High-dose boric acid (BA) exposure produces testicular lesions in adult rats characterized by inhibited spermiation (IS) that may progress to atrophy. In vivo and in vitro studies addressed possible mechanisms. In vivo, boron tissue disposition was examined, since no detailed data existed, and relevant boron concentrations for in vitro studies needed to be set. Since BA induces riboflavinuria and also affects calcium/phosphorus homeostasis, and testis zinc appears essential for normal testis function, we examined BA effects on flavin status and testis levels of phosphorus (P), calcium (Ca) and zinc (Zn). Data showed that the testicular toxicity and central nervous system (CNS) hormonal effect were not due to selective boron accumulation in testis or brain/hypothalamus, with testis boron concentrations at approximately 1 to 2 mM; that riboflavin deficiency is not involved, due to both the absence of overt signs of deficiency and effects on tissue flavin content during BA exposure; and that changes in testis P, Ca and Zn levels did not precede atrophy, and are therefore unlikely to be mechanistically relevant. In vitro studies addressed the hallmarks of the BA testicular toxicity: the mild hormone effect, the initial IS, and atrophy. No effect of BA on the steroidogenic function of isolated Leydig cells was observed, supporting the contention of a CNS-mediated rather than a direct hormone effect. Since increased testicular cyclic adenosine monophosphate (cAMP) produces IS, and a role for the serine proteases plasminogen activators (PAs) in spermiation has been proposed, we examined in vitro BA effects on both Sertoli cell cAMP accumulation and PA activity, respectively. Results showed that the IS is not due to BA effects on either process. To address the atrophy, we evaluated BA effects in Sertoli-germ cell cocultures on: Sertoli cell energy metabolism, since lactate, secreted by Sertoli cells, is a preferred energy source for germ cells; and also on DNA/RNA synthesis, since germ cells synthesize DNA/RNA and BA impairs nucleic acid synthesis in liver, and may do so in testis. The most sensitive in vitro endpoint was DNA synthesis of mitotic/meiotic germ cells, with energy metabolism in Sertoli or germ cells affected to a lesser extent. The in vitro effect on DNA synthesis was manifest in vivo as a decrease in the early germ cell/Sertoli cell ratio prior to atrophy in testes from BA-exposed rats. Overall, these combined studies revealed some changes offering a plausible explanation for the atrophy aspect of the BA testicular lesion. However, the mechanism for the IS is still undefined, and should be the subject of future work. -Environ Health Perspect 102(Suppl 7): 99-105 (1994)
