Ca2+-Induced Increases in Steady-State Concentrations of Intracellular Calcium Are Not Required for Inhibition of Parathyroid Hormone Secretion

It has been well established that increases in extracellular calcium concentration (Ca²⁺]) inhibit parathyroid hormone (PTH) secretion. The effects of Ca²⁺] are mediated through a G-protein-coupled receptor that has been cloned and characterized. Additionally, it has been demonstrated in parathyroid cells that an increase in Ca²⁺] results in an increase in steady-state levels of intracellular calcium (Ca²⁺]_i). At present, it has not been fully resolved whether changes in Ca²⁺]_i are related to changes in PTH secretion. In the current study, the effect of increased Ca²⁺] on PTH secretion and the connection regarding changes in concentrations of intracellular calcium Ca²⁺]_i have been examined in primary cultures of bovine parathyroid cells. PTH secretion was measured by radioimmunoassay and intracellular calcium was determined by single cell calcium imaging. Bovine parathyroid cells pre-incubated with either 0.5 or 1 mM calcium responded to rapid increases in Ca²⁺] (≥0.5 mM) with an immediate and sustained increase in steady-state levels of Ca²⁺]_i that persisted for time intervals greater than 15 minutes. Although the magnitude of the sustained increase in Ca²⁺]_i varied among individual cells (∼40% to >300%), the overall pattern and course of time were similar in all cells examined (n = 142). In all trials, Ca²⁺]_i immediately returned to baseline levels following the addition of the calcium chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA). Additional control studies, however, suggest that sustained increases in Ca²⁺]_i do not correlate with regulation of parathyroid hormone secretion. Sustained elevations of Ca²⁺]_i were not observed when Ca²⁺] was gradually increased by the addition of 0.1 mM increments at 1 minute intervals. Furthermore, the effect on inhibition of PTH secretion was the same regardless of whether Ca²⁺] was increased by gradual or rapid addition.