The control of cytosolic Ca2+ concentration: studies of high affinity Ca2+ transport in permeabilized acini of rat submandibular glands.
Active Ca2+ transport was studied in acini that had been permeabilized by incubation in buffer nominally free of Ca2+ in order to avoid any contribution to measured Ca2+ uptake by elements of the plasma membrane. In most experiments, ruthenium red was used to inhibit mitochondrial Ca2+ uptake. Non-mitochondrial Ca2+ transport was greatest at pH 7.0-7.5 and required Mg2+-ATP at concentrations typical of the cytosol (K0.5 = 1.36 +/- 0.53 mM). Other substrates were much less effective than ATP. Ca2+ uptake was stimulated by Ca2+ at concentrations near those measured in intact cells (K0.5 = 0.43 +/- 0.17 microM). Hill coefficients at subsaturating concentrations of Ca2+ (1.08 +/- 0.27) and Mg2+-ATP (0.81 +/- 0.22) indicated that cooperative interactions are not characteristic of the major cationic regulators of Ca2+ transporting activity. Na+ did not release Ca2+ from acinar mitochondria, but consistently reduced non-mitochondrial Ca2+ uptake by about 20% as compared to uptake in the presence of an equimolar K+ concentration. The properties of non-mitochondrial Ca2+ uptake in permeabilized acini are similar to those of high affinity Ca2+ uptake which, in broken cell preparations, has been found distributed in parallel with elements of the endoplasmic reticulum. The Ca2+-sequestering properties of a non-mitochondrial organelle in permeabilized submandibular gland acini are those expected of a principal regulator of cytosolic Ca2+ and could account for the ionized Ca2+ concentration measured in resting salivary acinar cells.
Hurley
T W
TW
Department of Child Health, University of Missouri Health Sciences Center, Columbia 65212.
Ryan
M P
MP
D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000493:pharmacokinetics*; D002463:Cell Membrane Permeability; D003600:Cytosol / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008297:Male; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000494:pharmacology; D013363:Submandibular Gland / Q000378:metabolism*; D013997:Time Factors
D000255:Adenosine Triphosphate; D012964:Sodium; D011188:Potassium; D002118:Calcium