Somatostatin precursors: evidence for presence in and release from rat median eminence and neurohypophysis

Characterization of somatostatin-like immunoreactivity in rat median eminence and neurohypophysis by gel chromatography yielded two high molecular weight forms in addition to the tetradecapeptide somatostatin. The two larger molecules comprised 5% and 35% of the total tissue immunoreactivity, showed molecular weights of 25000 and 4000 dalton and were both releaseable in vitro in response to depolarizing stimuli. Their characteristic elution volumes remained unchanged after treatment with dithiothreitol or boiling in 8 M urea. Gentle trypsinization of the 25000 dalton molecule resulted in partial conversion into immunoreactive material coeluting with the tetradecapeptide somatostatin. Since the antibody employed in these studies is specific for the central and C-terminal portions of the tetradecapeptide somatostatin the present data suggest that both high molecular weight forms represent N-terminal extensions of somatostatin and that the 25000 molecular weight material might represent a prohormone for somatostatin.