Stress-Induced Secretion of Pro-Opiomelanocortin-Derived Peptides in Rats: Relative Importance of the Anterior and Intermediate Pituitary Lobes

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Abstract
Stress stimulates secretion of the pro-opiomelanocortin (POMC)-derived peptides adrenocorticotropic hormone (ACTH), β-endorphin (β-END) and α-melanocyte-stimulating hormone (α-MSH) from the anterior lobe (AL) and intermediate lobe (IL) of the pituitary gland. The secretion of POMC-derived peptides from the AL and IL is differentially regulated and the relative contribution of the lobes may vary with the stimulus. We investigated (1) the relative importance of the AL and IL as source of POMC-derived peptides released in response to restraint and ether stress by selectively inhibiting the corticotropes of the AL by dexamethasone (DEX) or selectively inhibiting the melanotropes of the IL by bromocriptine (BR), and (2) whether β-adrenergic blockade by propranolol could be used to discriminate between the stress-induced effect on POMC secretion from the AL and IL as has previously been suggested. Selective inhibition of AL secretion by DEX totally blocked the ACTH response to restraint and ether stress, but only partially inhibited the β-END response. The α-MSH response to both stressors was not affected by DEX. Conversely, selective inhibition of IL secretion by BR totally blocked the α-MSH response to both stressors, partially inhibited the β-END response but did not influence the ACTH response. In response to restraint stress, β-END was secreted equally from the AL and IL, whereas the IL was the most important source of β-END in response to ether stress. Blockade of β-adrenergic receptors with propranolol inhibited the β-END- and α-MSH responses to restraint stress whereas the ACTH response was unaffected. The secretory response of both ACTH, β-END and α-MSH to ether stress was inhibited by propranolol. We conclude, that (1) restraint- and ether-stress-induced secretion of ACTH and α-MSH are of AL and IL origin, respectively, (2) β-END secreted in response to restraint stress originates almost equally from the AL and IL, whereas the source of β-END in response to ether stress is mainly the IL, and (3) β-adrenergic blockade by propranolol cannot be used to discriminate between the AL and IL as the source of POMC peptides secreted in response to stress.

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