Kinetics of thyrotrophs and somatotrophs during development of hypothyroidism and l-triiodothyronine treatment of hypothyroid rats

Two experiments were performed to assess the kinetics of changes in somatotroph (St) and thyrotrdph (Tt) populations in the rat anterior pituitary after thyroidectomy and during T₃ treatment (10 μtg/100 g BW) of thyroidectomized rats. St and Tt numbers were determined by a method combining cell counting and determination of DNA in dispersed cells as well as the percentage distribution of cells at the electron microscopic level. During development of hypothyroidism, the mean maximal increase in pituitary wet weight was reached 15 days after thyroidectomy. Pituitary weight remained relatively constant thereafter. Both pituitary DNA content and total pituitary cell number increased 30–40% above euthyroid values after 15 days. Thyroidectomy cells (TE) appeared within 4 days and progressively increased in number for 64 days. An initial decline in the normal appearing Tt was probably due to a shift to the TE form. After 64 days, Tt and TE comprised 38% of all pituitary cells, a net gain of 1.24 × 10⁶ cells/pituitary. St (1.71 × 10⁶ cells/pituitary at day 0) decreased progressively to 0.70 × 10⁶ cells/pituitary 64 days after thyroidectomy. During T₃ treatment of thyroidectomized rats, total pituitary cell number and DNA content remained relatively constant throughout the 20–24 days of T₃ injections. Both Tt and TE cells showed progressive declines, but 0.43 × 10⁶ TE cells still persisted at day 24. After 17–24 days, St number had increased from 0.82 × 10⁶ cell/pituitary to euthyroid values (1.7 × 10⁶ cells/pituitary). Graphic analysis and kinetic considerations suggested that the decrease in St during development of hypothyroidism and the decreases in Tt and TE during T₃ treatment occurred at a constant fractional rate. These data suggest that each of these cell lines may have a distinct rate of cell removal and division. St division appeared greatly augmented during T₃ treatment and Tt and TE division appeared similarly increased during development of hypothyroidism. The studies suggest that thyroidal status appears to be important in regulation of cell division in these cell lines.