D-ASPARTIC ACID IS IMPLICATED IN THE CONTROL OF TESTOSTERONE PRODUCTION BY THE VERTEBRATE GONAD - STUDIES ON THE FEMALE GREEN FROG, RANA-ESCULENTA

In the present study we report the occurrence of D-aspartic acid (D-As p) in the ovary of the green frog Rana esculenta and its putative invo lvement in testosterone production by the gonad, In the ovary, D-Asp c oncentrations undergo significant variations during the main phases of the sexual cycle. In spawning females (March), its concentration was low (2.5 +/- 1.1 nmol/g ovary) and during the postreproductive period (June) it increased and reached its peak level (58.0+/-10.1 nmol/g) in October. In that month, vitellogenesis occurs in a new set of ovarian follicles and continues until the next spring. The concentrations of D-Asp in the ovary and of testosterone in the ovary and in the plasma were inversely correlated during the reproductive cycle: when endogeno us D-Asp was low (March), testosterone was high (36.9+/-4.8 ng/g ovary ; 23.1+/-2.76 ng/ml plasma) and, in contrast, when the D-Asp concentra tion was high (October), the testosterone concentration was low (0.86/-0.21 ng/g ovary and 5/0+/-1.3 ng/ml plasma). In vivo experiments, co nsisting of injection of D-Asp (2.0 mu mol/g body weight) into the dor sal lymphatic sac of adult female hogs, demonstrated that this amino a cid accumulates significantly in the ovary. After 3 h, moreover, it ca used a decrease in testosterone level in the plasma of about 80%. This inhibition was reversible: within 18 h after the amino acid injection , as the D-Asp concentration in the ovary decreased, the testosterone titre was restored in both ovary and plasma. In vitro experiments, con ducted in isolated ovarian follicles, confirmed this phenomenon and id entified these gonadal components as the putative D-Asp targets. Other amino acids (L-Asp, D-Glu, L-Glu, D-Ala and L-Ala) used instead of D- Asp were ineffective. These findings indicate that D-Asp is involved i n the control of androgen secretion by the ovary in this amphibian spe cies, revealing a more complex system for control of this androgen syn thesis than was previously believed to exist.