The Acute Secretory Response to Alterations in Extracellular Calcium Concentration and Dopamine in Perifused Bovine Parathyroid Cells*

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Abstract
Perifusion of dispersed bovine parathyroid cells was used to examine the rapid kinetics of the secretory response to alterations in extracellular Ca and to dopamine and compared changes in hormone secretion with alterations in the putative intracellular mediators cytosolic Ca and cAMP. The increase in hormone secretion associated with a reduction in extracellular Ca from 2.0-0.75 mM occurred at least as rapidly as the change in Ca 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 Ca-sensitive dye QUIN 2, the initial activation of hormone secretion by low extracellular Ca was delayed by 30-40 s. In both QUIN 2-loaded and unloaded parathyroid cells, a subsequent increase in extracellular Ca concentration from 0.75-2.0 nM produced a rapid inhibition of hormone secretion which could not be separated temporally from the changes in extracellular Ca. In QUIN 2-loaded cells, the reduction in cytosolic Ca concentration associated with a decrease in extracellular Ca from 2.0-0.75 mM took place with a half-time of .apprx. 15 s. The increase in cytosolic Ca concentration on raising extracellular Ca from 0.75-0.2 mM had a half-time of .apprx. 20 s. Dopamine (10-5 M) also produced a nearly immediate 3- to 4-fold stimulation of PTH [parathyroid hormone] release. Although the increase in hormone secretion preceded the release of cAMP from perifused cells, intracellular cAMP increased 3.4-fold within 10 s of exposure to dopamine in parallel experiments. Preincubation of perifused cells with dopamine reduced the subsequent secretory response to low extracellular Ca. Dopamine-stimulated secretion was significantly greater in cells preincubated with 2.0 than in those incubated with 0.75 mM Ca. Apparently, perifused bovine parathyroid cells respond to secretagogues with a time course comparable to that observed in vivo and that the temporal changes in cytosolic Ca concentration and cellular cAMP are consistent with a mediatory role for these factors in low Ca- and dopamine-stimulated secretion, respectively. At least a portion of the secretory response to low Ca and dopamine may come from a common cellular pool of PTH.

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