Characterization of the kinetic and regulatory properties of high-affinity Ca2+-ATPase activity in acinar preparations of rat submandibular salivary glands.

High-affinity Ca2+-ATPase activity was characterized in the total particulate fraction of acinar preparations from rat submandibular glands. The Ca2+ concentration (as Ca2+-ATP) at half of maximal activity was 82 +/- 17 nM, the Hill coefficient was 2.36 +/- 0.6 and activity reached a steady level at approximately 200 pmol Pi min-1 microgram of membrane protein-1 from 1 to 20 microM Ca2+-ATP. High-affinity Ca2+-ATPase required micromolar concentrations of Mg2+ and was inhibited approximately 60 per cent by the phenothiazine derivative, fluphenazine, but was unaffected by ouabain, Na+, K+, La3+, ruthenium red, oligomycin and added calmodulin. Kinetics of adenosine diphosphate, guanosine triphosphate, uridine triphosphate and inosine triphosphate hydrolysis were similar to those of Ca2+-ATP but p-nitrophenylphosphate was a poor substrate. In the heavy microsomal fraction of whole glands, active Ca2+ uptake (ATP-dependent, oxalate-enhanced and abolished by A23187) was measurable in the absence of added Mg2+, was inhibited by fluphenazine and was stimulated by submicromolar concentrations of Ca2+-ATP. Thus rat submandibular glands contain the enzymic basis of active Ca2+ transport and can actively transport Ca2+. Both activities are stimulated at Ca2+ concentrations typical of the cytosol, appear to be positively cooperative and may be regulated, in part, by calmodulin.