EFFECT OF SPHINGOSINE DERIVATIVES ON CALCIUM FLUXES IN THYROID FRTL-5CELLS

The effects of sphingosine derivatives on Ca2+ fluxes were investigate d in thyroid FRTL-5 cells labelled with Fura 2. Addition of sphingosyl phosphocholine (SPC) or sphingosine (SP) increased intracellular free Ca2+ ([Ca2+](i)) in a dose-dependent manner. At the highest dose teste d (30 mu M), the response was biphasic: a rapid transient increase in [Ca2+](i), followed by a new, elevated. level of [Ca2+](i). Both phase s of the SPC-evoked increase in [Ca2+](i) were dependent on extracellu lar Ca2+, whereas only the SP-evoked elevated level of [Ca2+](i) was d ependent on the influx of Ca2+. Both compounds released sequestered Ca 2+ from thapsigargin- and inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ pools. In addition, the increase in [Ca2+](i) in response to SPC , but not to SP, was attenuated in cells treated with phorbol myristat e acetate or with the putative Ca2+-channel blocker SKF 96365, and in cells pretreated with pertussis toxin for 24 h. SPC did not activate t he production of IP3. Furthermore, both SPC and SP released sequestere d Ca2+ from permeabilized cells. We observed that SPC, but not SP, sti mulated release of [H-3]arachidonate from cells prelabelled with [H-3] arachidonate for 24 h. Both SPC and SP stimulated the incorporation of [H-3]thymidine into DNA in cells grown in the absence of thyroid-stim ulating hormone (TSH). The results suggest that sphingosine derivative s are putative regulators of Ca2+ fluxes in FRTL-5 cells, and that SP and SPC may act on [Ca2+](i) via different mechanisms. Furthermore, bo th SP and SPC may be of importance in modulating thyroid-cell prolifer ation.