The widely occurring1-3 tetradecapeptide somatostatin (SRIF-14) has been variously implicated4-8 as a neurotransmitter, a neurohormone, a cybernin (local regulatory factor) and a hormone. In the first isolation of SRIF-14 from hypothalamic extracts9,10 and subsequent extracts of other tissues11-15, peptides of higher molecular weight but with similar activity have been noted. Recently two such peptides have been characterized as the 28-amino acid SRIF-28 (from porcine gastro-intestinal tract16 and porcine and ovine hypothalamus17,18) and the 25-amino acid SRIF-25 (from ovine hypothalamus17), each of which consists of an N-terminal extension of SRIF-14. We now report that SRIF-28 and SRIF-25 are more potent than SRIF-14 in the inhibition of insulin release, but that SRIF-14 preferentially inhibits glucagon release. This suggests that SRIF-28 and SRIF-25 are not mere biosynthetic precursors of SRIF-14 and that their differential release may be physiologically important.
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