Seasonal Changes in Testicular Gonadotropin Receptors and Steroid Content in the Ram*

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Abstract
To determine if seasonal changes in plasma gonadotropin levels and testicular weight are accompanied by changes in the number of testicular gonadotropin receptors, testicular LH and FSH receptor numbers were measured every 3 months in the testes of 2-yr-old rams during 2 successive yr. The intratesticular contents of testosterone and 17β-estradiol were also measured. The binding affinity of testicular receptors for LH (1.9 ± 0.2 1010 M-1) and FSH (4.1 ± 0.2 109 M-1) did not vary with season. When expressed per milligram of membrane proteins or per gram of wet tissue, the concentrations of LH and FSH receptors were maximum in June. When expressed per testis, however, the number of LH receptors increased linearly from December to September (from approximately 2 to 5 pmol/ testis), then decreased rapidly from September to December. The number of FSH receptors per testis was minimum between December and March (∼4 pmol/testis), increased by J ne, was maximum between June and September (∼12 pmol/testis), and then decreased by December. These differences may be due to concomitant seasonal variations in testicular weight and/or membrane protein content. The mean testicular testosterone content was higher (P < 0.05) in September than in March or June. Mean 17β-estradiol content was higher (P < 0.02) in September than in December but did not differ from that in March and June. These results suggest: 1) The affinities of both LH and FSH for their respective receptors are independent of season. 2) The total number of LH and FSH receptors increases before and decreases during the breeding season. 3) The increase in LH receptor number precedes that of intratesticular testosterone content. 4) The intratesticular content of 17β-estradiol undergoes seasonal variations.