Calcium Release from Porcine Thyroid Microsomes by Phosphatidylinositol 4,5-Bisphosphate and Inositol 1,4,5- Trisphosphate*

We have demonstrated Ca2+ mobilization induced by inositol compounds from intracellular stores of isolated porcine thyroid cells that were permeabilized with saponin and from fractionated thyroid microsomes that were preloaded with Ca2+. In the presence of saponin and antimycin A, a mitochondrial inhibitor, the Ca2+ concentration in the cell suspension decreased after the addition of ATP; the addition of inositol 1,4,5-trisphosphate (IP3) elicited transient elevations in Ca2+. The half-effective concentration (EC50) of IP3 was about 1 microM. TSH, acetylcholine, and norepinephrine had no effect on the Ca2+ movement, but phosphatidylinositol 4,5-bisphosphate (PIP2) induced a similar Ca2+ release (EC50 = 3 microM). PIP2-induced release decreased as the Mg2+ concentration was raised. After the release induced by a maximal amount of IP3, PIP2 could induce a further Ca2+ release. We also measured Ca2+ efflux from fractionated microsomes preloaded with 45Ca2+ by ATP-dependent transport. IP3 induced a small efflux of 45Ca2+, but PIP2 was much more effective at low Mg2+ concentrations. More than half of the loaded 45Ca2+ was released by 10 microM PIP2 within 30 sec, and the EC50 was 0.7 microM. These results suggest that Ca2+ mobilization from the microsomes mediated by inositol compounds participates in the regulation of thyroid cell functions.