[Ca 2+ ] i rise at in vitro maturation in bovine cumulus–oocyte complexes

An intracellular calcium ([Ca 2+ ]i) rise has been described in cumulus–oocyte complexes (COCs) following luteinizing hormone (LH) exposure. Together with cAMP, Ca 2+ is a candidate signal for resumption of meiosis. Here, we analyzed if the most common hormones involved in oocyte maturation can induce the same Ca 2+ signal. In addition, we characterized the source of this signal. Immature, in vitro‐matured, and roscovitine‐meiotically arrested COCs were loaded with Fluo‐4 AM, stimulated with hormones/growth factors, and tested for [Ca 2+ ] i variations in cumulus cells. Reagents known to inhibit or stimulate [Ca 2+ ] i rises were used to characterize these [Ca 2+ ] i dynamics. Finally, expression of LH receptors (LHRs) in COCs was analyzed by immunofluorescence. In immature COCs, follicle‐stimulating hormone (FSH) elicited a single [Ca 2+ ] i rise that was higher than those induced by LH and growth hormone (GH), whereas epithelial growth factor failed to induce any changes in [Ca 2+ ] i . The [Ca 2+ ] i rise induced by FSH was higher in immature COCs; was reduced in roscovitine‐arrested, immature COCs; and was negligible in gonadotropin‐induced, in vitro‐matured COCs. In the case of spontaneous‐ and GH‐matured COCs, however, FSH stimulation caused a lower [Ca 2+ ] i rise. The hormone‐induced [Ca 2+ ] i rise was due to: (i) external Ca 2+ entry; (ii) intercellular communication; and (iii) intracellular Ca 2+ stores. Immunofluorescence revealed that LHRs were expressed throughout the cumulus cells. The above results show that: (i) gonadotropins and GH cause a [Ca 2+ ] i rise in cumulus cells; (ii) this [Ca 2+ ] i rise results from extra‐, inter‐, and intra‐cellular cumulative Ca 2+ fluxes; and (iii) LHRs are distributed on either outer or inner cumulus cells. Mol. Reprod. Dev. 79:369–379, 2012. © 2012 Wiley Periodicals, Inc.