Effect of dexamethasone and ACTH on oocyte growth and recruitment in the frog Rana cyanophlyctis during the prebreeding vitellogenic phase.

The effects of 5, 25, 50 and 75 micrograms of dexamethasone and 0.1 or 0.5 IU ACTH on oocyte growth and recruitment were studied in Rana cyanophlyctis during prebreeding vitellogenic phase (May). Injections (ip) were given 6 days a week for 31 days and frogs were killed on the 32nd day. Treatment with 5 micrograms dexamethasone had no effect on gonadosomatic index (GSI) or on the number and percentage of different oocytes. Administration of 25 micrograms dexamethasone caused a significant (P less than 0.05) increase in both number and percentage of medium second growth phase (MSGP) oocytes and atretic follicles (AF), and a numerical reduction in number and percentage of large second growth phase (LSGP) oocytes. Frogs which received 50 or 75 micrograms dexamethasone exhibited a significant (P less than 0.05) decrease in GSI, ovarian weight, and number and percentage of LSGP oocytes, while those of MSGP oocytes and AF increased. There was no effect of dexamethasone on first growth phase (FGP) oocytes. The administration of 0.1 IU ACTH had no effect on GSI, percentage of different oocytes, or MSGP and LSGP number. There was a numerical increase in number of FGP oocytes. Treatment with 0.5 IU ACTH caused a significant (P less than 0.05) decrease in GSI, ovary weight, and number and percentage of LSGP oocytes, while those of AF increased. The above findings suggest that lower doses of dexamethasone (5 or 25 micrograms) and of ACTH (0.1 IU) have no effect on oocyte recruitment and growth, but that higher doses (50 or 75 micrograms dexamethasone and 0.5 IU ACTH) impair vitellogenic growth of oocytes and increase follicular atresia.(ABSTRACT TRUNCATED AT 250 WORDS)