Calmodulin in Dispersed Human Parathyroid Cells*

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Abstract
We investigated the presence and several of the properties of calmodulin in human parathyroid cells. Boiled extracts of such cell preparations contained a heat-stable factor causing a 2- to 3-fold calcium-dependent stimulation of calmodulin- deficient phosphodiesterase activity, which was parallel to that due to pure porcine calmodulin. This activation could be totally blocked by 10−4 M trifluoperazine, with half-maximal inhibition at 3 × 10−5 M, similar to the effects of this phenothiazine on porcine calmodulin. These results suggested the presence of calmodulin in human parathyroid cells. By comparison with known quantities of porcine calmodulin, human parathyroid cells contained 9–208 ng calmodulin/106 cells. The content of calmodulin in 3 normal parathyroid glands [65 ± 8 (±SEM) ng/106 cells] did not differ significantly from that of 12 adenomas (61 ± 16 ng/106 cells). Cells from 7 glands showing secondary hyperplasia, however, had significantly greater levels of calmodulin (164 ± 11 ng/106 cells) than either normal cells or adenomas (P < 0.001 and P < 0.005, respectively). Extracts of human parathyroid cells caused half-maximal stimulation of phosphodiesterase activity at 1.1–4.8 JUM free calcium. The concentrations of calcium half-maximally activating phosphodiesterase (Ka) did not differ significantly for normal or abnormal cells (3.3 ± 0.03 vs. 2.6 ± 0.33; P > 0.3). Moreover, in 2 cases in which normal parathyroid tissue was obtained from patients with adenomas, the Ka values for calcium for the normal and abnormal cellswere similar (3.3 vs. 2.5 and 3.4 vs. 2.5 µM, respectively). Finally, there was no significant correlation between either the content of calmodulin or the Ka for calcium and the set-point for calcium [the calcium concentration causing half-maximal inhibition of parathyroid hormone (PTH) release] or the maximal rate of PTH secretion for dispersed parathyroid cells. These results suggest that human parathyroid cells contain calmodulin, but provide no evidence for a role of this protein in the abnormal calcium-regulated PTH release in hyperparathyroidism.

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