Ql#NEBAL AND COMPARATIVE ENDOCRINOLOGY i&i, 352-363 (1974) Sex-Steroid Formation in Gonadal Tissue Homogenates During the Testicular Cycle of the Redwinged Blackbird (Age/&us phoeniceus)

Fluctuations in the in vitro metabolism of pregnenolone-‘II and progesterone-“C were studied in the testes of captive, photoinduced, adult Redwinged Blackbirds (Age&w phoeniceus) during the gonadal cycle. Starting material utiliration was high 03799%) during the photosensitive stage (breeding period), decreased during the regressive stage, and leveled off at about 40% in the refractory stage. The extent of testosterone synthesis varied in a bimodal pattern during the testicular cycle. Peaks in testosterone synthesis occurred during the photosensitive and regressive stages. However, testosterone formation was not detected in the refractory stage. In addition to testosterone, 20fldihydropregnenoIone (3/3,2Ofldihydroxypregn5-ene), #)cudihyd.ropregnenolone (3,B,2On-dihydroxypreg&-ene), 2Opdihydroprogesterone @O/3-hydroxypregn+en&one), and %dihydroprogesterone (29a-hydroxypregn-Q-en&one) were isolated. The 2Oa-pregnenols were detected only during the regressive and refractory stages. Moreover, an inverse relationship was seen between the formation of U)or-pregnenols and testosterone in the photosensitive and refractory stages. The physiological significance of a bimodal pattern of testosterone synthesis and of a restricted occurrence of 2OOcy-pregnenol formation in blackbirds is discussed in reference to the proposal by Lacy that two major populations of steroidogenic cells are present in the testis.