Appearance of the rat testicular receptor for calcitriol (1,25-dihydroxyvitamin D3) during development

In the present study we have examined the developmental changes in the concentration of receptors for calcitriol in high-salt cytosol from the rat testis. Receptors for calcitriol were undetectable (less than 0.4 fmol/mg protein) until day 24, after which there was a rapid increase to reach adult levels (6-8 fmol/mg protein) between day 50-60. The lack of receptors in high-salt cytosol from the immature rat testis is not due to degrading enzymes, since cytosols prepared from the combination of equal volumes of testis homogenates from immature and adult rats had binding levels exactly half of that found in "adult controls". Furthermore, the increase in specific binding of [3H]calcitriol during development is due to an increase in the number of receptor sites, and is not due to a change in the apparent affinity of the receptors (Kd approximately equal to 1 X 10(-11) M at 0 degrees C). These results may explain why we previously were unable to demonstrate calcitriol receptors in cultured Sertoli cells and peritubular cells isolated from 19-day old rats. Furthermore, they indicate that calcitriol may be of minor importance for testicular function in the immature rat. The role of calcitriol in the pubertal and adult testis remains to be established.