We have studied the properties of bovine parathyroid cells, primarily because they have an unusual calcium dose-response curve for secretion: secretion decreases when calcium concentration increases. Thus, these cells are especially useful in testing models for calcium dependent secretion of hormones and neurotransmitters. We have previously shown a close correlation in these cells between the calcium dose-response curve for secretion and the calcium dose-response curve for the probability that a calcium-activated potassium channel is open, thus suggesting that these channels play a role in the secretion process. We have now addressed the question of the mechanism by which calcium enters the parathyroid cells. We found that several pharmacological agents that affect calcium channels also affect the amount of parathyroid hormone secreted. We also found that in the presence of these agents, high concentrations of extracellular potassium increase secretion. Taken together, these results suggest that calcium is transported across the plasma membrane of parathyroid cells by voltage-independent calcium channels.
