Leucine transport in Xenopus laevis oocytes: functional and morphological analysis of different defolliculation procedures

L-leucine uptake in stage V Xenopus laevis oocytes was affected by the specific methods used to remove the follicle cells. In the presence of 100 mM NaCl, L-leucine uptake was reduced by 67.5% ± 5.7 when defolliculation was performed enzymatically by collagenase treatment, whereas the reduction was 30.5% ± 6.4 after mechanical defolliculation. The Na+ -dependent uptake of 0.1 mM L-leucine was 18.6 ± 4.6 pmol oocyte-1 40 min-1 in folliculated oocytes and 5.6 ± 1.9 in collagenase defolliculated oocytes (means ± SE). L-leucine uptake was not affected by the removal of the follicular layer if defolliculation occurred after the transport period; radiolabeled L-leucine is therefore not taken up into a compartment that is removed by the defolliculation process. The different L-leucine uptake rates observed in folliculated and defolliculated oocytes were not due to non-specific L- leucine binding to membranes. L-leucine kinetics showed that the L-leucine V(max) and K(m) values were lower in oocytes deprived of the follicular layer than in control oocytes enveloped in intact follicular layers. The V(max) and K(m) values of Na+-dependent L-leucine transport, calculated from data obtained the day after defolliculation by collagenase treatment, were: 16 ± 1.5 pmol oocyte-1 40 min-1 and 57 ± 21 μmol (mean ± SD). The Na+- activation curve of 0.1 mM L-leucine was hyperbolic in folliculated oocytes and sigmoidal in defolliculated oocytes. The morphological analysis performed in parallel with the transport experiments showed that after defolliculation, the fibers forming the vitelline membrane tended to be arranged in a more regular orthogonal array, and the number of oocyte microvilli was reduced after collagenase treatment. Mechanical defolliculation did not appreciably affect the oocyte microvilli, however this procedure did not completely remove all follicle cells. The damage to collagenase treated oocytes was reversible, and the functional and structural features of most oocytes improved upon subsequent in vitro incubation. The recovery process seemed to involve protein synthesis in view of the increased value of L-leucine V(max), and microscopic observation showing recovery of the microvillar apparatus.