MinireviewMembrane-initiated actions of thyroid hormones on the male reproductive system

The presence of specific nuclear receptors to thyroid hormones, described in prepubertal Sertoli cells, implies the existence of an early and critical influence of these hormones on testis development. Although the mechanism of action thyroid hormones has been classically established as a genomic action regulating testis development, our research group has demonstrated that these hormones exert several effects in Sertoli cells lacking nuclear receptor activation. These findings led to the identification of non-classical thyroid hormone binding elements in the plasma membrane of testicular cells. Through binding to these sites, thyroid hormones could exert nongenomic effects, including those on ion fluxes at the plasma membrane, on signal transduction via kinase pathways, on amino acid accumulation, on modulation of extracellular nucleotide levels and on vimentin cytoskeleton. The evidence of the participation of different K+, Ca2+ and Cl− channels in the mechanism of action of thyroid hormones, characterizes the plasma membrane as an important microenvironment able to coordinate strategic signal transduction pathways in rat testis. The physiological responses of the Sertoli cells to hormones are dependent on continuous cross-talking of different signal transduction pathways. Apparently, the choice of the signaling pathways to be activated after the interaction of the hormone with cell surface binding sites is directly related to the physiological action to be accomplished. Yet, the enormous complexity of the nongenomic actions of thyroid hormones implies that different specific binding sites located on the plasma membrane or in the cytosol are believed to initiate specific cell responses.