Gonadotropin stimulates oocyte translation by increasing magnesium activity through intracellular potassium-magnesium exchange.

We previously showed that gonadotropin increases the K+ activity in Xenopus oocytes and that this is a signal for increased translation. However, K+ need not act to control synthesis directly but may act through an unidentified downstream effector. Using microinjection to vary the salt content of oocytes and concomitantly measuring [3H]leucine incorporation, we found that small changes in Mg2+ greatly affect translation rates. (Ca2+ had little influence.) By measuring intracellular ion activities, we found that oocyte cations existed in a buffer-like (ion-exchange) equilibrium in which K+ and Mg2+ are the preponderant monovalent and divalent cations. Hence, increasing cellular K+ activity might increase translation by causing Mg2+ activity to rise. If so, the increased translation rates produced by hormone treatment or K+ injection would be prevented by EDTA, a Mg2+ chelating agent. This prediction was tested and confirmed. We conclude that, when gonadotropin increases K+ activity, the cell's internal ion-exchange equilibrium is altered thereby increasing Mg2+ activity and this up-regulates translation.