Permissive Role of Thyroxine in the Ontogeny of Jejunal Sucrase*

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Abstract
The possibility that T4 [thyroxine] triggers the appearance of jejunal sucrase in the suckling rat was studied. When pups were made hypothyroid by administration of n-propylthiouracil (PTU), the timing of the appearance of sucrase was not delayed. The rate of the subsequent accumulation of the enzyme (from day 17-day 27) was markedly depressed compared with euthyroid controls. The PTU regimen had an even greater effect on total weight of the jejunum, there being a complete cessation of growth from day 17 onwards. All reported effects of PTU were prevented by concurrent administration of T4 (0.1 .mu.g/g BW [body weight] per day) every second day from day 5 onwards. Neither short term (days 9-11) nor long term (days 5-11) periods of physiological hyperthyroidism (0.1 .mu.g T4/g BW per day) caused precocious appearance of sucrase. In contrast, when pups were given hydrocortisone (50 .mu.g/g BW) on day 9 and killed on day 11, they had sucrase activity equivalent to normal 18-20 day old pups. This response to hydrocortisone was slightly enhanced by concurrent administration of T4 (days 9-11) and markedly enhanced by longer T4 treatment (days 5-11). T4 is not the primary cue for appearance of sucrase but it does play an important permissive role by regulating the rate at which the enzyme activity rises to adult levels.