Effects of the Calcium Ionophore A23187 on Dispersed Bovine Parathyroid Cells

Dispersed bovine parathyroid cells were employed to assess the effects of the divalent cation ionophore A23187 on parathyroid function. In the presence of ionophore, both basal parathyroid hormone (PTH) release and isoproterenol-stimulated cAMP accumulation were inhibited by significantly lower concentrations of calcium (half-maximal inhibition at 0.7–0.8 mm vs. 1.0 mm and 1.3–1.4 vs. 1.7–2.0 mm calcium, respectively). After preincubation with ionophore, there was also an alteration in the relationship between agonist-stimulated cAMP accumulation and PTH release similar to that observed previously with elevated extracellular calcium concentrations; with A23187, a given increase in secretion rate was associated with nearly a 10-fold greater increase in cAMP accumulation. A23187 effected a highly significant increase in the rate of efflux of 45Ca from dispersed cells. The uptake of 45Ca, in contrast, was decreased slightly by ionophore. A predominant effect of ionophore on calcium efflux was also suggested ...