Intravenous injection of the synthetic tripeptide (PyroGlu-His-Pro-NH₂: TRH) effected the prompt release of TSH and prolactin (PRL) from the pituitary of the goitrous rat. Plasma TSH and PRL levels increased 2-3-fold within 1 min after the injection of 0.4 and 2 µg TRH. Intravenous injection of 20 µg of L-thyroxine (T₄) induced repletion of TSH to supranormal levels in the adenohypophysis of goitrous rats without a significant change in PRL stores. The acute administration of TRH (2 and 50 µg) to rats after pituitary TSH rebound resulted in a simultaneous increase in circulating levels of the pituitary hormones; this was correlated with the prompt and vigorous extrusion of secretory granules from the pituitary cells. PRL content of the pituitary increased. A relationship was found in rats between the amount of TRH ingested in drinking water and plasma levels of PRL and TSH; hormonal stores in the adenohypophysis usually declined. Ingestion of large amounts of TRH (1,700 µg daily for 8 and 14 days) by the euthyroid rat resulted in a 2–3-fold elevation of the plasma TSH level. In PTU(propylthiouracil)-treated rats ingesting approximately the same amount of TRH, a plasma TSH increase failed to occur. The oral ingestion of TRH for 22–27 days by goitrous, TSH-rebounded rats resulted in a significant diminution in the circulating levels of TSH and PRL, and in ultrastructural manifestations suggestive of impaired release by the adenohypophysis. It is concluded that the acute administration of TRH causes the rapid release of TSH and PRL from the pituitary of the chronically hypothyroid rat. The intensity of the response to TRH is enhanced after pituitary TSH rebound, and synthesis of PRL appears to be augmented. Chronic oral administration of TRH to the goitrous rat results in a diminished release of the pituitary hormones, despite ample stores in the gland.

No abstract

The protein metabolism and [3H]-uridine uptake of thyroid and adenohypophysis and the kinetics of pituitary TSH rebound (PTR) were studied in goitrous female rats (fed propylthiouracil, PTU: for 7-12 weeks) following single, iv injections of L-thyroxine (T4: 0.8 to 200 mug). Goitrogenesis was associated with reduced protein concentration and enhanced uptake of [3H] uridine in both glands. Plasma levels of TSH were invariably elevated but stores in the adenohypophysis were consistently reduced. Small doses of T4 (4 mug) induced significant TSH repletion in the pituitary within 2-6 h following injection. Accumulations of pituitary TSH to supranormal levels (15-fold increases) were achieved with 20 mug T4 at 6 and 24 h; higher doses (100-200 mug) inhibited the PTR at all time intervals tested (0.5-24 h). Administration of puromycin or actinomycin D did not influence the PTR. Protein content and labeled uridine uptake of the pituitary bore no apparent relationship to T4-induced TSH repletion in the gland. Blood clearance rate of exogenous rat TSH was measured prior to and during PTR. Plasma half-life was determined to be 13.6 and 19.9 min in euthyroid and chronically hypothyroid rats, respectively; it was not significantly altered from the latter during rebound (18.7 min). Calculations of theoretical TSH secretory rates prior to (50.5 +/- 4.4 mU/h) and after rebound with 20 mug T4 (25.4 +/- 4.2 mU/H) revealed that the reaccumulation of TSH in the pituitary induced with T4 cannot be attributed solely to inhibition of release, but may also involve enhancement of synthesis. It is concluded that T4 administration at high dose levels inhibits both synthesis and release of TSH from pituitary thyrotrophs, whereas low critical doses of T4 suppress release, but augment synthesis and/or facilitate conformational change in a pituitary precursor(s) molecule which renders it detectable by bioassay.

Intensive physiologic ( 1, 2) and chemical (3, 4) researches during the last decade have recently culminated in the demonstration that the thyrotropin releasing hormone ( T R H ) of the hypothalamus in pigs, cattle, and sheep is a tripeptide, consisting of three amino acids, glu tamic aci d-hi s ti dine-proline (5-7) in that sequence. Bowers and associates (8) have shown further that Sprague Dawley rats (Charles River Breeding Laboratories, Wilmington, Mass.) were maintained in separate cages under controlled conditions of temperature (23 * 1') andartificial illumination ( 7 a.m. to 7 p.m.). Tap water and Purina Rat Chow were allowed ad libitum. On day 20 (a.m.) of gestation, each pregnant rat was injected ip with 8 pCi of I3'I to label iodothyronines in maternal and fetal thyroids. On the next day, pregnant rats were administered subcutaneously 2 50-500 pg of TRH,2 dissolved in 1 ml of saline, every 2 hr until spontaneous delivery occurred. Rats which had not delivered by 3 p.m. (after 3 injections) were subjected to cesarean section. Control pregnant rats received saline injections alone. Thyroidal radioiodine uptakes were determined in mothers and newborn 1-2 hr after the last injection of T R H and 24-28 hr after the labeling dose was administered. The thyroids were homogenized in vials containing 2 % NaOH and radioactivity was determined in a well-type scintillation counter. Blood from neonates was collected by decapitation and thoracic incision several minutes after a heparin injection (0.05 ml). Blood was obtained from mothers under light ether anesthesia by direct cardiac puncture. Radioactivity of the whole blood (0.2-ml samples) was measured immediately after collection. The thyroid and pituitary glands were removed from mothers and newborn (under 2The synthetic TRH was prepared by Dr. Karl Folkers.

The capacity of the fetal pituitary-thyroid system to respond to synthetic TRH (pyroGlu-His-ProNH₂) was tested by administration of the hypothalamic hormone on the last day of gestation to intact and hypophysectomized pregnant rats whose thyroids were labeled with ¹³¹I 24 h previously. Significant decrease in the ratios of thyroid-blood radioactivity occurred in newborn of TRH-treated mothers. Consistent reduction (30–70 %) in thyroidal ¹³¹I uptake and increase in blood radioactivity (12–60%) were observed. Bioassays (stasis tadpole) revealed significant elevation in plasma TSH levels and reduced hormone content of the pituitary in pups of mothers receiving TRH. In utero injections of the hypothalamic hormone into fetuses produced a similar pattern of TSH change within 15 min. Sufficient TSH release occurred to induce histologic activation in the thyroids of spontaneously delivered offspring. The effect of TRH on TSH release by the fetal hypophysis appeared to be specific. No indication of any substantial influence on maternal and fetal pituitary-adrenal system was observed.