Adenophostin A induces spatially restricted calcium signaling in Xenopus laevis oocytes

The activation of intracellular calcium release and calcium entry across the plasmalemma in response to intracellular application of inositol 2,4,5-trisphosphate and adenophostin A, two metabolically stable agonists for inositol 1,4,5-trisphosphate receptors, was investigated using Xenopus laevis oocytes and confocal imaging. Intracellular injection of inositol 2,4,5-trisphosphate induced a rapidly spreading calcium signal associated with regenerative calcium waves; the calcium signal filled the peripheral regions of the cell in 1-5 min. Injection of high concentrations of adenophostin A (250 nM) similarly induced rapidly spreading calcium signals. Injection of low concentrations of adenophostin A resulted in calcium signals that spread slowly (>1 h). With extremely low concentrations of adenophostin A (approximately 10 pM), stable regions of Ca2+ release were observed that did not expand to peripheral regions. When the adenophostin A-induced calcium signal was restricted to central regions, compartmentalized calcium oscillations were sometimes observed. Restoration of extracellular calcium caused a rise in cytoplasmic calcium restricted to the region of adenophostin A-induced calcium mobilization. The limited diffusion of adenophostin A provides an opportunity to examine calcium signaling processes under spatially restricted conditions and provides insights into mechanisms of intracellular calcium oscillations and capacitative calcium entry.