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Abstract:
Regulation of steady-state free Ca2+ concentration at rest and at stimulation have been studied in isolated permeabilized pancreatic acinar cells by measuring the free Ca2+ concentration of the surrounding incubation medium with a Ca2+-specific electrode. Ca2+ transport mechanisms have been further characterized in subcellular membrane fractions by measuring 45Ca2+ uptake into membrane vesicles and protein phosphorylation using polyacrylamide gel electrophoresis. (a) In permeabilized isolated acinar cells from exocrine glands, inositol-1,4,5-trisphosphate (IP3) releases Ca2+ from endoplasmic reticulum. (b) Secretagogue-induced Ca2+ release from permeabilized cells is accompanied by increased production of IP3. At rest, steady-state free Ca2+ concentration is regulated at 4 X 10 mol/L by the rough endoplasmic reticulum (RER). Ca2+ uptake into this pool is promoted by a (Ca2+ + Mg2+)-ATPase, and is dependent on cations and anions in the incubation medium in the order K+ greater than Na+ greater than Li+ greater than choline+ and Cl- greater than Br- greater than SO42- = NO3- greater than I- greater than cyclamate- greater than SCN-, respectively. Similarly, Ca2+-stimulated 32P incorporation from [gamma 32P]ATP into a 130 kD protein intermediate of (Ca2+ + Mg2+)-ATPase, as well as 32P liberation, indicating (Ca2+ + Mg2+)-ATPase activity, are cation dependent. While 32P incorporation is highest in the presence of choline, 32P liberation is higher with K+, as compared with Na+ or choline, indicating that K+ ions facilitate dephosphorylation
