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.
Horowitz
S B
SB
Department of Physiology and Biophysics, Michigan Cancer Foundation, Detroit 48201.
Tluczek
L J
LJ
D000818:Animals; D006063:Chorionic Gonadotropin / Q000494:pharmacology*; D004533:Egtazic Acid / Q000494:pharmacology; D004594:Electrophysiology / Q000379:methods; D005260:Female; D007700:Kinetics; D007930:Leucine / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism* / Q000494:pharmacology; D008839:Microelectrodes; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D011188:Potassium / Q000378:metabolism*; D014176:Protein Biosynthesis / Q000187:drug effects*; D014316:Tritium; D014982:Xenopus laevis
D006063:Chorionic Gonadotropin; D014316:Tritium; D004533:Egtazic Acid; D007930:Leucine; D008274:Magnesium; D011188:Potassium