The bioactivity of rat placental extracts was evaluated using a fetal rat testis testosterone (FRTT) bioassay. It is based on the measurement of the increase in testosterone secreted in vitro by testes from 18.5-day-old fetuses in response to the addition of placental extracts. Placentas obtained on days 11.5, 12.5, 13.5, 14.5, 15.5, 17.5 and 19.5 and homogenized with a Potter homogenizer (1 placenta per ml incubation medium), increased the secretion of testosterone by 460, 690, 500, 300, 220, 200 and 170%, respectively. These extracts showed a high proteolytic activity capable of suppressing the bioactivity of 0.8 ng/ml LH added prior to the extraction procedure. However, the bioactivity of the placental extracts did not increase after inhibiting placental proteases by the addition of benzamidine. Heat treatment did not decrease the bioactivity of placental extracts obtained without inhibition of proteases. Ultrafiltration of the placental extracts obtained with inhibition of the proteases showed that most of the bioactivity was dialyzable. The levels of bioactive material with molecular weights > 10,000 were very low (0.06 ng LH equivalent/placenta on day 12.5 and < 0.02 ng LH equivalent/placenta on day 14.5). These results suggest that rat placental bioactivity as measured with the FRTT bioassay is due, for the most part, to steroid precursors of testosterone and, for a very small part, to an LH/chorionic gonadotrophin (CG)-like molecule. Furthermore, after day 14.5, the placental LH/CG-like bioactivity, if it exists, is synthesized in too low levels to be able to control the testicular activity in the rat fetus.

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