Effects of age and ovarian function on the pituitary-thyroid system in female rats

Sexual dimorphism in autonomic changes and in the renin-angiotensin system in the hearts of mice subjected to thyroid hormone-induced cardiac hypertrophy

Based on the relevance of the renin-angiotensin system and the ongoing controversy regarding the role of the sympathetic nervous system in thyroid hormone-induced cardiac hypertrophy, the aim of the present study was to establish whether the putative difference in the degree of cardiac hypertrophy exhibited by males and females might be related to differences in the sympathetic-vagal balance and/or in the cardiac renin-angiotensin system in mice of different genders. Male and female mice (n = 117) were given 0.1 mg kg(-1) of triiodothyronine or normal saline each day for 10 days consecutively. At the end of that period, study of the heart rate variability, spectral analysis and histopathological examination were performed to assess the sympathetic-vagal balance and the diameter of cardiomyocytes. The cardiac levels of angiotensin I and II were also measured. Treatment with triiodothyronine induced a greater degree of cardiac hypertrophy in male (~73%) than in female mice (~42%). This difference was attributed to greater modulation of the sympathetic nervous system and higher levels of angiotensin I and II in male than in female mice. Our data indicate that thyroid hormone-induced cardiac hypertrophy was more intense in male mice due to the synergic effect of the sympathetic nervous system and the cardiac renin-angiotensin system.

The importance of sodium/iodide symporter (NIS) for thyroid cancer management

The thyroid gland has the ability to uptake and concentrate iodide, which is a fundamental step in thyroid hormone biosynthesis. Radioiodine has been used as a diagnostic and therapeutic tool for several years. However, the studies related to the mechanisms of iodide transport were only possible after the cloning of the gene that encodes the sodium/iodide symporter (NIS). The studies about the regulation of NIS expression and the possibility of gene therapy with the aim of transferring NIS gene to cells that normally do not express the symporter have also become possible. In the majority of hypofunctioning thyroid nodules, both benign and malignant, NIS gene expression is maintained, but NIS protein is retained in the intracellular compartment. The expression of NIS in non-thyroid tumoral cells in vivo has been possible through the transfer of NIS gene under the control of tissue-specific promoters. Apart from its therapeutic use, NIS has also been used for the localization of metastases by scintigraphy or PET-scan with 124I. In conclusion, NIS gene cloning led to an important development in the field of thyroid pathophysiology, and has also been fundamental to extend the use of radioiodine for the management of non-thyroid tumors.

Estrogen effects on thyroid iodide uptake and thyroperoxidase activity in normal and ovariectomized rats

Sex steroids interfere with the pituitary-thyroid axis function, although the reports have been controversial and no conclusive data is available. Some previous reports indicate that estradiol might also regulate thyroid function through a direct action on the thyrocytes. In this report, we examined the effects of low and high doses of estradiol administered to control and ovariectomized adult female rats and to pre-pubertal females. We demonstrate that estradiol administration to both intact adult and pre-pubertal females causes a significant increase in the relative thyroid weight. Serum T3 is significantly decreased in ovariectomized rats, and is normalized by estrogen replacement. Neither doses of estrogen produced a significant change in serum TSH and total T4 in ovariectomized, adult intact and pre-pubertal rats. The highest, supraphysiological, estradiol dose produced a significant increase in thyroid iodide uptake in ovariectomized and in pre-pubertal rats, but not in control adult females. Thyroperoxidase activity was significantly higher in intact adult rats treated with both estradiol doses and in ovariectomized rats treated with the highest estradiol dose. Since serum TSH levels were not significantly changed, we suggest a direct action of estradiol on the thyroid gland, which depends on the age and on the previous gonad status of the animal.

