Changes in somatotropin and thyrotropin secretory patterns in aging rats

In order to clarify whether pituitary enlargement influences the secretory patterns of growth hormone (GH) and thyrotropin (TSH) in old rats, we studied the correlation between pituitary weight and plasma levels of GH and TSH in Sprague-Dawley rats of different age and sex. Young female (3-4 months; YF), old female (25 months; OF), and senescent female (33-35 months; SF) rats and young male (3-4 months; YM) and old male (24-26 months; OM) rats carrying chronic intraatrial cannulas were used. Sequential blood samples were removed through the cannulas while the animals remained conscious and undisturbed. Plasma TSH and GH as well as serum thyroxine (T₄) and triiodothyronine (T₃) were measured by radioimmunoassay. At two years of age, both males and females showed a consistent decline in GH pulse amplitude without change in trough levels. By 33-35 months of age, females showed a reversal in the previous pattern of change for GH secretion: pulse amplitude, trough levels, and mean plasma GH increased significantly with respect to the old females. The correlation between mean plasma GH and anterior pituitary (AP) weight was positive and significant (p<0.01) for females but nonsignificant for males. Old and senescent rats showed significantly lower serum T₄, but not T₃, than young animals while plasma TSH increased with age in both sexes. The present results show for the first time that senescent females hypersecrete GH and suggest that the age-related alteration of TSH secretion in rats may be due to the low levels of T₄ present in the aged animals. The correlation analysis shows that pituitary enlargement is in general associated with increased secretion of both GH and TSH in senescent female rats.