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 Ca-45(2+) from cells loaded with Ca-45(2+). Replacement of extracellular Na+ with choline+, significantly decreased the adenosine triphosphate-induced efflux of Ca-45(2+). Furthermore, adenosine triphosphate-induced uptake of Ca-45(2+) 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 < 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 < 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 < 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 < 0.05). Readdition of Na+ to the plateau phase decreased cytosolic free Ca2+ to 152 +/- 5 nM. Stimulating the cells with 10-mu-M of the Na+-selective monovalent ionophore monensin increased cytosolic free Ca2+ from 53 +/- 9 nM to 124 +/- 16 nM (P < 0.05). This increase in cytosolic free Ca2+ was dependent on both extracellular Na+ and extracellular Ca2+. If cells were first stimulated with monensin, and then with adenosine triphosphate, the transient increase in cytosolic free Ca2+ was 1027 +/- 24 nM (P < 0.05, compared with control cells). The results thus indicate, that FRTL-5 cells have a functional Na+/Ca2+ exchange mechanism and that this mechanism is of importance in restoring adenosine triphosphate-induced transient increase in cytosolic free Ca2+ to resting cytosolic free Ca2+ levels.