Developmental and patterns of pituitary and plasma TSH in the normal and hypothyroid female rat.

The developmental patterns of thyroid-stimulating hormone (TSH) was examined in female Long-Evans rats by radioimmunoassay of pituitary and plasma from 5 through 80 days of age. The effects of hypothyroidism on normal TSH levels were examined in animals radiothyroidectomized at birth. Control animals showed a peak in circulating TSH levels at day 12 followed by lower levels through day 30, and even lower levels at day 80 when values were those of adult animals. Hypothyroid females had higher plasma TSH levels than controls from days 10 through 30 with no distinct peak at day 12. Pituitary content reached a peak at day 18 and was always higher in control than hypothyroid animals.

Developmental patterns of FSH and LH in female rats deprived of light before puberty.

Plasma FSH and LH levels were examined in female rats reared in the dark at different ages from birth until sexual maturation to investigate whether, and to what extent, external factors such as light, influence gonadotropin levels during development. Control animals were raised in diurnal lighting consisting of 12 hours of light and 12 hours of dark. Light deprivation did not eliminate the characteristic peak of gonadotropins seen in early postnatal development but significantly increased levels of FSH and slightly decreased levels of LH (except for a transient rise at day 12). Constant darkness tended to lower whole body, ovarian and pituitary weights but to increase pineal weight. Whereas the time of eye-opening was the same in control and light-deprived animals, puberty (as judged by vaginal opening and first ovulation) was delayed in animals raised in the dark. The data suggest that environmental light has a mediating action on patterns of gonadotropin release, particularly on FSH, during prepuberal development.

Pituitary hormones and growth retardation in rats raised at simulated high altitude (3800m).

Growth and the endocrine status of the pituitary and thyroid glands were studied in rats born and raised in a hypobaric chamber at a simulated high altitude of 3800 m (SHA); comparisons were drawn with similar rats at sea level. From birth until 40 days of age, SHA rats weighed significantly less than controls with the most striking growth impairment found in female SHA rats. Relative organ weights of anterior pituitary glands, ovaries and uteri from 40-day-old female SHA rats were significantly less than controls. Pituitary content of growth hormone (GH) was reduced in 40-day-old female SHA rats while the content of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) were significantly increased over sea level controls. Plasma levels of GH, LH, FSH and thyrotropin (TSH) and pituitary TSH levels did not differ from control values. However, thyroidal uptake of 131I and plasma protein-bound 131I were significantly reduced in SHA rats as compared with controls. It is suggested that (1) the continuous exposure of developing female rats to hypoxia significantly impairs pituitary function and reproductive maturation, and (2) that despite other environmental factors acting on the developing organism at high altitude, growth retardation in rats born and raised at high altitudes is primarily a consequence of hypoxia.

Effects of simulated high altitude (3800 m) on reproductive function in the pregnant rat.

Ovarian appearance, uterine contents (including numbers of fetuses, size and weight), and levels of gonadotropic hormones were studied in pregnant rats exposed to simulated altitude (3800 m). Comparisons were made at 11, 20 and 21 days of gestation between sea level controls rats bred at sea level and placed at reduced barometric pressure on confirmation of pregnancy (SHA). Although fewer fetuses were present in SHA rats on day 11 of gestation, litter size at days 20 and 21 was similar to controls. Body weight and length of SHA fetuses were significantly reduced by day 21 of gestation while placental weights remained similar to controls. The follicle-stimulating hormone (FSH) content in the pituitaries of SHA rats was significantly reduced at days 11 and 21 of pregnancy as compared with controls; luteinizing hormone (LH) content was lower at day 20 than at day 11 among SHA rats. Plasma gonadotropin levels were similar between SHA and control rats all all three stages of gestation. It is suggested that, despite lack of placental adaptation and altered endocrine functions, exposure of rats to hypoxic conditions only during gestation permits better maintenance of pregnancy than reported in rats with longer exposure to high altitude.