The somatostatin analogue octreotide modulates Iodothyronine deiodinase activity and pituitary neuromedin B.

Somatostatin inhibits growth hormone and thyrotropin (TSH) secretion. It also enhances the inhibitory effect of thyroid hormone (TH) on TSH by poorly understood mechanisms. We investigated the acute effect of the long-acting somatostatin analogue, octreotide (OCT), on anterior pituitary type 1 (D1) and 2 (D2) deiodinase activity, on liver D1, and on pituitary content of neuromedin B (NB), an autocrine inhibitor of TSH secretion, which is positively regulated by thyroid hormones. Euthyroid or hypothyroid rats were sacrificed at different times after a single subcutaneous injection of OCT (1 microg/kg body weight [BW]). D1 and D2 activities were measured by the release of 125I from 125I reverse triiodothyronine (rT3) under different assay conditions. NB, TSH, T3, and thyroxine (T4) were quantitated by radioimmunoassay (RIA). In euthyroid rats, liver and pituitary D1 activities were decreased (50%) 6 hours after OCT injection; pituitary D2 and NB remained unchanged. In hypothyroid rats, OCT increased near to the level of normal rats both pituitary D1 activity (but not liver) and NB content, at 24 hours and at 6 and 24 hours, respectively (p < 0.05). Pituitary D2, greatly increased by hypothyroidism, showed a small (25%) but significant reduction at 3 hours, persisting at 24 hours (p < 0.01), although it remained higher than that of euthyroid control. Serum thyroid hormones were not affected by OCT injection. The results show that octreotide acutely regulates pituitary deiodinases and NB content, both representing mechanisms that potentially can contribute to somatostatin and octreotide actions on pituitary growth hormone (GH) and TSH secretion and to modulate these cells sensitivity to thyroid hormone action.