Stimulus-secretion coupling in bovine parathyroid cells. Dissociation between secretion and net changes in cytosolic Ca2+.
The relationship between the concentration of cytosolic free Ca2+ ([Ca2+]i) and secretion of parathyroid hormone (PTH) was investigated in isolated bovine parathyroid cells using the fluorescent Ca2+ indicator, quin 2. Increasing the concentration of extracellular Ca2+ from 0.5 to 2.0 mM caused a 3-fold increase in [Ca2+]i (from 183 +/- 4 to 568 +/- 21 nM) which was associated with a 2-4-fold decrease in secretion of PTH. Decreasing extracellular Ca2+ to about 1 microM caused a corresponding fall in [Ca2+]i to 60-90 nM. Extracellular Ca2+-induced changes in [Ca2+]i were not affected by omission of extracellular Na+. Depolarizing concentrations of K+ (30 mM) depressed [Ca2+]i at all concentrations of extracellular Ca examined, and this was associated with increased secretion of PTH. Ionomycin (0.1 or 1 microM) increased [Ca2+]i at extracellular Ca2+ concentrations of 0.5, 1.0, and 2.0 mM, but inhibited secretion of PTH only at Ca concentrations near the "Ca2+ set point" (1.25 microM). In contrast, dopamine, norepinephrine (10 microM each), and Li+ (20 mM) potentiated secretion of PTH without causing any detectable change in [Ca2+]i. The results obtained with these latter secretagogues provide evidence for a mechanism of secretion which is independent of net changes in [Ca2+]i. The phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA) did not alter [Ca2+]i or secretion of PTH at low (0.5 mM) extracellular Ca2+ concentrations. At 2.0 mM extracellular Ca2+, however, TPA (20 nM or 1 microM) depressed [Ca2+]i and potentiated secretion of PTH. The addition of TPA prior to raising the extracellular Ca2+ concentration reduced the subsequent increase in [Ca2+]i. The results show that the effects of TPA on secretion in the parathyroid cell are not readily dissociated from changes in [Ca2+]i and suggest that some TPA-sensitive process, perhaps involving protein kinase C, may be involved in those mechanisms that regulate [Ca2+]i in response to changes in extracellular Ca2+.
D000634:Aminoquinolines / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002417:Cattle; D003600:Cytosol / Q000378:metabolism; D004298:Dopamine / Q000494:pharmacology; D004837:Epinephrine / Q000494:pharmacology; D004987:Ethers / Q000494:pharmacology; D015759:Ionomycin; D008094:Lithium / Q000494:pharmacology; D008274:Magnesium / Q000378:metabolism; D010280:Parathyroid Glands / Q000166:cytology / Q000378:metabolism*; D010281:Parathyroid Hormone / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D000634:Aminoquinolines; D004987:Ethers; D010281:Parathyroid Hormone; D015759:Ionomycin; D008094:Lithium; D008274:Magnesium; D013755:Tetradecanoylphorbol Acetate; C035739:Quin2; D002118:Calcium; D004298:Dopamine; D004837:Epinephrine