Expressed ryanodine receptor can substitute for the inositol 1,4,5-trisphosphate receptor in Xenopus laevis oocytes during progesterone-induced maturation.

Two structurally related forms of intracellular calcium release channels that can mediate the release of intracellular calcium have been identified: the ryanodine receptors (RyR) and the inositol 1,4,5-trisphosphate receptors (IP3R). Each channel responds to distinct pathways for activation. The IP3R is activated by IP3 and the RyR is thought to be activated by calcium or by another second messenger cADP ribose. It has been proposed that each type of channel subserves a specialized pool of intracellular calcium, and it is not understood why some cell types require more than one form of intracellular calcium release channel. The present study was designed to examine whether the RyR can substitute for the IP3R during oocyte maturation. IP3R expression was inhibited in Xenopus laevis oocytes using antisense oligonucleotides. These oocytes, with reduced levels of IP3R, demonstrated a marked delay in the time course of progesterone-induced maturation. The cloned skeletal muscle RyR1 was then expressed in X. laevis oocytes that were deficient in IP3R. Functional studies showed that the properties of the cloned RyR1, expressed in oocytes, were comparable to those of the native RyR1. X. laevis oocytes deficient in IP3R, but expressing RyR1, were able to undergo progesterone-induced maturation with a time course comparable to that seen in wild-type oocytes when caffeine was used to activate RyR and induce intracellular calcium release. These studies show that RyR1 can substitute for the IP3R as the intracellular calcium release channel required for Xenopus oocyte maturation and that intracellular calcium release is important for controlling the rate of progesterone-induced maturation.