1,25-Dihydroxyvitamin D3Receptors in the Seminiferous Tubules of the Rat Testis Increase at Puberty*

Abstract

Whether 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptor levels correlate with the rapid in vivo growth rate of the testes in the prepubertal rat was examined. Low salt chromatin-localized 1,25-(OH)2D3 receptors were compared in the testes and intestinal mucosa (control) of prepubertal, peripubertal, and mature rats (37, 49 and 90 days old, respectively). The number of 1,25-(OH)2D3 receptors per g wet wt was significantly (P < 0.02) reduced in the testes of the prepubertal rats compared to those in the peripubertal and mature groups. Conversely, no changes were observed in the 1,25-(OH)2D3 receptor levels in the control tissue intestinal mucosa among these age groups. Further experiments confirmed the identity of the testicular 1,25-(OH)2D3 receptors. The specific [3H]1,25-(OH)2D3-binding component was predominantly localized in the nuclei/chromatin fraction in hypotonic buffers. Scatchard analysis of [3H]1,25-(OH)2D3 binding to the testicular chromatin of adult rats yielded a single specific binding component with a Kd of 0.33 .+-. 0.06 nM and a Nmax of 102.3 .+-. 6.4 fmol/g tissue (n = 6), which was inhibited by excess 1,25-(OH)2D3, but only minimally by 50 nM 25-hydroxyvitamin D3. Sucrose gradient analysis required hydroxylapatite treatment of fractions after centrifugation to remove free 3H-labeled steroid. With this modification, a discrete 3.6S peak of [3H]1,25-(OH)2D3 was unmasked, which was eliminated by excess 1,25-(OH)2D3, but not by 50 nM 25-hydroxyvitamin D3, or 1 .mu.M cortisol, or the progesterone analog promegestone. In spite of its seemingly ubiquitous distribution, the 1,25-(OH)2D3 receptor does exhibit tissue specificity, since it appears to be absent in the prostate and, at best, greatly reduced in the epididymis. The cellular localization of the testicular 1,25-(OH)2D3 receptors was examined by mechanically separating interstitial cells (93.7% of the total [125I]hCG binding) from the tubules. Under these conditions, 91.3% of the specific [3H]1,25-(OH)2D3 binding occurred in the tubular chromatin preparation. Thus, these data provide evidence for the presence of a specific 1,25-(OH)2D3 receptor in the seminiferous tubules of the rat testis. The temporal correlation of increased 1,25-(OH)2D3 receptor levels with testicular maturation suggests a better correlation to testicular function and spermatogenesis than to growth of the organ in vivo.