Evidence for Two Different Turnover Pools of Adrenocorticotropin, α-Melanocyte-Stimulating Hormone, and Endorphin-Related Peptides Released by the Frog Pituitary Neurointermediate Lobe

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Abstract
The neurointermediate lobes of dark adapted frogs (Xenopus laevis) were incubated for 1 h in [3H]arginine or [>3H]phenylalanine and then chased for various periods, ranging from 1–10 h, in the absence or presence of an inhibitor of secretion, dopamine. The labeled peptides synthesized and secreted by the neurointermediate lobes were analyzed by acidurea polyacrylamide gels. Using this pulse-chase paradigm, two pools of peptides synthesized from the ACTH-endorphin precursor, which differed in their turnover and regulation of release were observed. About 76% of the newly synthesized, releasable, labeled peptides belonged to a fast turnover pool. The peptides in this pool included ACTH with molecular weights (K = 1000 daltons) of approximately 21K and 13K, αMSH, and β-endorphin, and they were rapidly released after synthesis. The release of these peptides was inhibited by dopamine but not by Lisoproterenol. If the lobe was inhibited from release by dopamine, peptides in this pool were degraded intracellularly within 6 h of synthesis. Long term pulse-chase experiments in the presence of dopamine revealed another pool of processed peptides in the neurointermediate lobe that has a slow turnover. About 24% of the total releasable labeled peptides belonged to this slow turnover pool. The peptides in this pool consisted of approximately 21K ACTH, approximately 13K ACTH, αMSH, β-lipotropin, and βendorphin, but unlike the fast turnover pool, β-endorphin was a minor processed product in this pool. These peptides were secreted together with the fast turnover pool within 6 h of synthesis. In contrast to the fast turnover pool, the release was inhibited not only by dopamine but also by Lisoproterenol. If the lobes were inhibited from release by dopamine, the peptides in this slow turnover pool were stored and appear to be stable for at least 10 h. These two pools of peptides, which differ in the turnover, regulation of release by L-isoproterenol, and β-endorphin content, may exist to facilitate the preferential release of certain peptides of the ACTH-endorphin family during different physiological states of the animal.

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