Evidence that the TRP-1 protein is unlikely to account for store-operated Ca²⁺ inflow in Xenopus laevis oocytes

The role of the TRP-1 protein, an animal cell homologue of the Drosophila transient receptor potential Ca²⁺ channel, in storeoperated Ca²⁺ inflow in Xenopus laevis oocytes was investigated. A strategy involving RT-PCR and 3′ and 5′ rapid amplification of cDNA ends (RACE) was used to confirm and extend previous knowledge of the nucleotide and predicted amino acid sequences of Xenopus TRP-1 (xTRP-1). The predicted amino acid sequence was used to prepare an anti-TRP-1 polyclonal antibody which detected the endogenous oocyte xTRP-1 protein and the human TRPC-1 protein expressed in Xenopus oocytes. Ca²⁺ inflow (measured using fura-2) initiated by 3-deoxy-3-fluoroinositol 1, 4, 5-trisphosphate (InsP₃F) or lysophosphatidic acid (LPA) was completely inhibited by low concentrations of lanthanides (IC₅₀ = 0.5 μM), indicating that InsP₃F and LPA principally activate store-operated Ca²⁺ channels (SOCs). Antisense cRNA or antisense oligodeoxynucleotides, based on different regions of the xTRP-1 cDNA sequence, when injected into Xenopus oocytes, did not inhibit InsP₃F-, LPA- or thapsigargin-stimulated Ca²⁺ inflow. Oocytes expressing the hTRPC-1 protein, which is 96% similar to xTRP-1, exhibited no detectable enhancement of either basal or InsP₃F-stimulated Ca²⁺ inflow and only a very small enhancement of LPA-stimulated Ca²⁺ inflow compared with control oocytes. It is concluded that the endogenous xTRP-1 protein is unlikely to be responsible for Ca²⁺ inflow through the previously-characterised Ca²⁺-specific sacs which are found in Xenopus oocytes. It is considered that xTRP-1 is likely to be a receptor-activated non-selective cation channel such as the channel activated by maitotoxin.