Rapid responses to thyroxine in the testis: Active protein synthesis-independent pathway

We investigated the involvement of protein synthesis in the stimulatory action of thyroid hormones on amino acid accumulation and characterized K+ currents involved in the hyperpolarizing effect of thyroxine (T4) on Sertoli cells. Immature rat testes were incubated in Krebs Ringer-bicarbonate buffer (KRb) in the presence of [14C]methylaminoisobutyric acid with and without T4, 3,5,3′-l-triiodothyronine (T3) and/or cycloheximide. Sertoli cells were monitored by intracellular recording in a chamber perfused with KRb with and without T4, T3 and/or blockers, and the membrane potential was monitored. T4 and T3 stimulated amino acid accumulation and protein synthesis. Treatment with cycloheximide diminished T3 stimulatory actions on amino acid accumulation but had no effect on T4 action. Both hormones elicited a hyperpolarization of the Sertoli cell membrane potential which involved K+ channels, since TEA and apamin abolished this effect. These findings on rapid membrane actions of thyroid hormone in the testis suggest that some effects of T4 are modulated by non-genomic mechanisms.