Adrenal medullary responses to insulin-induced hypoglycaemia in the young rat. Influence of thyroid hormones.

The adrenal medulla of normal, hypothyroid and hyperthyroid young rats was stimulated by insulin-induced hypoglycaemia. In normal rats, insulin-induced adrenal epinephrine secretion increases during the first 10 days of post-natal life. Hypothyroidism retards the development of adrenal response; hyperthyroidism facilitates the development of this response. At 14 days, when insulin-induced adrenal epinephrine depletion is the same for all groups, the recovery of adrenal catecholamines stores after depletion is linear and takes less than 48 h. Recovery rate is slightly slower for hyperthyroid rats than for either hypothyroid or control rats at 14 days. Following epinephrine depletion, adrenal tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH) activities are increased for a few days in the control rats, corresponding to a transsynaptic induction. Hypothyroidism impairs TH induction and completely suppresses DBH induction; hyperthyroidism impairs TH induction, but has no effect on DBH induction. These data show that the various processes related to CA synthesis, in the adrenal medulla of the developing rat, are controlled in different ways by the thyroid hormones.