Stimulus-secretion coupling in parathyroid cells deficient in protein kinase C activity

The role of protein kinase C (PKC) in regulating cytosolic Ca2+ concentrations ([Ca2+]i) and parathyroid hormone (PTH) secretion was studied in bovine parathyroid cells rendered deficient in PKC activity by incubation with phorbol 12-myristate 13-acetate (PMA). Pretreatment with PMA caused a time- and concentration-dependent loss of functional PKC activity as assessed by the failure of [Ca2+]i and PTH secretion to respond to the subsequent addition of PKC activators or the inhibitor staurosporine. Pretreatment for 24 h with 1 microM PMA caused a loss of 85% of the total and 98% of the cytosolic PKC activity. Cells so pretreated were considered to be "PKC downregulated." Increasing the concentration of extracellular Ca2+ or Mg2+ caused corresponding increases in [Ca2+]i that were similar in control and in PKC-downregulated cells. PTH secretion regulated by extracellular Ca2+ or Mg2+ was likewise similar in control and PKC-downregulated cells. Stimulus-secretion coupling is thus unimpaired in parathyroid cells deficient in PKC activity. Cytosolic Ca2+ responses remained depressed in cells incubated for 24 h with low concentrations of PMA (30 or 100 nM). However, under these conditions, extracellular Ca2+ still suppressed PTH secretion similarly to control cells. These results reveal a dissociation between cytosolic Ca2+ and PTH secretion and suggest that signals other than cytosolic Ca2+ are involved in the regulation of PTH secretion.