Effect of medroxyprogesterone acetate on thyrotropin secretion in adult and old female rats

Steroid hormones have been implicated in the modulation of TSH secretion; however, there are few and controversial data regarding the effect of progesterone (Pg) on TSH secretion. Medroxyprogesterone acetate (MPA) is a synthetic alpha-hydroxyprogesterone analog that has been extensively employed in therapeutics for its Pg-like actions, but that also has some glucocorticoid and androgen activity. Both hormones have been shown to interfere with TSH secretion. The objective of the present study was to investigate the effects of MPA or Pg administration to ovariectomized (OVX) rats on in vivo and in vitro TSH release and pituitary TSH content. The treatment of adult OVX rats with MPA (0. 25 mg/100 g body weight, sc, daily for 9 days) induced a significant (P<0.05) increase in the pituitary TSH content, which was not observed when the same treatment was used with a 10 times higher MPA dose or with Pg doses similar to those of MPA. Serum TSH was similar for all groups. MPA administered to OVX rats at the lower dose also had a stimulatory effect on the in vitro basal and TRH-induced TSH release. The in vitro basal and TRH-stimulated TSH release was not significantly affected by Pg treatment. Conversely, MPA had no effect on old OVX rats. However, in these old rats, ovariectomy alone significantly reduced (P<0.05) basal and TRH-stimulated TSH release in vitro, as well as pituitary TSH content. The results suggest that in adult, but not in old OVX rats, MPA but not Pg has a stimulatory effect on TSH stores and on the response to TRH in vitro.

Effect of testosterone propionate treatment on thyrotropin secretion of young and old rats in vitro

The aim of this study was to evaluate the influence of androgens on TSH secretion during aging in Dutch rats. Male young (2 months) and old (16-21 months) rats were castrated (Cx) or sham-operated (C) and received testosterone propionate (TP--4 mg/Kg B.W., i.m., 7 days) or vehicle. Female adult (3 months) and old (12 and 17 months) intact rats received TP or corn oil in the same dose. The rats were decapitated, trunk blood was collected and anterior pituitaries were dissected out for in vitro incubation. In Cx young male rats, only TSH pituitary content showed lower levels than in their controls. Cx TP-treated rats showed higher serum TSH and in vitro basal and TRH-induced TSH secretion, but TP only partially reversed the decrease in pituitary TSH promoted by castration. The old male rats showed lower basal in vitro TSH secretion and pituitary TSH content. In Cx old male rats, serum and basal in vitro TSH concentrations were higher than those of old controls and TP treatment further increased basal in vitro TSH secretion, as well as, stimulated TRH-induced TSH secretion. Interestingly, TP had no effect on intact young or old male rats. However, in intact old female rats, TP stimulated in vitro TSH secretion but, as observed in the intact male, TP had no effect on adult female rats. These results suggest a stimulatory role of testosterone on TSH secretion of young and old male rats. Thereafter, it seems that the testes of old rats secrete some testicular factor that inhibits TSH secretion. However, in male rats with normal testosterone levels TP treatment did not increase further TSH secretion, but in old female rats it had a stimulatory effect.

Dose-dependent effects of 17-beta-estradiol on pituitary thyrotropin content and secretion in vitro

We studied the basal and thyrotropin-releasing hormone (TRH) (50 nM) induced thyrotropin (TSH) release in isolated hemipituitaries of ovariectomized rats treated with near-physiological or high doses of 17-beta-estradiol benzoate (EB; sc, daily for 10 days) or with vehicle (untreated control rats, OVX). One group was sham-operated (normal control). The anterior pituitary glands were incubated in Krebs-Ringer bicarbonate medium, pH 7.4, at 37 degrees C in an atmosphere of 95% O2/5% CO2. Medium and pituitary TSH was measured by specific RIA (NIDDK-RP-3). Ovariectomy induced a decrease (P < 0.05) in basal TSH release (normal control = 44.1 +/- 7.2; OVX = 14.7 +/- 3.0 ng/ml) and tended to reduce TRH-stimulated TSH release (normal control = 33.0 +/- 8.1; OVX = 16.6 +/- 2.4 ng/ml). The lowest dose of EB (0.7 microgram/100 g body weight) did not reverse this alteration, but markedly increased the pituitary TSH content (0.6 +/- 0.06 microgram/hemipituitary; P < 0.05) above that of OVX (0.4 +/- 0.03 microgram/hemipituitary) and normal rats (0.46 +/- 0.03 microgram/hemipituitary). The intermediate EB dose (1.4 micrograms/100 g body weight) induced a nonsignificant tendency to a higher TSH response to TRH compared to OVX and a lower response compared to normal rats. Conversely, in the rats treated with the highest dose (14 micrograms/100 g body weight), serum 17-beta-estradiol was 17 times higher than normal, and the basal and TRH-stimulated TSH release, as well as the pituitary TSH content, was significantly (P < 0.05) reduced compared to normal rats and tended to be even lower than the values observed for the vehicle-treated OVX group, suggesting an inhibitory effect of hyperestrogenism. In conclusion, while reinforcing the concept of a positive physiological regulatory role of estradiol on the TSH response to TRH and on the pituitary stores of the hormone, the present results suggest an inhibitory effect of high levels of estrogen on these responses.

