Thyrotropin upregulates alpha 1-adrenergic receptors in rat FRTL-5 thyroid cells.

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Abstract
FRTL-5 rat thyroid cells grown and maintained in a medium containing 0.05 nM thyrotropin have a 10-fold higher number of .alpha.1-adrenergic receptors on their cell surface than FRTL-5 cells maintained in the absence of thyrotropin in their medium. The increased number of .alpha.1-adrenergic receptors, measured as increased specific [3H]prazosin binding per microgram of DNA, is not associated with any changes in Kd values of prazosin. Thyrotropin increases the number of .alpha.1-adrenergic receptors by inducing their biosynthesis, as evidenced by the inhibitory effects of cycloheximide or actinomycin D; the effect on biosynthesis is cAMP-mediated, since 8-bromoadenosine 3'',5''-cyclic monophosphate, cholera toxin, forskolin, or 3-isobutyl-1-methylxanthine can mimic the thyrotropin effect in both extent and time course. The .alpha.1-adrenergic receptors on FRTL-5 thyroid cells have been functionally linked to iodide efflux into the follicular lumen and to the iodination of thyroglobulin-i.e., to the formation of thyroid hormones; the .alpha.1-adrenergic receptor signal is mediated by Ca2+ rather than by cAMP and involves arachidonic acid intermediates. The present data thus describe a unique upregulation phenomenon wherein the sequential expression of two receptors (thyrotropin and .alpha.1-adrenergic) and two distinct signal systems (cAMP and Ca2+) are apparently a necessary prelude to thyroid hormone homoeostasis.