Abstract
Although vitamin D acts in various biological processes, it plays a critical role in the mainte-nance of bone health, and regulates calcium homeostasis. In humans and rodents, the main tissues involved in vitamin D metabolism are the liver and the kidneys, however it has been shown that the testis has strongly participated in its bioactivation. Indeed, in these differ-ent species, enzymes metabolizing vitamin D (CYP27A1, CYP27B1 and CYP2R1) have been demonstrated in this tissue. Moreover, men with hypogonadism have shown a decrease in circulating levels of vitamin D. In equine species, the castration of males is a regular practice to reduce the behavior of stallions deemed too aggressive. Castration is carried out at various ages: in foals during their growth or in adulthood once they have reached their optimum size. Although horses exhibit atypical vitamin D metabolism with low cir-culating levels of vitamin D, it was suggested that testis may contribute to its activation as has been described in rodents and humans; castration could therefore be likely to affect its metabolism. In this study, blood levels of bioactive form of vitamin D (1 alpha,25[OH] 2 vitamin D 3 ) were measured before and after castration at different ages: 1 wk, after puberty (2 yr) and at adulthood (6 yr). The gene expression of enzymes involved in vitamin D metabolism has been sought in the testis of different experimental groups. No change in bioactive vi-tamin D3 levels was observed after castration regardless of the age at the time of surgery. The exceptional status of equine species is confirmed with a low or a lack of testis contri-bution to vitamin D metabolism, regardless of testicular development. This is demonstrated by a low or a lack of signal from enzymes involved in vitamin D bioactivation. Therefore, horses constitute a unique model in comparative endocrinology. (c) 2021 Elsevier Inc. All rights reserved.