Abstract
The application of thapsigargin (TG) or cyclopiazonic acid (CPA) to Fura-2-loaded chromaffin cells in the perfused rat adrenal medulla abolished a transient [Ca2+]c rise induced by muscarine in Ca2+-free medium due to the depletion of Ca2+ stores. Either TG or CPA induced a sustained increase of [Ca2+]c in Ca2+-containing medium, indicating that store-operated Ca2+ entry (SOCE) mechanism exists in this cell type. The TG-induced [Ca2+]c increase was inhibited completely with 2 mM Ni2+ but only by 18% with 100 μM D600, whereas maintained [Ca2+]c increases during prolonged stimulations with muscarine (100 μM) and high-K+ (40 mM) were inhibited by 100% and 51% with Ni2+, by 53% and 80% with D600, respectively. In cells to which muscarine and Ni2+ had been co-applied, Ca2+ stores remained depleted to induce a sustained SOCE after the two agents were washed out. In isolated chromaffin cells, CPA induced a much smaller extent of elevation in [Ca2+]c, compared with that induced in cells being in the adrenal medulla, which possibly suggests that the mechanism involved in SOCE may be fragile and was impaired in dissociation. TG or CPA applied alone to the adrenal medulla did not elicit a detectable amount of catecholamine secretion despite the elevation of [Ca2+]c, nor promoted secretory responses to a significant extent when applied during stimulation with high-K+. These results suggest that SOCE in rat chromaffin cells may not produce a sufficient increase in [Ca2+]c near the secretory vesicles to trigger exocytosis. [J Physiol Sci. 2006;56 Suppl:S108]
