The relative contribution of inositol-trisphosphate(InsP(3))-sensitive and InsP(3)-insensitive Ca2+ stores to the agonist-evoked oscillatory release of Ca2+ in HeLa cells was investigated using fura-2 cytosolic Ca2+ measurements and whole-cell recordings of Ca2+-activated K+ currents [K(Ca2+)]. The experimental approach chosen consisted in studying the effects on Ca2+ oscillations of a variety of pharmacological agents such as ryanodine, ruthenium red, caffeine and theophylline, which are known to affect the Ca2+ channels responsible for Ca2+-induced Ca2+ release (CICR) in excitable cells. The results obtained essentially indicate (a) that neither ryanodine nor ruthenium red affects the generation of periodic K(Ca2+) current pulses in whole-cell experiments, and (b) that histamine-induced Ca2+ oscillations are inhibited by caffeine and theophylline in a dose-dependent manner. However, these methylxanthines were unable, at concentrations ranging from 0.1 mM to 10 mM, either to mobilize Ca2+ from internal stores or to block the initial Ca2+ rise evoked by histamine. In addition, both methylxanthines showed at high concentrations (10-20 mM) a moderate inhibitory action on the production of InsP(3) induced by histamine. This effect was not essential to the action of caffeine on the oscillatory release of Ca2+, since an inhibition by caffeine of InsP(3)-induced Ca2+ oscillations was still observed in whole-cell experiments where the InsP(3) concentration was kept constant. The results also show (c) that the application of either caffeine or theophylline being stronger than that resulting from the application of caffeine, and finally (d) that the action of caffeine and theophylline is not related to an increase in cAMP concentration since neither forskolin (10-50 mu M) nor 8-Br-cAMP (1 mM) caused an inhibition of the InsP(3)-induced Ca2+ oscillations. It is concluded on basis of these results that the agonist-evoked Ca2+ oscillations in HeLa cells do not involve directly or indirectly a ryanodine-sensitive Ca2+-release channel with CICR properties, but rather arise from a control by Ca2+ of the InsP(3) Ca2+-release process.
