Calcium-Regulated Phosphodiesterase in Bovine Parathyroid Cells*

Abstract

The potential role of calmodulin-activated phosphodiesterase in regulating parathyroid function was assessed in dispersed bovine parathyroid cells. Boiled parathyroid cell sonicates contained 40–50 ng/10⁶ cells of calmodulin, as determined by the activation of calmodulin-deficient phosphodiesterase. Bovine parathyroid calmodulin appeared to be similar to, if not identical with, pure porcine brain calmodulin by a number of criteria. 1) Both eluted as single peaks at similar ionic strengths on DEAE-cellulose ion exchange chromatography. 2) Both activated calmodulin-deficient phosphodiesterase over a similar range of calcium concentrations. 3) In both cases, calmodulinactivated phosphodiesterase activity was specifically inhibited by similar concentrations of the phenothiazine trifluoperazine. In sonicates of bovine parathyroid cells, both cAMP and cGMP phosphodiesterase activities were inhibited by EGTA and restored by the addition of excess calcium. Moreover, calmodulinactivated phosphodiesterase could be directly demonstrated in cell sonicates subjected to DEAE-cellulose chromatography. When chromatography was carried out in the absence of EGTA, calmodulin and calcium-activated phosphodiesterase comigrated at 0.22 M NaCl. In the presence of EGTA, calmodulin-activated phosphodiesterase eluted at 0.13 M NaCl, while calmodulin eluted between 0.25–0.4 M NaCl. These results directly demonstrate the presence of calmodulin and calmodulin-activated phosphodiesterase in bovine parathyroid cells and suggest that this enzyme complex may contribute to the calcium-induced reduction of intracellular cAMP content as well as parathyroid hormone release in this cell type.