Presence of calmodulin in parathyroid adenomas

A significant percent of parathyroid adenomas demonstrate decreased sensitivity to the suppressive effects of calcium on parathyroid hormone secretion. The recognition that calmodulin mediates a large number of calcium-dependent cellular events suggests that calmodulin may play a key role in the normal regulation by calcium of parathyroid activity. We therefore undertook to determine if parathyroid adenomas contained calmodulin and if so, if the calmodulin content of adenomas could be correlated with the serum concentrations of calcium and immunoreactive parathyroid hormone (IPTH) in hyperparathyroid patients prior to surgery. Calmodulin was assayed by activation of a calmodulin-stimulatable phosphodiesterase (PDE) obtained from rat brain. We found PDE-stimulating activity in each of six adenoma extracts examined. The stimulation had the following properties, which are characteristic of calmodulin-mediated processes: (1) it required the presence of calcium; (2) the dose response to adenoma extract and human red blood cell calmodulin were parallel, and (3) the stimulation was phenothiazine-inhibitable. The amount of calmodulin present in the adenoma extracts ranged from 0.4 to 1.25 units/micrograms protein. No correlation was found between the calmodulin content of the adenomas and the presurgical levels of either serum calcium or serum IPTH.