Expression of Ca²⁺ mobilizing receptors in xenopus oocytes: A tool for receptor characterization

In order to investigate the molecular characteristics of gastrointestinal hormone receptors, we have expressed their mRNAs in Xenopus laevis oocytes. Xenopus oocytes possess intrinsic muscarinic cholinergic receptors which couple to intracellular Ca²⁺ release. Release of intracellular Ca²⁺ was detected by an increase in⁴⁵Ca²⁺ release from preloaded oocytes or by using the Ca²⁺-sensitive fluorescent dye fura-2. Similarly, cholecystokinin (CCK) receptors expressed on the surface of oocytes which were injected with mRNA prepared from rat pancreatic acinar AR42J cells were readily detected by their ability to mobilize intracellular Ca²⁺. The CCK receptors expressed in oocytes showed similar binding characteristics as the native receptors. CCK receptor expression in the oocytes could be specifically blocked by hybridizing the mRNA with antisense oligonucleotides based on the highly conserved second transmembrane region of the HM4 muscarinic cholinergic receptor before injection. These latter results strongly suggest that the CCK receptor is a member of the G protein-linked receptor family. Thus, expression in Xenopus laevis oocytes provides a powerful tool for elucidation of the molecular characteristics of gastrointestinal hormone receptors.