Evidence for persistent binding of biologically active thyrotropin to thyroid in vitro

Differences exist in the rates at which hormones are inactivated by, or dissociate from, their target tissues. The present studies examined the binding of biologically active TSH to thyroid slices and compared its characteristics to those of PGE ₁. Canine thyroid slices were initially incubated with 5 mU/ml of bovine TSH (TSH-Initial) for 15 min, washed and incubated in media free of hormone for 3 hr. At the conclusion of this second incubation period, all slices were again washed. Some were then transferred to media containing 10^-2 M theophylline for a final 10 min incubation and subsequent measurement of cAMP and protein kinase, while others were transferred to media containing [l-^l4 C]glucose without theophylline fora final 45 min incubation to assess glucose oxidation. Identically treated slices never exposed to TSH served as controls, while others were exposed to TSH only during the final 10 or 45 min incubation periods (TSH-Final). cAMP content determined after the final 10 min incubation in theophylline was significantly increased in TSH-Initial (mean 2.98 ± 0.36 (SE) pmol/mg wet wt) compared to control (0.35 ± 0.04), but was less than that in TSH-Final (5.76 ± 0.51). This phenomenon was not unique to canine thyroid, since comparable results were noted in studies of human, bovine or porcine thyroid slices. The protein kinase activity ratio (-cAMP/+cAMP) and glucose oxidation of TSH-Initial were also significantly increased above control following the final 10 min or 45 min incubations respectively. Addition of trypsin to the 3 h incubation abolished the subsequent increase in cAMP in TSH-Initial, while addition of TSH antiserum appreciably reduced this increase. These results are consistent with the persistent binding of biologically active TSH to thyroid. By contrast, evidence of similar persistent binding of PGE ₁ to thyroid, glucagon to liver, or parathyroid hormone to renal cortex was lacking when assessed by an identical experimental procedure. Differences between the duration of interaction of TSH and PGE ₁, with thyroid may be dependent on a more gradual dissociation of tissue bound TSH, a more rapid inactivation of bound-PGE ₁, or both.