Changes in the multiple components of rat pituitary TSH and TSH beta subunit following thyroidectomy

Heterogeneity of pituitary TSH was investigated in rats following thyroidectomy. Adult male rats were sacrificed at varying periods (2-28 days) after thyroidectomy. In another experiment, thyroidectomized rats were injected daily with various doses of L-T4 (0.3-7.5 micrograms/100 g body weight, ip) and sacrificed 2 weeks later. The homogenate of the pituitaries was applied on an isoelectric focusing column or a Sephacryl S-200 column. The normal rat pituitary contained 5 major components of immunoreactive (IR) TSH in isoelectric focusing, in which the isoelectric point (pI) ranged from 6.6 to 8.8. The multiple components of IR-TSH beta were observed almost in the same areas as those of IR-TSH. Following thyroidectomy IR-TSH components with more acidic pI, associated with IR-TSH beta, were evident. A large amount of IR-TSH beta in the pituitaries of thyroidectomized rats appeared near the void volume in gel filtration, suggesting the presence of big TSH beta. Supplement of L-T4 minimized these thyroidectomy-induced changes in isoelectric focusing and gel filtration. Furthermore, big IR-TSH beta was little affected by ultracentrifugation and was relatively stable after treatment with 6 M guanidine hydrochloride. We demonstrated that the rat pituitary gland contained multiple components of IR-TSH and IR-TSH beta, both of which became variegated after thyroidectomy. It is suggested that a discernible degree of heterogeneity of TSH, particularly of TSH beta, is dependent upon the increased rate of TSH biosynthesis at the pituitary level.