Response of porcine oocytes to electrical and chemical activation during maturation in vitro

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Abstract
These studies were conducted to examine activation of in vitro‐matured porcine oocytes in response to an electrical stimulus or to an ionophore. Cumulus‐enclosed porcine oocytes were incubated in maturation medium supplemented with either FSH and LH (MM:Exp.1) or pregnant mare serum gonadotropin (PMSG; MMP: experiments 2–4) at 39°C in 5% CO2:95% air with high humidity. In experiment 1, groups of oocytes were stripped of cumulus and then shampulsed (control) or electrically pulsed with a Zimmerman Cell Fusion unit at 24, 31, 41, 48, and 65 h of incubation. Control oocytes were exposed to the activation medium for 20 sec, whereas oocytes to be pulsed were subjected to a single activation pulse (120 V, 30 μsec). Oocytes were cultured for an additional 24 h and then fixed and examined. For oocytes pulsed at 24, 31, 41, 48, and 65 h, the proportions which activated were 0, 0, 87, 88, and 83%, respectively. In experiment 2, oocytes were electrically or sham‐pulsed with a BTX 200 Embryomanipulation System at 24, 30, and 40 h of incubation and respective proportions of oocytes activating were 27%, 39%, and 72%. In experiment 3, oocytes were subjected to 0, 1, or 2 activation pulses after 41 h of incubation in MM‐P. Double‐pulsing halved the proportion of activated oocytes (P<.0001). In experiment 4, oocytes were subjected to 0, 25, 50, or 100 μM ionophore at 48 h of incubation. Proportions of oocytes activated by ionophore were greater than for control (P < .05), but activation was not increased by increasing dose of ionophore. These results indicate that porcine oocytes may respond to an activation stimulus by 24 h of incubation, long before maturation is normally completed. Activation can be achieved also with an ionophore, as described previosly for other species.