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The growth-retarded (grt) mouse, originally isolated from a closed colony of Snell's dwarf mouse (DW/J strain), shows growth retardation that is inherited in a recessive manner. We have already reported that this strain exhibits severe primary hypothyroidism with significantly reduced plasma levels of thyroxine (T4), dramatically elevated plasma titers of thyroid-stimulating hormone (TSH) and an increase in the number of immunoreactive TSH cells in the pituitary gland. The thyroid gland of the grt mouse exhibits characteristically numerous smaller follicles with poor colloid accumulation. In order to elucidate the possible site of the defect in the grt mouse, and in particular to clarify the discrepancy between elevated plasma TSH titers and reduced T4 levels, we examined the bioactivity of TSH recovered from the plasma sample of the grt animal and the responses of the grt thyroid gland to exogenous TSH and other thyroid hormone secretagogues. Plasma samples from the grt mice invariably exhibited significant levels of TSH bioactivity following injection into normal test mice. Thus, the reduced responsiveness to TSH exhibited by the grt mice is not due to the reduced bioactivity of TSH. Administration of exogenous TSH to the grt mice failed to elevate the plasma T4 and triiodothyronine (T3) levels in vivo or to stimulate free T4 and free T3 releases from the grt thyroid gland in vitro. The thyroid gland of the grt mouse exhibited a markedly diminished response of adenylate cyclase to exogenous TSH as compared to the gland of euthyroid littermates. Production of cAMP in the grt mouse was significantly increased following stimulation of the thyroid glands with forskolin, cholera toxin, prostaglandin (PG) E1 and isoproterenol. These results strongly suggest a defect in TSH responsiveness, particularly in TSH-TSH receptor-Gs protein-adenylate cyclase signalling system including the expression and the function of TSH receptor and the TSH receptor-Gs protein coupling, in the thyroid gland of the grt mice.

Responses of the Thyroid Gland to TSH and Other Thyroid Stimulators in the Growth-Retarded (grt) Mouse