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Developmental Changes in the Management of Acid Loads During Preimplantation Mouse Development¹

^a Department of Physiology, University of Sydney, New South Wales 2006, Australia ^b Department of Anatomy, University of Cambridge, Cambridge CB2 3DY, United Kingdom ^c Medical Foundation of the University of Sydney, New South Wales 2006, Australia

Intracellular pH recovery in Quackenbush Swiss mouse preimplantation embryos following acid loading was investigated under conditions of H⁺-monocarboxylate cotransporter inactivity. Isoform-sensitive inhibitors of Na⁺-H⁺ exchange (NHE) were used to block the Na⁺-dependent component of the response. A biphasic dose-response curve for HOE-694 and N-methylisopropylamiloride (MIA) suggested that two isoforms (putatively NHE1 and NHE3) are active in the oocyte, 1-cell, and 2-cell stages. By the blastocyst stage, loss of one of the MIA-sensitive NHE activities (putatively NHE3) was observed in isolated inner cell masses, and an MIA-resistant component of the recovery was identified. The MIA-resistant component was inhibited by 2 mM amiloride and enhanced by external K⁺ and by 4,4'-diisothiocyanostilbene-2,2'-disulfonate, suggesting NHE4 activity. However, unlike NHE4 in other tissues, the MIA-resistant component did not transport Li⁺ in exchange for H⁺, and reverse transcription-polymerase chain reaction detected NHE4 mRNA in the oocyte but not in later stages. Trophoblast, whether in intact or collapsed blastocysts, did not show measurable NHE activity or MIA-sensitive activity during recovery from acid load. Both trophoblast and pluriblast manifested an H⁺ conductance in response to acid load. This H⁺ conductance was first detected at the 8-cell stage and was blocked by zinc in the isolated inner cell mass but not in trophoblast. No other effective inhibitors of its activity were found.

¹ Supported by the NHMRC Australia, Medical Foundation of the University of Sydney, The Royal Society, U.K., and The Wellcome Trust, U.K.

² Correspondence: M.L. Day, Department of Physiology (F13), University of Sydney, NSW 2006, Australia. FAX: 61 2 9351 2058; margotd{at}physiol.usyd.edu.au

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