Effect of aging on cold tolerance and thyroid activity in CBA/Ca inbred mice

Relationships between cold tolerance, serum levels of thyroxine (T4), thyronine (T3), and thyrotropine (TSH), and thyroid morphometry have been investigated in male CBA/Ca inbred mice at various ages through their life span. From the data obtained it appeared that there was an age-related decrease in cold tolerance up to 18 months of age which was followed by an increase, the age effect being most apparent in relation to cold resistance and cold tolerance during the recovery period following cold exposure. The age-related changes in cold tolerance appeared to be associated with changes in the serum concentrations of T3, T4, and TSH. In contrast to the T3 serum levels which showed a decrease at 36 months, the thyroxine contents showed a perceptible decrease from the age of 12-18 months onwards. A similar pattern was observed for the TSH levels, with a peak at 21 months, followed by a decline at 30 months. A relationship with age between serum T4 level and thyroid weight was indicated together with structural changes in the thyroid gland, particularly during senescence, for example the size and number of thyroid epithelial cells had become enlarged by 30 months of age.

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)

Age-induced decreases in the messenger RNA coding for the sarcoplasmic reticulum Ca2(+)-ATPase of the rat heart

Age-associated slowing of cardiac relaxation related to the decline in the Ca2+ pump function of cardiac sarcoplasmic reticulum (SR) has been previously described. It is unclear if the decreased Ca2+ pump function results from a lower amount of Ca2(+)-ATPase protein or a decreased pumping activity of the enzyme. To determine if these alterations could be mediated by changes in the amount of the protein itself, the level of the messenger RNA (mRNA) coding for the Ca2(+)-ATPase of the SR of Fischer rat hearts (4- and 30-month-old rats) were quantitated with a Northern blotting technique. We observed that the levels of SR Ca2(+)-ATPase mRNA were 60% lower in old rats as compared with young rats, suggesting that a quantitative reduction in the levels of the corresponding protein could occur during aging to explain the delayed diastolic relaxation documented in old animals as opposed to a change in the specific activity of this enzyme. The thyroid hormone responsiveness of SR Ca2(+)-ATPase mRNA has been previously established. We have found in this study that the thyroxine levels were consistently lower in old rats; however, this difference was relatively small (4.3 +/- 0.7 and 3.1 +/- 0.8 micrograms/dl [mean +/- SD), respectively, in young and old rats). In addition, no age-induced decrease in 3,5,3'-triiodothyronine levels was observed, suggesting that the aging process itself may be responsible for the changes in SR Ca2(+)-ATPase mRNA levels.

