Calcium-regulated secretion of tissue plasminogen activator and parathyroid hormone from human parathyroid cells

The effects of Ca and other agents on secretion of plasminogen activator (PA) and PTH have been examined and compared, using parathyroid cells obtained from the glands of chronic renal patients. During 2 weeks culture at different [Ca], the secretory rates of PA activity and PTH were parallel; steady-state secretion over 24-h periods was maximal at 0.5-0.9 mM Ca, minimal at 1.5-2.5 mM Ca, and the [Ca] at 50% suppression was 1.1 mM. At 2.5 mM Ca, two inhibitors of cellular proteolysis, 3-methyladenine and chloroquine, stimulated secretion of both PA activity and PTH. The results indicated that secretion of PA from human parathyroid cells is regulated similarly to that of PTH. The characteristics of human parathyroid PA were also examined using human parathyroid adenoma tissue. In homogenates, the highest specific activity of PA was in microsomal fractions. The Mr of PA from tissue and from culture media was 70 kilodalton by sodium dodecyl sulfate gel electrophoresis followed by zymography, or by Western blotting using antisera to human tissue PA (tPA). Enzyme activity was inhibited by incubation with antisera to tPA but not to urokinase. In contrast to bovine parathyroid cells that secrete a urokinase, human parathyroids apparently contain and secrete tPA.