Iodothyronine Release from the Perfused Canine Thyroid following Cessation of Stimulation

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Abstract
The kinetics of thyroid secretion after termination of stimulation by 100 μU/ml bovine thyroid stimulating hormone (TSH) or 5 mM cyclic AMP (cAMP) were studied using perfused canine thyroid lobes. All experiments were performed as paired comparisons, one thyroid lobe acting as a control continuing to receive infusion of the stimulator. 2.5 h after termination of TSH infusion, the secretion of thyroxine (T4), 3,5,3′-triiodothyronine (T3), and 3,3′,5′-triiodothyronine (rT3) was not significantly different from that of the control lobes. After cessation of cAMP infusion, the secretion of T4 continued unaffected for ∼40 min. Then a gradual decline in T4 release occurred. The secretion of T3 and rT3 decreased somewhat earlier leading to a transient phase with increases in the T4:T3 and T4:rT3 ratios in the thyroid effluent. The persistently high secretion of iodothyronines despite cessation of TSH infusion is most likely the result of a continued stimulation by receptor-bound TSH. Because the clearance of intracellular cAMP is rapid and the concentration of cAMP used for stimulation in these experiments only exceeded the concentration necessary for eliciting a secretory response modestly, it is reasonable to assume that stimulation of colloid droplet formation stopped shortly after termination of cAMP infusion. The bulk of iodothyronines secreted thereafter thus originated from continued hydrolysis of thyroglobulin engulfed by the follicular cells during the preceding cAMP infusion. The pattern of an earlier decline in secretion of T3 and rT3 than of T4 from this intracellular pool of thyroglobulin points to a more rapid liberation of triiodothyronines than of thyroxine from thyroglobulin during intracellular hydrolysis.