Properties of amino acid transport in preimplantation rabbit embryos.

The kinetics, specificity, time dependency, Na+ dependency, and exchange behavior of the uptake of methionine, as a model for the uptake of the neutral amino acids, was investigated in rabbit eggs, morulae, and 6-day blastocysts. Uptake by the blastocyst was physically resolved into components representing uptake into the cellular and cavity compartments. The kinetic data for all material, except the cellular compartment of the blastocyst, could be described mathematically by the parallel activity of a saturable and a linear transport system. The Km decreased from 91 microM in the unfertilized egg to 38 microM in the fertilized egg to 13 microM in the 69-hr morula. The Km of uptake into blastocysts was 131 microM. The Vmax of uptake into the whole blastocyst was calculated to be about 5000 times that of uptake into the egg. Competition of methionine uptake by other amino acids indicated that the egg had somewhat more "L-like" transport specificity than subsequent developmental stages, while the system(s) in the blastocyst were reactive with most neutral amino acids. Uptake into the egg, 69-hr morula, and the cellular compartment of the blastocyst reached a steady state with time, in contrast to uptake into the 76-hr morula and the cavity compartment of the blastocyst. Compatible with this, was the observation that eggs and 69-hr morula, but not blastocysts, exhibited considerable efflux into amino acid-containing media. The cavity compartment of the blastocyst may, however, act as a reservoir of amino acids for the cellular compartment. The Na+ dependency of transport was unchanged throughout the developmental period studied in this investigation.