Variant forms of immunoreactive thyrotropin in aged rats

Specific inhibition of pituitary prolactin production by energy restriction in C3H/SHN female mice

Pituitaries were excised from control (C; 95 kcal/week) or energy restricted (ER; 48 kcal/week) female mice of 2, 3, 7, and 18 months of age. The total RNA and relative actin mRNA amounts in the pituitary were significantly greater in C than in ER mice both at 7 and 18 months. Prolactin (PRL) mRNA, standardized with actin mRNA, was significantly less in ER mice of 7 (50%) and 18 (51%) months of age than in age-matched controls, suggestive of specific inhibition of PRL mRNA transcription. Pituitary RNA and actin mRNA increased from 7 to 18 months in C mice but not in ER mice. Similarly, mean pituitary volumes increased between 2 and 18 months in C mice but not in ER mice. PRL mRNA, standardized with actin mRNA, did not change in either C or ER mice 7-18 months of age. All examined C mice of 3, 7, and 18 months of age had estrous cycles but none of the ER mice of the same ages. After 1 month of ER, the pituitary volumes and serum insulin concentrations in 2-month-old female mice were reduced. Thus net reduction of PRL mRNA per pituitary by ER is attributable to decreases in pituitary size and specific inhibition of PRL production, both of which may be due to low estrogen and insulin levels.

Age-related decreases in the thyrotropin (TSH) responsiveness to thyrotropin-releasing-hormone (TRH) stimulation and to the inhibitory effect of triiodothyronine (T3); in vitro study on superfused rat pituitaries

The effect of age on the thyrotropic function was investigated in vitro by superfusing pituitary fragments obtained from 2-3-month- and 24-month-old male Wistar rats with medium 199 (GIBCO) and by measuring basal TSH secretion and TSH response to a 6-min pulse of TRH (10 nM): a/ in the absence and b/ in the presence of T3 (100 nM). TSH was measured by RIA in 3-min fractions with rat TSH materials from the NIADDK. The TRH-induced TSH release elicited by pituitary fragments from the old rats was decreased in comparison to that found in young animals. Addition of T3 to the superfusion medium did not alter basal TSH release but significantly decreased the TSH secretory response to TRH in the young rats. This response was not modified in the old animals. Our results suggest that aging induces not only a TSH hyporesponsiveness to TRH stimulation but also a decrease of this responsiveness to the inhibitory effect of T3 which could be related to a decreased TSH synthesis and to an age-related impairment of T3 action on the thyrotrophs.

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 less than 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.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of age on myosin and creatine kinase isoforms in left ventricles of Fischer 344 rats

Left ventricles of hearts from male Fischer 344 rats of 2, 8 and 23 months of age were analyzed to determine if aging results in significant alterations in the isoform distribution of myosin and creatine kinase protein and mRNAs. Left ventricles of maturing (2-month) rats contained almost exclusively alpha-myosin heavy chain (MHC) mRNA and protein. In adults (8 months) there was a 5-fold increase in beta-MHC (fetal isoform) and an approximate 10% decrease in alpha-MHC mRNA levels, relative to 2 months. By 23 months (senescence), beta-MHC mRNA levels had increased by 11-fold and alpha-MHC mRNA levels had decreased by about 30%. These changes corresponded to an increase in the relative proportion of beta-MHC protein, from undetectable levels at 2 months, to about 40% by 8 months and to about 60% by 23 months. Increased levels of beta-MHC and its mRNA in older rats correlated with decreased serum thyroid hormone levels. The specific activity of creatine kinase in crude homogenates decreased with age, as has been reported previously. Relative to 2-month controls, the specific activity of creatine kinase had decreased by 21% at 8 months and by 37% at 23 months. Analysis of creatine kinase activity showed no large increase in levels of the fetal (B) isoform with age, as was found for myosin. Levels of mRNAs encoding the B and M isoforms of creatine kinase were significantly reduced in senescent rats. Thus, the decreased levels of creatine kinase in aging rats is correlated with decreased levels of mRNA encoding the BCK and MCK isoforms but not an isoform shift.

Effects of aging on the pituitary-thyroid axis in the dog

The effects of advancing age on basal serum concentrations of thyroxine (T4) and thyrotropin (TSH), on T4 responses to TSH, and on TSH responses to thyrotropin-releasing hormone (TRH) were studied in beagle dogs. A total of 27 female dogs belonging to four age-groups were used: prepubertal (11.4 +/- 0.2 (SD) weeks), adult (2.1 +/- 0.3 years), middle-aged (6.5 +/- 0.2 years), and old (12.4 +/- 0.3 years). There was no significant difference between serum T4 concentrations of prepubertal (4.3 +/- 0.1 microgram/dl, mean +/- SE) and adult dogs (4.1 +/- 0.1 microgram/dl). Also, the decreases between adulthood and middle age (16.01%) and between middle age and old age (23.5%) were not significant, but serum T4 levels in the old dogs (2.6 +/- 0.2 micrograms/dl) were significantly (p less than 0.01) lower than those in the adult dogs. Serum cortisol levels showed a progressive increase with advancing age and were significantly higher in old animals (20.1 +/- 2.4 ng/ml) compared to those in the adults (13.4 +/- 1.2 ng/ml). There were no significant differences in serum TSH concentrations among the four age-groups. A single i.v. injection of TSH (0.15 IU/kg B.W.) raised serum T4 levels in the prepubertal animals by more than 45% in 1 h and by more than 100% in 2 h. T4 responses to TSH in the adult dogs were similar to those in the prepubertal dogs but were greatly delayed and subdued in the middle-aged and old animals. A single i.v. injection of TRH (5 micrograms/kg B.W.) increased serum TSH levels in the prepubertal animals by more than 120% in 15 min. These increases were less in the adult (74.8%) and middle-aged (21.3%) animals, and a significant increase (24.6%) in the old animals did not occur until 30 min after TRH treatment. This study demonstrates that, with advancing age, marked alterations occur in the regulation of the T4-TSH system in the dog.

Endocrine changes associated with the human aging process: II. Effect of age on the number and size of thyrotropin immunoreactive cells in the human pituitary

The thyrotropin immunoreactive cells in the pituitaries of 40 patients of varying ages were quantified after immunocytochemical staining. Thyrotroph hypertrophy and relative hyperplasia were both present in aged pituitaries. No consistent relation with the histologic features of the thyroid or adrenal, or with the cause of death, was demonstrable.