Inhibition of cyclic ovarian activity in rats treated chronically with vitamin A

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Abstract
The effects of altering the dietary level of vitamin A on cyclic ovarian activity and serum and hepatic vitamin A concentrations were studied in female rats. High but non-toxic oral doses of retinyl palmitate (5000 IU, 3 times/wk) significantly advanced the age at vaginal opening but had no effect on vaginal and ovarian cycles after 15 wk of treatment. Hepatic stores of vitamin A were significantly elevated, but serum levels were unchanged. Retinol-deficient rats given a supplement of retinoic acid had a normal vaginal opening age but displayed cornified vaginal cells for many of the days that smears were taken. Hepatic and serum vitamin A concentrations were markedly reduced. When treatment with high doses of retinyl palmitate was continued for 9 mo., animals developed polycystic anovulatory ovaries which were significantly lighter than those of controls. Retinol-deficient rats given retinoic acid and demonstrating depleted hepatic vitamin A reserves had ovaries of normal wt and containing corpora lutea, indicating that retinol is not necessary for ovulatory activity. Short-term treatment (16 days) of rats with toxic doses of retinyl palmitate (50,000 IU daily) sufficient to raise serum and hepatic retinol levels significantly did not alter ovarian cyclicity; corpora lutea were present and ovarian weight was normal. Long-term treatment with high but non-toxic doses of vitamin A apparently inhibits cyclic ovulatory activity, perhaps via alteration of a steroidogenic mechanism in the ovary or adrenal since other studies indirectly support the existence of such a relationship. Retinoic acid is a sufficient form of vitamin A replacement to maintain cyclic ovarian activity.