Ca-ATPase activity in bovine parathyroid cells

Ca-ATPase is thought to function as a calcium extrusion pump that may regulate cytosolic calcium concentration. Because the parathyroid gland is among the few tissues that are directly regulated by extracellular calcium and because cytosolic calcium may be a mediator of the effects of extracellular calcium on parathyroid hormone secretion, we have investigated the presence of this enzyme in homogenates of parathyroid cells. High performance liquid chromatography (HPLC) was used to quantify the formation of ADP from ATP following incubation of ATP with cellular homogenate in a buffer containing ethylenedioxy- (diethylenedinitrilo) tetra acetic acid (EGTA), ouabain, and calcium. Enzyme activity was calcium-dependent, with Ca-ATPase showing two Km (Ca) values, 31 and 853 nM. High affinity Ca-ATPase activity was reduced by the calmodulin inhibitor, trifluoperazine (TFP), with half-maximal inhibition occurring at 7 X 10(-5) M. Monovalent cations stimulated high affinity Ca-ATPase activity (K+ greater than Na+ greater than Rb+ greater than Li+) in the presence of calcium. Magnesium (0.8 mM) also stimulated cleavage of ATP. Sodium increased Ca-dependent ATPase activity by 82% but had no significant effect on Mg-stimulated activity. Furthermore, azide, an inhibitor of mitochondrial ATPase(s), had a significantly greater inhibitory effect on Mg-dependent than on Ca-dependent activity. In summary, a high affinity Ca-ATPase is present in bovine parathyroid cells which has a Km in the range of the cytosolic calcium concentration that is found in other cells. Ca-ATPase(s) may be of importance in regulating the cytosolic calcium concentration and, therefore, hormonal secretion in this cell type.