AJP - Gastrointestinal and Liver Physiology, Vol 240, Issue 2 122-G129, Copyright © 1981 by American Physiological Society
Characterization of an ATP-dependent Ca2+ uptake system in mouse pancreatic microsomes
B. C. Ponnappa, R. L. Dormer and J. A. Williams
The uptake of 45Ca2+ was studied in microsomes prepared from isolated mouse
pancreatic acini. These microsomes accumulated 45Ca2+ in the presence of
ATP; uptake was potentiated by addition of oxalate. Sequestered microsomal
45Ca2+ was only gradually removed by
ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) but
was readily released by the divalent cation ionophore A23187. Inhibitors of
mitochondrial oxidation and mitochondrial calcium transport had little
effect on microsomal 45Ca2+ uptake. A separate subcellular fraction
enriched in plasma membranes took up 45Ca2+ poorly compared with the
microsomal fraction. Half-maximal 45Ca2+ uptake by the microsomal fraction
was observed at a free Ca2+ concentration of 1.1 microM. 45Ca2+ uptake was
dependent on Mg-ATP and showed a pH optimum at 6.8-7.0. Subfractionation of
the total microsomes into "heavy" and "light" microsomal fractions
indicated higher 45Ca2+ uptake activity associated with the heavy fraction.
A Ca2+-activated, Mg2+-dependent ATPase was demonstrated in this fraction.
Stimulation of pancreatic acini with the cholecystokinin analogue caerulein
prior to homogenization increased the subsequent rate of 45Ca2+ uptake by
the microsomal fraction.
