Measurement of rat plasma and pituitary TSH levels by radioimmunoassay and its application to the study of TSH secretion with special reference to the effect of thyroidectomy, cold exposure, synthetic TRH and aging

Abstract

Radioimmunoassay of rat TSH was established with purified rat TSH for iodination, standard rat TSH and anti-rat TSH supplied by NIAMDD. Iodination of rat TSH was performed with the method of Greenwood et al. This assay was successful in application to determine the plasma and pituitary TSH level in rats. In this assay the minimum detectable level was 10 ng/tube of NIAMD-Rat TSH-RP-1. Neither rat GH, prolactin, LH, nor synthetic ACTH interferred with the binding of rat TSH to anti-rat TSH. The recovery rate of TSH added to rat plasma was 80-114%. Intraassay and interassay coefficients of variation were 3.45-7.83% and 3.14-11.33% respectively. Plasma TSH level was 0.47+/-0.04 mug/ml in intact male adult rats and 0.35+/-0.03 mug/ml in pentobarbital-anesthetized ones. Plasma and pituitary TSH levels after thyroidectomy were investigated every second day during 2 weeks. The former gradually elevated until 2 weeks after operation, while the latter, 290+/-30 mug/mg gland before thyroidectomy, were reduced to 70+/-21 mug/mg gland on the 4th postoperative day, and thereafter restored to a certain degree. The changes of plasma TSH level immunoassayed in thyroidectomized rats paralleled those obtained by the bioassay within their detectable ranges. Cold exposure for 60 minutes at 0+/-1 degrees C resulted in the increasing plasma TSH from 0.35+/-0.05 mug/ml to 0.97+/-0.06 mug/ml. After the cessation of cold exposure, plasma TSH levels were restored to the resting levels within 60 minutes. Intravenous administration of synthetic TRH over the range from 0.5 ng to 500 ng resulted in a rapid plasma TSH increment with peak response between 6 and 12 minutes. Minimum effective dose of TRH on TSH release was 0.5 ng. The effect of aging was examined, revealing that plasma TSH levels were 0.35+/-0.03 mug/ml and 0.74+/-0.07 mug/ml at 7-9 weeks and 120-150 weeks after birth respectively, while pituitary TSH were 292+/-17 mug/mg gland and 66+/-13 mug/mg gland in the corresponding week with age respectively. TRH-induced TSH response in the aged rats was attenuated and delayed.