title: Evidence that ATP-dependent Ca2+ transport in rat parotid microsomal membranes requires charge compensation.
abstract: ATP-dependent Ca2+ transport was investigated in a rat parotid microsomal-membrane preparation enriched in endoplasmic reticulum. Ca2+ uptake, in KCl medium, was rapid, linear with time up to 20 s, and unaffected by the mitochondrial inhibitors NaN3 and oligomycin. This Ca2+ uptake followed Michaelis-Menten kinetics, and was of high affinity (Km approximately 38 nM) and high capacity (approximately 30 nmol/min per mg of protein). In the presence of oxalate, Ca2+ uptake continued to increase for at least 5 min, reaching an intravesicular accumulation approx. 10 times higher than without oxalate. Ca2+ uptake was dependent on univalent cations in the order K+ = Na+ greater than trimethylammonium+ greater than mannitol and univalent anions in the order Cl- greater than acetate- greater than Br- = gluconate- = NO3- greater than SCN-. Ca2+ uptake was not elevated if membranes were incubated in the presence of a lipophilic anion (NO3-) and carbonyl cyanide p-trifluoromethoxyphenylhydrazone. Ca2+ transport was altered by changes in the K+-diffusion potential of the membranes. A relatively negative K+-diffusion potential increased the initial rate of Ca2+ accumulation, whereas a relatively positive potential decreased Ca2+ accumulation. In the presence of an outwardly directed K+ gradient, nigericin had no effect on Ca2+ uptake. In aggregate, these studies suggest that the ATP-dependent Ca2+-transport mechanism present in rat parotid microsomal membranes exhibits an electrogenic Ca2+ flux which requires the movement of other ions for charge compensation.
authors.affiliation: Clinical Investigations and Patient Care Branch, National Institute of Dental Research, Bethesda, MD 20892.
authors.forename: B J
authors.initials: BJ
authors.lastname: Baum
authors.forename: I S
authors.initials: IS
authors.lastname: Ambudkar
authors.forename: V J
authors.initials: VJ
authors.lastname: Horn
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D000838:Anions / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000493:pharmacokinetics*; D002412:Cations / Q000494:pharmacology; D007425:Intracellular Membranes / Q000187:drug effects / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects; D008861:Microsomes / Q000187:drug effects / Q000378:metabolism; D010306:Parotid Gland / Q000187:drug effects / Q000378:metabolism*; D011522:Protons; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D000838:Anions; D002412:Cations; D011522:Protons; D000255:Adenosine Triphosphate; D002118:Calcium