CYCLOPIAZONIC ACID INDUCES ACCELERATED PROGRESS OF MEIOSIS IN PIG OOCYTES

In mammalian oocytes, calcium plays an important role in the regulatio n of meiotic maturation. In our study, we used the mycotoxin cyclopiaz onic acid (CPA), an inhibitor of calcium-dependent ATPases, to mobilis e intracellular calcium deposits during in vitro maturation of pig ooc ytes. The CPA treatment of maturing oocytes significantly accelerated the progress of their maturation. Oocytes entered the CPA-sensitive pe riod after 21 h of in vitro culture. A very short (5 min) exposure to CPA (100 mM) is sufficient to accelerate maturation and it seems that accelerated maturation can be triggered by a transient elevation of in tracellular calcium levels. The effect of CPA is not mediated through the cumulus cells, because maturation is accelerated by CPA treatment even in oocytes devoid of cumulus cells. Culture of oocytes with the c alcium channel blocker verapamil (concentrations ranging from 0.01 to 0.04 mM) blocked the progress of oocyte maturation beyond the stage of metaphase I. This block can be overcome by the mobilisation of intrac ellular calcium deposits after CPA treatment (100 nM). The microinject ion of heparin (20 pl, 0.1 mg/ml), the inhibitor of inositol triphosph ate receptors, before CPA treatment prevented the acceleration of oocy te maturation. This indicates that CPA mobilises the release of calciu m deposits through inositol trisphosphate receptors. On the other hand , the microinjection of procaine (20 pl, 200 nM) or the microinjection of ruthenium red (20 pl, 50 mM), both inhibitors of ryanodine recepto rs, did not prevent accelerated maturation in CPA-treated oocytes. If present in pig oocytes, ryanodine receptors evidently play no part in the liberation of calcium from intracellular stores after CPA treatmen t.