Rapid stimulatory effect of thyroxine on plasma membrane transport systems: Calcium uptake and neutral amino acid accumulation in immature rat testis

Although the biological effects of thyroid hormones are mediated by nuclear receptors (genomic mechanisms), interactions with receptors associated with the plasma membrane (non-genomic mechanisms) of target cells are not clear. In this study we investigated the rapid stimulatory effect of thyroxine (T4) on 45Ca2+ uptake as well as ionic currents and intracellular messengers involved in the stimulatory action of T4 in amino acid accumulation in immature rat testes. Results indicated that 10−9 M or 10−6 M T4 was able to increase immediately 45Ca2+ uptake after 60 s of hormone exposure. These results indicate for the first time that voltage-dependent Ca2+ channels and ATP-dependent K+ channels can be seen as a set-point in the stimulatory effect of T4 on amino acid accumulation. Apamin-sensitive small-conductance Ca2+-activated K+ channels (SKCa) and chloride channels were shown to be partially involved in this mechanism. The amino acid accumulation triggered by the PKC pathway suggests a functional link between different ion channel activities and the stimulatory effect of T4 on amino acid accumulation. In conclusion, we show in this study a rapid and stimulatory effect of T4 on calcium uptake and on amino acid accumulation, both events initiated at the plasma membrane, which strongly characterizes a non-genomic effect of T4 in immature rat testes.