[Ca(2+) ](i) rise at in vitro maturation in bovine cumulus-oocyte complexes.

An intracellular calcium ([Ca2+]i) rise has been described in cumulus–oocyte complexes (COCs) following luteinizing hormone (LH) exposure. Together with cAMP, Ca2+ is a candidate signal for resumption of meiosis. Here, we analyzed if the most common hormones involved in oocyte maturation can induce the sameCa2+ 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 [Ca2+]i variations in cumulus cells. Reagents known to inhibit or stimulate [Ca2+]i rises were used to characterize these [Ca2+]i dynamics. Finally, expression of LH receptors (LHRs) in COCs was analyzed by immunofluorescence. In immature COCs, follicle-stimulating hormone (FSH) elicited a single [Ca2+]i rise that was higher than those induced by LH and growth hormone (GH), whereas epithelial growth factor failed to induce any changes in [Ca2+]i. The [Ca2+]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 [Ca2+ ]i rise. The hormone-induced [Ca2+ ]i rise was due to: (i) external Ca2+ entry; (ii) intercellular communication; and (iii) intracellular Ca2+ stores. Immunofluorescence revealed that LHRs were expressed throughout the cumulus cells. The above results show that: (i) gonadotropins and GH cause a [Ca2+]i rise in cumulus cells; (ii) this [Ca2+]i rise results from extra-, inter-, and intracellular cumulative Ca2þ fluxes; and (iii) LHRs are distributed on either outer or inner cumulus cells.