Mitotic counts in rat adenohypophysial thyrotrophs and somatotrophs: Effects of short-term thyroidectomy, thyroxine, and thyrotropin-releasing hormone.

The question of whether thyroxine (T₄) and thyrotropin-releasing hormone (TRH) affect mitoses in pituitary thyrotrophs (Tt) and somatotrophs (St) of hypothyroid rats was investigated. Fifteen day thyroidectomized (Tx) rats were used. Groups of Tx animals received T₄ or TRH or both. Except 6 and 24 h TRH groups, the animals were sacrificed 12 h after injections. Unoperated euthyroid rats served as controls. In Tx group adenohypophysial mitoses were significantly increased. T₄ diminished mitoses in Tx rats. Mitotic counts were decreased in 6 and 24 h Tx groups, but increased in 12 h TRH group. TRH plus T₄ in Tx animals had a synergistic effect on adenohypophysial mitoses. In unoperated controls few mitoses were observed in Tt and more mitoses in St. In Tx rats more mitoses were seen in Tt than in St. T₄ alone failed to reduce mitoses in Tt but increased them in St. We concluded that T₄ affects Tt and St replication. In normal rats mitoses occur mainly in St. In Tx rats mitotic activity increased in Tt. TRH plus T₄ have a synergistic motogenic effect on St. T₄ but not TRH affects St replication. It appears that the presence, of T₄ is necessary for St multiplication. [ABSTRACT FROM AUTHOR]