The presence of the capacitative Ca2 entry mechanism was investigated in porcine oocytes. In vitro-matured oocytes were treated with thapsigargin in Ca2 -free medium for 3 h to deplete intracellular calcium stores. After restoring extracellular calcium, a large calcium influx was measured by using the calcium indicator dye fura-2, indicating capacitative Ca2 entry. A similar divalent cation influx could also be detected with the Mn2 -quench technique after inositol 1,4,5-triphosphate-induced Ca2 release. In both cases, lanthanum, the Ca2 permeable channel inhibitor, completely blocked the influx caused by store depletion. Heterologous expression of Drosophila trp in porcine oocytes enhanced the thapsigargin-induced Ca2 influx. Polymerase chain reaction cloning using primers that were designed based on mouse and human trp sequences revealed that porcine oocytes contain a trp homologue. As in other cell types, the capacitative Ca2 entry mechanism might help in refilling the intracellular stores after the release of Ca2 from the stores. Further investigation is needed to determine whether the trp channel serves as the capacitative Ca2 entry pathway in porcine oocytes or is simply activated by the endogenous capacitative Ca2 entry mechanism and thus contributes to Ca2 influx.
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