ISSN:
1432-1424
Keywords:
electrogenic Ca2+ transport
;
plasma membrane
;
rough endoplasmic reticulum
;
pancreatic acinar cells
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
Notes:
Summary ATP-dependent45Ca2+ uptake was investigated in purified plasma membranes from rat pancreatic acinar cells. Plasma membranes were purified by four subsequent precipitations with MgCl2 and characterized by marker enzyme distribution. When compared to the total homogenate, typical marker enzymes for the plasma membrane, (Na+,K+)-ATPase, basal adenylate cyclase and CCK-OP-stimulated adenylate cyclase were enriched by 43-fold, 44-fold, and 45-fold, respectively. The marker for the rough endoplasmic reticulum was decreased by fourfold compared to the total homogenate. Comparing plasma membranes with rough endoplasmic reticulum, Ca2+ uptake was maximal with 10 and 2 μmol/liter free Ca2+, and half-maximal with 0.9 and 0.5 μmol/liter free Ca2+. It was maximal at 3 and 0.2 mmol/liter free Mg2+ concentration, at an ATP concentration of 5 and 1 mmol/liter, respectively, and at pH 7 for both preparations. When Mg2+ was replaced by Mn2+ or Zn2+ ATP-dependent Ca2+ uptake was 63 and 11%, respectively, in plasma membranes; in rough endoplasmic reticulum only Mn2+ could replace Mg2+ for Ca2+ uptake by 20%. Other divalent cations such as Ba2+ and Sr2+ could not replace Mg2+ in Ca2+ uptake. Ca2+ uptake into plasma membranes was not enhanced by oxalate in contrast to Ca2+ uptake in rough endoplasmic reticulum which was stimulated by 7.3-fold. Both plasma membranes and rough endoplasmic reticulum showed cation and anion dependencies of Ca2+ uptake. The sequence was K+〉Rb+〉Na+〉Li+〉choline+ in plasma membranes and Rb+≥K+≥Na+〉Li+〉choline+ for rough endoplasmic reticulum. The anion sequence was Cl−≥Br−≥I−〉SCN−〉NO 3 − 〉isethionate− 〉cyclamate−〉gluconate−〉SO 4 2− ≥glutarate− and Cl−〉Br〉gluconate〉SO 4 2− 〉NO 3 − 〉I−〉cyclamate−≥SCN−, respectively. Ca2+ uptake into plasma membranes appeared to be electrogenic since it was stimulated by an inside-negative K+ and SCN diffusion potential and inhibited by an inside-positive diffusion potential. Ca2+ uptake into rough endoplasmic reticulum was not affected by diffusion potentials. We assume that the Ca2+ transport mechanism in plasma membranes as characterized in this study represents the extrusion system for Ca2+ from the cell that might be involved in the regulation of the cytosolic Ca2+ level.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01871647
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