Adrenergic Regulation of β-Endorphin Secretion from Anterior Pituitary in Conscious Rats: Effects of Thyroid State

In conscious, chronically cannulated, unrestrained rats, systemic administration of catecholamines increases the plasma levels of β-endorphin-like immunoreactivity (βEi). In euthyroid rats, this effect is mediated by both βi and β-adrenergic receptors; the rise in plasma βEi caused by isoproterenol is blocked by 1 mg/kg propranolol, and the similar effects of norepinephrine and phenylephrine are blocked by 0.1 mg/kg prazosin, Both types of responses are completely suppressed by a 4-h pretreatment of rats with 0.1 mg/kg dexamethasone, indicating the anterior pituitary origin of the j3Ei released. Prior sectioning of the pituitary stalk does not significantly reduce the response to either phenylephrine or isoproterenol, suggesting that both agents act directly on the pituitary. Hypothyroidism induced by surgical thyroidectomy does not influence the βEi response to isoproterenol, which remains sensitive to block by propranolol or suppression by dexamethasone. However, neither norepinephrine nor phenylephrine...