Inhibition of thyrotropin response to TSH-releasing hormone by thyroxine in hypothyroid rats

Conference · Wed May 01 04:00:00 EDT 1985 · J. Nucl. Med.; (United States)

OSTI ID:7028136

Boado, R J; Zaninovich, A A; Ulloa, E R; Fernandez Pol, J A

Pharmacological amounts of throxine (T4) can inhibit the thyrotropin (TSH) response to TSH-releasing hormone (TRH) before its conversion to triiodothyronine (T3) in the hypophysis of euthyroid rate. The present work tested physiological doses of T4 in hypothyroid rats. Rats were treated with iopanoic acid (IOP) 5 mg/100 g BW 24, 12 and 1.5 hours preceding the study, to prevent intrapituitary conversion of T4 to T3. Nonradioactive T4 was injected iv at time 0. At 20 min a 1 ..mu..g/100 g BW dose of TRH was injected iv. Blood samples were drawn at times 0, 20, and 30 min for determination by radioimmunoassay of plasma T4, T3, and TSH. In untreated rats basal TSH was 1450 +- 200 (SEM) ..mu..U/ml. At 20 min it was 105 +- 12% the basal value and at 30 min (10 min post-TRH) plasma TSH rose to 165 +- 14%. In T4-treated rats, those injected with IOP or with the vehicle alone both had the TSH response suppressed. IOP reduced intrapitutiary T3 from 4.6 +- 2.4 to 0.5 +- 0.2 fmol/min/gland. Thirty min. following the iv injection of 150 ..mu..Ci of double-labeled /sup 125/I-T4, the in vitro cytoplasmic radioactivity in control rats was 1.3 +- 0.13 x 10-/sup 2/% of the injected dose (75% T4, 17% T3), while in nuclei it was 4.2 +- 3.6 x 10-/sup 3/% (5l% T4, 28% T3). The injection of 25 ..mu..g of nonradioactive T4 decreased /sup 125/I-T4 in cytoplasm with no changes in nuclei. These findings suggest an intrinsic capacity of T4 to control TRH stimulation of TSH through binding to cytoplasmic receptors.

• Research Organization: Nuclear Medicine Center, Univ. of Buenos Aires
• OSTI ID: 7028136
• Report Number(s): CONF-850611-
• Conference Information: Journal Name: J. Nucl. Med.; (United States) Journal Volume: 26:5
• Country of Publication: United States
• Language: English

Related Subjects

550501 -- Metabolism-- Tracer Techniques
551001* -- Physiological Systems-- Tracer Techniques
59 BASIC BIOLOGICAL SCIENCES
AMINO ACIDS
ANIMALS
BETA DECAY RADIOISOTOPES
BIOCHEMICAL REACTION KINETICS
BIOCHEMISTRY
BODY
BRAIN
CARBOXYLIC ACIDS
CENTRAL NERVOUS SYSTEM
CHEMISTRY
DAYS LIVING RADIOISOTOPES
DISEASES
DOSES
DRUGS
ELECTRON CAPTURE RADIOISOTOPES
ENDOCRINE DISEASES
ENDOCRINE GLANDS
FUNCTIONS
GLANDS
HORMONES
HYPOTHALAMUS
HYPOTHYROIDISM
IMMUNOASSAY
IMMUNOLOGY
INHIBITION
INTERMEDIATE MASS NUCLEI
IODINE 125
IODINE ISOTOPES
ISOTOPE APPLICATIONS
ISOTOPES
KINETICS
LABELLED COMPOUNDS
LABELLING
MAMMALS
MEDICINE
MEMBRANE PROTEINS
NERVOUS SYSTEM
NUCLEAR MEDICINE
NUCLEI
ODD-EVEN NUCLEI
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC HALOGEN COMPOUNDS
ORGANIC IODINE COMPOUNDS
ORGANS
PEPTIDE HORMONES
PHYSIOLOGY
PITUITARY GLAND
PITUITARY HORMONES
PROTEINS
RADIATION DOSES
RADIOASSAY
RADIOIMMUNOASSAY
RADIOIMMUNOLOGY
RADIOIMMUNOTHERAPY
RADIOISOTOPES
RADIOLOGY
RADIOPHARMACEUTICALS
RADIOTHERAPY
RATS
REACTION KINETICS
RECEPTORS
RESPONSE FUNCTIONS
RODENTS
SURGERY
THERAPY
THYROID
THYROID HORMONES
THYROIDECTOMY
THYROXINE
TRACER TECHNIQUES
TRH
TSH
VERTEBRATES