Intracellular pH regulation in human preimplantation embryos

We report here that intracellular pH (pH(i)) in cleavage-stage human embryos (2-8-cell) is regulated by at least two mechanisms: the HCO3-/Cl- exchanger (relieves alkalosis) and the Na+/H+ antiporter (relieves acidosis). The mean pH(i) of cleavage-stage embryos was 7.12 +/- 0.008 (n = 199) with little variation between different stages. Embryos demonstrated robust recovery from alkalosis that was appropriately Cl--dependent, indicating the presence of the HCO3-/Cl- exchanger. This was further confirmed by measuring the rate of intracellular alkalinization upon Cl- removal, which was markedly inhibited by the anion transport inhibitor, 4,4'-diisocyanatostilbene-2,2'-disulphonic acid, disodium. salt. The set-point of the HCO3-/Cl- exchanger was between pH(i) 7.2 and 7.3. Embryos also exhibited Na+-dependent recovery from intracellular acidosis. Na+/H+ antiporter activity appeared to regulate recovery up to about pH(i) 6.8; this recovery was HCO3--independent and amiloride-sensitive, with a pH(i) set-point of similar to 6.8-6.9. A second system that was both Na+- and HCO3--dependent appeared to mediate further recovery from acidosis up to aboit pH(i) 7.1, Thus, pH(i) of early human preimplantation embryos appears to be regulated by opposing mechanisms (HCO3-/Cl- exchanger, Na+/H+ antiporter, and possibly a third acid-alleviating transporter that was both Na+- and HCO3--dependent) resulting in the maintenance of pH(i) within a narrow range.