Calcium fluxes in rat thyroid FRTL-5 cells. Evidence for a functional Na+/Ca2+ exchange mechanism.
The effect of extracellular Na+ on cytosolic free Ca2+ and on influx and efflux of Ca2+ was investigated in FRTL-5 thyroid cells. Stimulating the cells with the purinergic agonist ATP induced a rapid efflux of 45Ca2+ from cells loaded with 45Ca2+. Replacement of extracellular Na+ with choline+, significantly decreased the adenosine triphosphate-induced efflux of 45Ca2+. Furthermore, adenosine triphosphate-induced uptake of 45Ca2+ was increased when extracellular Na+ was replaced with choline+, compared with the uptake seen in Na+ buffer. Replacing extracellular Na+ with choline+, increased resting levels of cytosolic free Ca2+ from 50 +/- 2 nM (mean +/- SE) to 81 +/- 3 nM (P less than 0.05) in Fura 2 loaded cells. In cells preincubated with 1 mM ouabain for 30 min, resting cytosolic free Ca2+ increased to 73 +/- 3 nM (P less than 0.05). In a Na+ buffer, the adenosine triphosphate-induced transient increase in cytosolic free Ca2+ was 872 +/- 59 nM, compared with 1070 +/- 63 nM in choline+ buffer (P less than 0.05). The plateau level of cytosolic free Ca2+ in response to adenosine triphosphate was 130 +/- 16 nM in Na+ buffer, compared with 209 +/- 9 nM in choline+ buffer (P less than 0.05). Readdition of Na+ to the plateau phase decreased cytosolic free Ca2+ to 152 +/- 5 nM. Stimulating the cells with 10 microM of the Na(+)-selective monovalent ionophore monensin increased cytosolic free Ca2+ from 53 +/- 9 nM to 124 +/- 16 nM (P less than 0.05). This increase in cytosolic free Ca2+ was dependent on both extracellular Na+ and extracellular Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)
Törnquist
K
K
Endocrine Research Laboratory, University of Helsinki, Minerva Foundation Institute for Medical Research, Finland.
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D007474:Ion Exchange; D008985:Monensin / Q000494:pharmacology; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D013961:Thyroid Gland / Q000187:drug effects / Q000378:metabolism*
D000255:Adenosine Triphosphate; D008985:Monensin; D012964:Sodium; D002118:Calcium