Rapid mobilization of cellular Ca2+ in bovine parathyroid cells evoked by extracellular divalent cations. Evidence for a cell surface calcium receptor.
The concentration of intracellular free Ca2+ ([Ca2+]i) was measured in dissociated bovine parathyroid cells using the fluorescent indicator quin-2 or fura-2. Small increases in the concentration of extracellular Ca2+ produced relatively slow, monophasic increases in [Ca2+]i in quin-2-loaded cells, but rapid and transient increases followed by lower, yet sustained (steady-state), [Ca2+]i increases in fura-2-loaded cells. The different patterns of change in [Ca2+]i reported by quin-2 and fura-2 appear to result from the greater intracellular Ca2+-buffering capacity present within quin-2-loaded cells, which tends to damp rapid and transient changes in [Ca2+]i. In fura-2-loaded parathyroid cells, other divalent cations (Mg2+, Sr2+, Ba2+) also evoked transient increases in [Ca2+]i, and their competitive interactions suggest that they all affect Ca2+ transients by acting on a common site. In contrast, divalent cations failed to cause increases in steady-state levels of cytosolic Ca2+. Low concentrations of La3+ (0.5-10 microM) depressed steady-state levels of cytosolic Ca2+ elicited by extracellular Ca2+ but were without effect on transient increases in [Ca2+]i elicited by extracellular Ca2+, Mg2+ or Sr2+, suggesting that increases in the steady-state [Ca2+]i arise from the influx of extracellular Ca2+. Mg2+- and Sr2+-induced cytosolic Ca2+ transients persisted in the absence of extracellular Ca2+ but were abolished by pretreatment with ionomycin. These results show that cytosolic Ca2+ transients arise from the mobilization of cellular Ca2+ from a nonmitochondrial pool. Extracellular divalent cations thus appear to act at some site on the surface of the cell, and this site can be considered a "Ca2+ receptor" which enables the parathyroid cell to detect small changes in the concentration of extracellular Ca2+.
D000634:Aminoquinolines / Q000378:metabolism; D001572:Benzofurans / Q000378:metabolism; D001665:Binding Sites; D002021:Buffers; D002118:Calcium / Q000378:metabolism*; D002413:Cations, Divalent / Q000494:pharmacology*; D004987:Ethers / Q000494:pharmacology; D016257:Fura-2; D015759:Ionomycin; D007811:Lanthanum / Q000494:pharmacology; D008274:Magnesium / Q000494:pharmacology; D010280:Parathyroid Glands / Q000166:cytology* / Q000378:metabolism; D010281:Parathyroid Hormone / Q000378:metabolism; D013324:Strontium / Q000494:pharmacology; D013499:Surface Properties
D000634:Aminoquinolines; D001572:Benzofurans; D002021:Buffers; D002413:Cations, Divalent; D004987:Ethers; D010281:Parathyroid Hormone; D015759:Ionomycin; D007811:Lanthanum; D008274:Magnesium; C035739:Quin2; D002118:Calcium; D016257:Fura-2; D013324:Strontium