Microheterogeneity of thyroid-stimulating hormone from the pituitaries of euthyroid, hypothyroid and hyperthyroid rats

Abstract

The microheterogeneity of pituitary thyroid-stimulating hormone (TSH) is dependent on variations in the hormone's carbohydrate moieties. In this study, changes in the pattern of heterogeneity have been assessed by chromatofocusing, which separates the isospecies on the basis of their isoelectric points (pI). Rats (n = 6 per group) were either untreated or rendered hypo- or hyperthyroid by including in the drinking water either propylthiouracil (0.05% for 8 weeks) or thyroxine (T4; 4 mg/l for 6 weeks) before they were killed at 16 weeks. On autopsy, serum TSH and total T4 were (means +/- S.E.M.): 2 +/- 0.3 micrograms TSH/l and 64 +/- 5 nmol T4/l (control); < 1 microgram TSH/l and 133 +/- 6 nmol T4/l (hyperthyroid); 58 +/- 6 micrograms TSH/l and 32 +/- 6 nmol T4/l (hypothyroid). The pituitaries were individually homogenized and the TSH isoforms separated by chromatofocusing over a pH range of 7-4. Fractions were assayed for TSH by radioimmunoassay. TSH from the control group was distributed into seven major peaks with pI values of (means +/- S.E.M., n = 6) 6.9 +/- 0.1, 6.6 +/- 0.1, 6.2 +/- 0.1, 5.8 +/- 0.1, 5.5 +/- 0.1, 5.2 +/- 0.1 and 4.8 +/- 0.1; 7 +/- 3% of the TSH had a pI of < 4.0. Six peaks of TSH were conserved in the hypothyroid group (with pI values of 6.8 +/- 0.1, 6.5 +/- 0.1, 6.2 +/- 0.1, 5.8 +/- 0.1, 5.4 +/- 0.1 and 5.2 +/- 0.1), and 11 +/- 4% of the hormone had a pI of < 4.0.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyrotropin-releasing hormone time-dependently influences thyrotropin microheterogeneity--an in vivo study in euthyroidism

“…In accordance with the literature, we showed a maximal TSH concentration 30 min after intravenous, 60 min after nasal and 180 min after oral TRH stimulation (Schurr et al 1985). Corresponding to previous results, IEF analysis of IRP hTSH revealed ] 25% alkaline, ] 53% neutral and ]22% acidic TSH isoforms (Pickles et al 1992, Magner 1994, Schaaf et al 1997.…”

“…In humans and experimental animals, TSH secreted during hypothyroidism was found to be more sialylated than in euthyroid individuals (Miura et al 1989, Pickles et al 1992, Trojan et al 1994. Further studies on the cellular mechanisms underlying this phenomenon have shown increased expression of -2,6-sialyltransferase, -1,4-galactosyltransferase, and the Golgi enzyme -mannosidase-II in hypothyroid mice (Pickles et al 1992, Helton & Magner 1994.…”

“…Further studies on the cellular mechanisms underlying this phenomenon have shown increased expression of -2,6-sialyltransferase, -1,4-galactosyltransferase, and the Golgi enzyme -mannosidase-II in hypothyroid mice (Pickles et al 1992, Helton & Magner 1994. As sialylation has an important effect on TSH metabolic clearance rate and bioactivity, it can be speculated that the increased percentage of the sialylated isoforms in hypothyroidism is part of a mechanism that provides an enhanced stimulation of the impaired thyroid function.…”

“…TSH isoforms separated by IEF showed significantly enhanced bioactivity for the basic isoforms and only low activity for the acidic forms. However, the attempt to establish a simple correlation between biochemical structure and biological activity of isoforms leads to controversial results (Pickles et al 1992, Szkudlinski et al 1993, Metcalfe et al 1998, Persani et al 1998. The lessprocessed basic isoforms such as desialylated, highmannose, and core-fucosylated glycosylation variants have higher in vitro bioactivity.…”