Autoimmune thyroiditis associated with mild "subclinical" hypothyroidism in adults with Down syndrome: a comparison of patients with and without manifestations of Alzheimer disease

Serum tests of thyroid function were compared in Down syndrome (DS) patients with and without manifestations of Alzheimer disease (AD). Relative to control individuals, DS patients had, overall, lower mean total T4 (P = 0.070) and T3f (P = 0.015), higher T3U (P = 0.013) and TSH (P = 0.020), no difference in free T4, and higher thyroid antithyroglobulin (ATA) (P = 0.033) and antimicrosomal autoantibody (AMA) titres (P = 0.0097). Similar trends were apparent in DS males and females, and in DS patients off all drugs. In an analysis of case/control pairs with corrections for age and sex, DS patients with AD manifestations (n = 9) had significantly lower T3 (P = 0.029) and higher AMA (P = 0.043) than paired control individuals, whereas DS patients without AD manifestations (n = 20) had significantly lower T3 (P = 0.013) but higher ATA (P = 0.0065). T3 was significantly lower in the DS patients with AD manifestations than in the unaffected (P = 0.0013). These data suggest that autoimmune thyroiditis associated with a mild "subclinical" form of hypothyroidism is common in adult DS patients and more pronounced in patients with AD manifestations than in those without. This "subclinical" hypothyroidism may contribute to cognitive deficits in ageing DS patients.

Growth hormone and thyrotropin secretory profiles and provocative testing in aged rats

The impact of aging on both basal and induced GH and TSH secretion in male and female rats was investigated. Analysis of the individual GH secretory profiles in young (3-4 month) and old (19-20 month) rats indicated that sex-dependent patterns of GH secretion was preserved in old animals. However, we observed a reduction of individual GH peak amplitudes of 66% in old males and 53% in old females when compared to their respective young animals. Further, the GH response to an intravenous bolus of GH-releasing factor (GRF), morphine and clonidine was dramatically blunted or absent in old male and female animals. In contrast to GH, basal TSH secretion was elevated, while the TSH response to thyrotropin-releasing hormone (TRH) was not significantly affected in old animals of either sex. The present data provide evidence that reduced pituitary sensitivity to GRF may be a possible cause for reduced GH secretion in old animals. Further, the elevation in plasma TSH observed in old animals is not the result of an increased pituitary sensitivity to TRH.

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.

Influence of sex and age on T3 receptors and T3 concentration in the pituitary gland of the rat: consequences on TSH secretion

A reduced secretion of thyroid hormones with age has been documented in humans and animals with no substantial increase in TSH secretion, which may be indicative of an age-related impairment of the pituitary sensitivity to the negative control exerted by thyroid hormones. We have evaluated in rats the influence of sex and age on pituitary T3 nuclear receptors--known to be determinant in the regulation of TSH secretion--as well as on T3 concentration in the pituitary gland. As regards sex, the density of T3 receptors and the concentration of T3 in pituitary gland and plasma were greater in females than in males whereas pituitary and plasma TSH concentrations were less. As for age, the density of T3 receptors was greater in old male rats than in young ones with no changes in pituitary T3 and plasma TSH concentrations. In old female rats in contrast, there was no significant increase in T3 receptors but pituitary T3 was less and plasma TSH greater than in young female rats. In both sexes plasma thyroid hormones and pituitary TSH were reduced with age whereas TSH response to TRH was not altered. These results illustrate sex and age differences in pituitary T3 receptors and pituitary T3 concentration as well as in TSH secretion. In young animals of both sexes an inverse correlation is observed between the density of pituitary T3 receptors and plasma TSH. In contrast, in old animals the absence of this correlation is suggestive of an age-related impairment of T3 action on the thyrotrophs or of changes pertaining to other factors modulating TSH secretion.

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.

Thyroxine deiodination, cytoplasmic distribution and nuclear binding of thyroxine and triiodothyronine in liver and brain of young and aged rats

This study examines (a) the effects of aging on plasma thyroid hormone concentration and (b) in vivo binding and cytoplasmic distribution of thyroid hormones as well as the conversion of thyroxine (T4) to triiodothyronine (T3) in liver and cerebral hemispheric tissue. The results show that (a) in male Long-Evans rats aging decreases plasma T4 concentration but does not affect plasma T3 concentration and (b) the in vivo nuclear T3 binding does not change significantly. However, nuclear T3 binding derived from T4 is decreased as a consequence of reduced T4 to T3 conversion in both tissues. The nuclear T4 binding is also depressed, perhaps due to the decrease in the T4 of the protein free cytoplasmic compartment. Aging was also found to change protein free and protein bound cytoplasmic distribution of T4. That is, an increase was observed in protein bound cytoplasmic T4 and a decrease in the protein free cytoplasmic T4 of both tissues. These results indicate an overall alteration in thyroid hormone production and peripheral tissue binding and processing of thyroid hormones with a consequent suboptimal thyroid state with aging.

