Store-operated Ca²⁺ entry in first trimester and term human placenta

We have examined whether store-operated Ca²⁺ entry, a common pathway for Ca²⁺ entry in non-excitable tissue, is apparent in the syncytiotrophoblast of both first trimester and term human placenta. Expression of transient receptor potential (TRPC) homologues, a family of channels thought to be involved in store-operated Ca²⁺ entry, was also studied at the mRNA and protein levels. [Ca²⁺]_i in syncytiotrophoblast of first trimester and term placental villous fragments was measured by microfluorimetry using the Ca²⁺-sensitive dye fura-2. Store-operated Ca²⁺ entry was stimulated using 1 μm thapsigargin in Ca²⁺-free Tyrode buffer (no added Ca²¹ + I mM EGTA followed by superfusion with control (Ca²⁺-containing) buffer. In term fragments, this protocol resulted in a rapid increase in [Ca²⁺]_i, which was inhibited in the presence of 150 μM GdCl₃, 200 μM NiCl₂, 200 μM CoCl₂ or 30 μM SKF96365 but was unaffected by addition of 10 μM nifedipine. It was not possible to stimulate such a rise in [Ca²⁺]_i in first trimester fragments. Messenger RNA encoding TRPC1, TRPC3, TRPC4, TRPC5 and TRPC6 was identified in both first trimester and term placentas. From Western blotting, TRPC3 and TRPC6 proteins were detected in term, but not in first trimester, placentas, while TRPC1 protein was not detected. By immunocytochemistry, TRPC3 and TRPC4 were localised to cytotrophoblast cells in first trimester placentas and to the syncytiotrophoblast in term placentas. TRPC6 staining was present in the syncytiotrophoblast of both first trimester and term placenta, but the intensity was much greater in the latter. We propose that store-operated Ca²⁺ entry may be an important route for Ca²⁺ entry into the syncytiotrophoblast of term, but not first trimester placentas, and that in human placenta TRPC channels may underlie this entry mechanism.