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Subcutaneous injection of cortisol (10 μg/g body wt daily for 5 days) into normal neonatal rats increased the activity of stomach pepsinogen after day 5. l-Thyroxine (T4; 0·2 μg/g body wt daily for 5 days) alone did not affect the activity but it somewhat enhanced the effect of cortisol. Even before day 5, when no effect of cortisol alone was observed, T4 plus cortisol increased pepsinogen activity. In adrenalectomized, thyroidectomized neonatal rats, injection of cortisol alone did not induce enzyme activity but cortisol together with T4 did induce it. Moreover, the increase in pepsinogen activity was depressed in rats thyroidectomized on day 10 but not in those thyroidectomized on day 15. These results suggest that T4 does not have a direct effect on the stomach but plays an important role in making the stomach responsive to glucocorticoids, resulting in increased pepsinogen activity.
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The effect of l-thyroxine (T4) on amylase activity in the developing rat pancreas has been investigated. Administration of T4 (0·2 μg/g body wt) alone to intact rats on days 5–10 after birth did not induce pancreatic amylase but the enzyme was induced significantly by daily injection of cortisol (10 μg/g body wt) alone into intact rats over the same period. In thyroidectomized, adrenalectomized rats pancreatic amylase was not induced by the injection of cortisol alone but it was induced by the administration of cortisol plus T4. Increase in enzyme activity was much less in thyroidectomized animals than in intact animals. These results suggested that T4 does not have a direct effect in increasing pancreatic amylase activity but plays a permissive role in increasing enzyme activity.
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The effects of cortisol (10 μg/g body weight) and l-thyroxine (T4; 0·2 μg/g body weight) on the activity of parotid gland amylase in young rats were investigated. Administration of cortisol or T4 for 5 consecutive days from day 5 after birth caused the precocious appearance of amylase, T4 having almost twice the effect of cortisol. Cortisol and T4 did not have synergistic effects. In thyroidectomized-adrenalectomized rats, T4 increased amylase activity but cortisol did not. The increase in enzyme activity after day 20 was much less in rats thyroidectomized on day 10 than in rats adrenalectomized on day 10. These results suggest that T4 has a direct effect on the early increase of amylase activity (days 15–25) and that the action of glucocorticoid requires the presence of endogenous thyroid hormones. The hormone-induced level of amylase in intact rats was less than that of normal adult rats. Forced weaning of intact rats resulted in a further increase in amylase activity, suggesting that further amylase accumulation (after day 25) may be due to dietary factors.