Plasma concentrations of thyroid hormones in dogs: influence of sampling hour, breed and age

Thyroxine (T4) and triiodothyronine (T3) plasma concentrations have been determined during 24-hr sampling periods in six mongrels (age 12-36 months), six beagles (age 35-37 months), three labradors (age 3.5 months) and three beagles (age 5 months). The mean T4 levels of the labradors were significantly lower than the values found for mongrels or older beagles (P less than 0.05), whereas T3 was higher in the 5 month old beagles compared to the mongrels (P less than 0.001), young beagles (P less than 0.05) or labradors (P less than 0.01). Circadian and ultradian rhythmicities have been evaluated by cosinor and Fourier analysis. Mongrels and older beagles did have a 12-hr rhythmicity in plasma T4 (P less than 0.05), whereas 5 month old beagles had a circadian one (P less than 0.01). A 12-hr rhythmicity was also found for T3 in the older Beagles (P less than 0.05). However, Fourier analysis indicated that the daily variation in T4 and T3 plasma levels was inadequately mathematically described by single sinusoidal rhythm and that more harmonic components are to be taken into account. The obtained data during a 24-hr period indicate that T4 and T3 concentrations in plasma may vary according to breed, age and sampling hour.

Thyroid hormone levels in an egg-laying mammal, the platypus Ornithorhynchus anatinus

Serum thyroxine (T4) and triiodothyronine (T3) levels were measured in wild-caught platypuses in southeastern Australia in summer (February) and late winter (September). There was little seasonal difference in thyroid hormone levels. Males had lower levels of both T4 and T3 than did females, but this was only statistically significant for T3. Juveniles had higher levels than adults. The levels of thyroxine ranged from 37 to 136 nmol . liter-1, and triiodothyronine from 0.3 to 1.7 nmol . liter-1. These thyroxine levels are much higher than found in marsupials or the other egg-laying mammal, the echidma, and it is suggested that they may be associated with the platypus' relatively high metabolism and growth rate.

Age-related changes in pituitary/thyroid function in chickens

Pituitary cyclic AMP-dependent protein kinase (cAMP-PK) activity ratios are not increased in response to thyrotropin-releasing hormone (TRH) in old chickens (retired White Leghorn breeders). This reduced responsiveness may be due to reduced hypothalamic function, reduced thyrotrope function, or to a reduction in TRH membrane receptors. The thyroid cAMP-PK activity ratio of old males does not respond to TRH treatment but the thyroid of old females does have an increased activity ratio ater TRH injection. Both males and females have a much higher basal cAMP-PK activity ratio than young birds. This higher basal level is thought to be due to an increased thyroid-stimulating hormone (TSH) stimulation, and the failure of old males to increase activity after TRH injections may be due to a loss of the thyroid's ability to respond to direct TRH stimulation.

Serum thyroxine, triiodothyronine, and TSH levels and TSH release after TRH in aging male and female rats

Serum levels of TSH, T3 and T4, pituitary responsiveness to TRH, and T3-resin uptake were examined in aging male and female rats. Despite normal serum TSH levels, serum T3 and T4 levels were significantly reduced in old male rats (19-26 months of age) when compared to young (120-150 days of age) and middle-aged males (12-1- months of age). In old female rats, serum TSH and T3 levels were not significantly (p greater than 0.05) different from those of young or middle-aged females. Serum T4 levels were significantly lower in old females when compared to younger rats. The T3-resin uptake of old rats was not significantly different from the value of young rats. Serum T4 levels, 3 hours after TRH induced TSH release, were significantly depressed in old male and female rats. Pituitary sensitivity to TRH was unaltered in old rats. These findings suggest that thyroidal release of T4 and T3 is depressed, and that the hypothalamic-pituitary axis is insensitive to diminished thyroid hormone feedback in old rats.

