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Changes in thyrotropin (TSH) carbohydrate structure and response to TSH-releasing hormone during postnatal ontogeny:
Abstract:
We have studied the carbohydrate structure of TSH as well as its response to TRH during postnatal ontogenesis in the rat using Concanavalin-A (Con A)-Sepharose chromatography of labeled glycopeptides. Pituitaries from neonatal (5-day-old) rats with low levels of endogenous TRH and mature (56-day-old) rats were incubated for 24 h in medium containing [3H] glucosamine in the presence or absence of 10(-7) M TRH. Both intracellular and secreted TSH were immunoprecipitated, treated with Pronase to generate glycopeptides, and analyzed by chromatography on Con A-Sepharose. The total amount of [3H]glucosamine-labeled TSH was greater per pituitary in mature rats compared to that in neonatal rats (P less than 0.05), while there was no significant difference between the groups in the concentration of total labeled TSH per microgram pituitary DNA. RIA determination of total TSH was greater in the older animals than in the younger animals when normalized both per pituitary and per microgram pituitary DNA (P less than 0.01 and P less than 0.02, respectively). However, for both labeled and unlabeled TSH the percentage of TSH secreted was greater in mature rats than in neonatal rats (P less than 0.02 and P less than 0.01, respectively), indicating a less active hormonal secretory process in the younger animals. In control animals, the proportion of labeled TSH glycopeptides that did not bind to Con A was greater in 56- than in 5-day-old animals for both intrapituitary and secreted forms (P less than 0.01), reflecting a shift toward more multiantennary and/or bisected biantennary complex carbohydrate structures in the older animals. In response to TRH in vitro, the total amount of labeled secreted TSH was increased more than 2-fold in both 5-day-old (P less than 0.05) and 56-day-old (P = NS) animals. However, there was a marked difference in the glycopeptide distribution between these two ages. Five-day-old animals showed a small but not significant decrease in the percentage of secreted TSH glycopeptides that bound to Con A-Sepharose, while 56-day-old animals had a specific increase in the glycopeptide fractions that bound and corresponded to biantennary complex and/or unusual hybrid forms (P less than 0.01). These studies in the rat suggest differences in TSH carbohydrate structure and secretion as well as a differential response to TRH during postnatal ontogenesis.