Changes of alanine-sodium co-transport during maturation of Xenopus laevis oocytes.

Maturation of full grown Xenopus oocytes is associated with a decrease of their capacity to transport amino acids and nucleosides. Nevertheless, the transport continues at a reduced rate until the time of metaphase arrest when the oocytes are ready for shedding. We have found that the L-alanine transport in unfertilized, shed oocytes is a saturable process which is Na+-dependent. The stoichiometrical ratio between sodium and alanine is 2:1. Comparison with the alanine uptake in full grown oocytes shows that during maturation Vmax is strongly reduced while Km and the stoichiometrical ratio of sodium and alanine is unaffected. Lowering the osmolarity of the shedding medium reduces the rate of uptake of alanine below the rates seen after shedding into isotonic media. The transport rates amongst the individual oocytes differ. They do not fit to a Gaussian distribution. In addition, in individual oocytes, there exists no correlation between the variations of the transport rates for amino acids and nucleosides, which are also taken up by the unfertilized, shed oocytes.