibition of Ca2+ uptake than stimulation of Ca2+ release). When Ca2+ uptake and release assays were carried out at pH 7.4, the Ca2+ release promoting action of the inhibitor became more pronounced, such that the magnitude of enhancement in Ca2+ release at varying concentrations of the inhibitor (20-200 micrograms/mL) was not markedly different from the magnitude of inhibition of Ca2+ uptake. In the absence of oxalate in the assay medium, inhibition of Ca2+ uptake was observed at alkaline but not acidic pH.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Physiology, University of Western Ontario, London, Canada.
authors.forename: N
authors.initials: N
authors.lastname: Narayanan
authors.forename: P
authors.initials: P
authors.lastname: Bedard
authors.forename: T S
authors.initials: TS
authors.lastname: Waraich
mesh_terms: D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D002132:Calcium Radioisotopes; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D006321:Heart / Q000187:drug effects; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D009124:Muscle Proteins / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism* / Q000648:ultrastructure; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*
chemical_list: D002132:Calcium Radioisotopes; D009124:Muscle Proteins; D002118:Calcium