Summary
In the present experiments the effects of the interruption of a prolonged T3 treatment
on somatotroph population and GH synthesis and secretion were studied. The treatment
with pharmacological doses of T3 provokes marked ultrastructural changes in somatotrophs
compatible with a stimulated synthesis of GH. These results can be correlated with
the significant increase in pituitary GH content and the normal values of serum GH.
Twenty-four hours after T3 withdrawal, somatotrophs exhibited a marked depletion of
secretory granules by exocytosis. These changes were concurrent with a significant
discharge of pituitary GH and a twofold increase in GH serum levels. The serum concentrations
attained the highest values on the second and the third day after the T3 suppression,
while the pituitary GH contents recovered the control levels.
Morphometry of somatotroph population revealed a clear proliferation of cells and
increased areas immunostained for GH, 24 h after withdrawal of the T3 treatment. The
effects of T3 on somatotrophs were persistent for several days and at least five days
were required for all parameters to return to control values. These results provide
a new insight on the residual activity of thyroid hormones on both functional activity
and morphological organization of somatotrophs.
Key words
T3 - somatotroph - morphometry - immunocytochemistry - GH secretion