Effects of aging on luteinizing hormone release in different physiological states of the female golden hamster

Effects of aging on estrous cycles and LH release in response to luteinizing hormone releasing hormone (LHRH), castration, and estradiol benzoate were studied in the female golden hamster (Mesocricetus auratus). About 80% to 90% of female golden hamsters still cycled regularly when reaching 19-22 months of age. However, some animals showed age-induced irregularity of the estrous cycle which included an interruption or complete absence of estrous vaginal discharge. Young female hamsters (3-5 months) had significantly (p less than 0.01) higher basal LH concentration than old animals (19-22 months) in the morning of each stage of estrous cycle. LHRH elicited about 20-30 fold increase in serum LH concentrations in both young and old hamsters. No significant difference in LH release was observed between young and old hamsters in response to LHRH. In acyclic hamsters, the peak of LH release in response to LHRH was delayed. LHRH-induced LH release was greater in the morning of proestrus than during diestrus in both young and old hamsters. LH increase was significantly greater in the young than in old hamsters on the 13th and 15th day after castration. However, positive feedback stimulation of LH release by estradiol benzoate was the same in both young and old hamsters. These results indicate that in the female hamster, LH response to acute stimuli such as LHRH and estrogens is the same in the young as in the old animal and that circulating basal LH concentration may decrease or its degradation or clearance may increase during the aging process in female golden hamsters. Irregularity of estrous cycles in aging hamsters may be related to delayed responsiveness of pituitary LH to LHRH stimulation.

Changes in the control of enzyme clusters in the liver of adult and senescent rats

The activities of a number of enzymes in rat liver have been measured at different times during adulthood and senescence and expressed as a percentage of maximal activity that can be attained after hormonal stimulation. Three different profiles can be detected. Type I profile shows decreasing activities during adolescence (1--3 months of age), increasing activities during adulthood (4--12 months of age) and relatively high activities thereafter. Enzymes of this group are carbamoyl-phosphate synthase and arginase; DNA content shows the same pattern. Type II profile shows decreasing activities during adolescence and relatively low activities thereafter. Enzymes of this group are tyrosine aminotransferase, glucose-6-phosphatase, and glucokinase. Type III profile shows relatively high activities during adolescence, adulthood and senescence. Enzymes of this group are ornithine transcarbamoylase, glutamate dehydrogenase and hexokinase. Some enzymes are constant with age in females, but slowly decrease in activity with age in males; decreasing levels of androgens and possibly also thyroid hormones can explain this decrease in males. Decreasing activities of carbamoyl-phosphate synthase and arginase during adolescence can be attributed to a depressant effect of gonadal hormones. The difference between relatively high and relatively low basal activities of enzymes in adult and senescent rats corresponds with their relatively long and short half-lives, respectively. This relation implicates a similar rate of synthesis of glucocorticosteroid hormone-dependent enzymes.

Capacity of old versus young male rats to release thyrotropin (TSH), thyroxine (T4) and triiodothyronine (T3) in response to different stimuli

Serum TSH, T4 and T3 were measured by radioimmunoassay (RIA) in 20-24 month old and 6-8 month old Long-Evans male rats during basal conditions, and upon exposure to cold temperature (4 degrees C), injection of ovine TSH, thyroidectomy, and T4 or T3 treatment. Basal serum TSH values in the old and young rats were similar, but serum T4 and T3 levels in the old rats were significantly lower than in the young rats. No differences were observed in TSH release between old and young rats in response to cold temperature, but old rats whowed no rise in serum T4, TSH administration produced a significantly smaller increase in serum T4 in old rats, but similar increases in serum T3 in old and young rats. No differences were observed between old and young rats in the rise of serum TSH after thyroidectomy, or in the fall of serum TSH after T4 or T3 administration. These results are believed to indicate that the primary cause for reduced thyroid function in old rats lies in the thyroid gland itself.