Activation of amino acid accumulation in delayed implantation mouse blastocysts.

Delayed implantation blastocysts remained in a diapausing-like state when incubated in Na+ -depleted (55 mM) medium. Accumulation of radioactive amino acids remained low when embryos were in low Na+ medium, but increased in the manner characteristic of these embryos when placed in high (124 mM) Na+ medium. Differentiation of blastocysts was apparently arrested in medium containing only 55 mM Na+ since no progress toward formation of trophoblastic outgrowths was made in this medium even when it contained all other compounds required for this process. Low Na+ may prevent uptake of amino acids, such as lysine, essential to outgrowth formation. Accumulation of lysine was Na+ -dependent in blastocysts even though this amino acid is not transported via a Na+-symport system in most cell types. Glucose deprivation did not affect the increase in the capacity of delayed implantation blastocysts to take up amino acids when incubated in high Na+ medium, and glucose accumulation was not Na+-dependent. The capacity of delayed implantation blastocysts to take up amino acids was inversely related to the time before these embryos began forming outgrowths in vitro. The latter process occurred more rapidly if diapausing blastocysts had been stimulated in utero by injecting overiectomized mice with progesterone and estrogen. However, these hormones slowed outgrowth formation slightly when present in the culture medium in vitro.