The acute secretory response to alterations in extracellular calcium concentration and dopamine in perifused bovine parathyroid cells

Abstract

We used perifusion of dispersed bovine parathyroid cells to examine the rapid kinetics of the secretory response to alterations in extracellular calcium and to dopamine and compared changes in hormone secretion with alterations in the putative intracellular mediators cytosolic calcium and cAMP. The increase in hormone secretion associated with a reduction in extracellular calcium from 2.0 to 0.75 mM occurred at least as rapidly as the change in calcium concentration, suggesting that the lag time for the stimulation of secretion was of the order of seconds or less. In cells loaded with the intracellular calcium-sensitive dye QUIN 2, on the other hand, the initial activation of hormone secretion by low extracellular calcium was delayed by 30-40 sec. In both QUIN 2-loaded and unloaded parathyroid cells, a subsequent increase in extracellular calcium concentration from 0.75 to 2.0 mM produced a rapid inhibition of hormone secretion which could not be separated temporally from the changes in extracellular calcium. In QUIN 2-loaded cells, the reduction in cytosolic calcium concentration associated with a decrease in extracellular calcium from 2.0 to 0.75 mM took place with a half-time of about 15 sec. The increase in cytosolic calcium concentration on raising extracellular calcium from 0.75 to 2.0 mM had a half-time of approximately 20 sec. Dopamine (10(-5) M) also produced a nearly immediate 3- to 4-fold stimulation of PTH release. Although the increase in hormone secretion preceded the release of cAMP from perifused cells, intracellular cAMP increased 3.4-fold within 10 sec of exposure to dopamine in parallel experiments. Preincubation of perifused cells with dopamine reduced the subsequent secretory response to low extracellular calcium. Conversely, dopamine-stimulated secretion was significantly greater in cells preincubated with 2.0 than in those incubated with 0.75 mM calcium. These results indicate that perifused bovine parathyroid cells respond to secretagogues with a time course comparable to that observed in vivo and that the temporal changes in cytosolic calcium concentration and cellular cAMP are consistent with a mediatory role for these factors in low calcium- and dopamine-stimulated secretion, respectively. In addition, at least a portion of the secretory response to low calcium and dopamine may come from a common cellular pool of PTH.