Secretion of plasminogen activator from bovine parathyroid cells

The characteristics and secretion of plasminogen activator (PA) were examined in fresh and cultured bovine parathyroid cells. Release of PA activity was maximal at low Ca concentrations ([Ca]) and suppressed at physiological [Ca]. PA secretion at high [Ca], like that of PTH, was increased by treatment of cells with chloroquine and/or 3-methyladenine. Secretion from organoids was not affected by hydrocortisone hemisuccinate, but was strongly increased by 1,25-dihydroxyvitamin D3. In cell homogenates, PA specific activity was highest in microsomes, from which less than 50% could be solubilized by Triton X-100. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of microsomes and media followed by zymography on fibrin-agarose gels showed that PA from both sources had an apparent mol wt of 44,000. No inhibitors of PA were detected by reverse zymography. PA activity was inhibited by placental urokinase (uPA) inhibitor and amiloride, which indicated that it was a uPA, but the secreted form required tissue-type PA stimulator (fibrin peptides) or denatured microsomes for full activity. Neither the microsomal nor the secreted forms of PA were active with S-2444, a substrate specific for active uPA. By comparison with the characteristics of human activators, the results suggested that parathyroid cells secrete uPA that is primarily in the precursor form single chain urokinase or scuPA.