Reproduction biotechnologies depend on the use of fully meiotically competent oocytes. Growing oocytes without full meiotic competence are an interesting potential source due to their quantity, but the mechanisms regulating the processes of acquisition of meiotic competence have not been clarified to date. Nitric oxide synthase (NOS) and its product, nitric oxide (NO), may possibly play a role. Understanding the precise NO regulatory mechanism is therefore important for the development of in vitro growth methods. The objective of this work was to detect changes in the expression of NOS isoforms and their mRNA expression and changes in the intracellular localization of separate NOS isoforms during the growth period of the porcine oocyte, and also to determine whether these changes are related to the process of meiotic competence acquisition. mRNA for all NOS isoforms was already detected in oocytes at the beginning of their growth and was present in them until they completed their growth period. mRNA for iNOS and eNOS was also observed in granulosa and cumulus cells from these oocytes. But nNOS mRNA was not demonstrated in these types of cells. Pig oocytes and their surrounding cells contained all NOS proteins. Their amounts increased and localization changed with the acquisition of meiotic competence. nNOS was localized mainly in the cortex in meiotically incompetent oocytes, while meiotically competent oocytes contained nNOS in the nucleus as well. iNOS protein was distributed in the cytoplasm and nucleus in all oocytes, and meiotically incompetent oocytes contained iNOS in the nucleolus as well. eNOS protein was distributed in oocytes in the form of fine granules with a strong fluorescence signal. Protein was concentrated in the nuclear area in meiotically incompetent oocytes and also in the periphery in oocytes with partially and fully-developed meiotic competence. All these findings indicate that NOS isoforms may significantly influence the acquisition of meiotic competence in porcine oocytes.

Pig oocytes matured in vitro were parthenogenetically activated (78%) after treatment with 2 mM nitric oxide-donor (±)-S-nitroso-N-acetylpenicillamine (SNAP) for 24 h. Inhibition of soluble guanylyl cyclase with the specific inhibitors 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) or 6-anilino-5,8-quinolinequinone (LY83583) suppressed the SNAP-induced activation in a dose-dependent manner (23% of activated oocytes after treatment with 400 μM ODQ; 12% of activated oocytes after treatment with 40 μM LY83583). 8-Bromo-cyclic guanosine monophosphate (8-Br-cGMP), a phosphodiesterase-resistant analogue of cGMP, enhances the effect of suboptimal doses (0.1 or 0.5 mM) of the NO donor SNAP. DT3, a specific inhibitor of cGMP-dependent protein kinase (PKG, PKG), is also able to inhibit the activation of pig oocytes after NO donor treatment. Involvement of the cGMP-dependent signalling pathway is specific for NO-induced oocyte activation, because both the guanylyl cyclase inhibitor ODQ and the PKG inhibitor DT3 are unable to inhibit activation in oocytes treated with the calcium ionophore A23187. These data indicate that the activation of pig oocytes with an NO donor is cGMP-dependent and that PKG plays an important role in this mode of oocyte activation.

The culture of pig oocytes in the presence of the calcium channel blocker verapamil (0.02 mM) resulted in the blocking of meiosis at the metaphase I stage, and only a small fraction (about 28%) of the oocytes were able to continue their maturation to the stage of metaphase II. Hence, meiotic maturation in pig oocytes is a calcium-dependent process. After isolation of the pig oocytes from their follicles, the intracellular calcium deposits in the oocyte and granulosa cells, detectable using the combined oxalate-pyroantimonate method, are depleted. The amount of calcium deposits in the oocyte and granulosa cells increased during oocyte meiotic maturation in vitro, especially in the nucleus, mitochondria, vacuoles and cytoplasm. The replenishment of calcium deposits is significantly changed under the effect of verapamil. The increase in calcium deposits in the oocyte nucleus was delayed, a much larger amount of deposits was formed in the mitochondria, and the amount of deposits in the vacuoles was demonstrably smaller. A significant peak in the accumulation of calcium deposits was observed in the cytoplasm of verapamil-treated oocytes after 16 h of in vitro culture. We propose that an altered pattern in the replenishment of calcium deposits can disturb intracellular signalling and prevent the exit of oocytes from the metaphase I stage.