Effect of inositol trisphosphate and calcium on oscillating elevations of intracellular calcium in Xenopus oocytes

Stimulation of many nonexictable cells by Ca²⁺-mobilizing receptor agonists causes oscillating elevations of the intracellular free Ca²⁺ concentration ([Ca²⁺](i)), rather than a continuous increase. It has been proposed that the frequency at which [Ca²⁺](i) oscillates determines the biological response. Because the occurrence of [Ca²⁺] oscillations is observed together with endogenous inositol polyphosphate (InsPs) production or following InsPs application, we injected Xenopus laevis oocytes with InsPs and monitored Ca²⁺-activated Cl^- currents as an assay of [Ca²⁺](i). Microinjection of the poorly metabolizable inositol trisphosphate (InsP₃) derivatives inositol 2,4,5-trisphosphate (Ins(2,4,5)P₃) and inositol 1,4,5-trisphosphorothioate (Ins(1,4,5)P₃S₃) induced [Ca²⁺](i) oscillations. The frequency at which [Ca²⁺](i) oscillated increased with the injected dose, indicating that the frequency-generating mechanism lies distal to InsP₃ production and that generation of oscillations does not require either oscillation of InsP₃ levels or InsP₃ metabolism. Injections of high doses of Ins(1,4,5)P₃ or Ins(2,4,5)P₃ inhibited ongoing oscillations, whereas Ca²⁺ injections decreased the amplitude of Ins(2,4,5)P₃-induced oscillations without altering their frequency. Injections of the Ins(1,4,5)P₃ metabolite inositol 1,3,4,5-tetrakisphosphate also caused oscillations whose frequency was related to the injected dose, although inositol tetrakisphosphate injection induced an increase in the cellular level of Ins(1,4,5)P₃. The results suggest a multicomponent oscillatory system that includes the InsP₃ target as well as a Ca²⁺-sensitive step that modulates amplitude.