The biological activity of catfish pancreatic somatostatin

Catfish pancreatic somatostatin, which contains eight additional amino acids on the amino terminus of a tetradecapeptide with considerable homology to tetradecapeptide somatostatin (SRIF), is a naturally occurring homolog of the hypothalamic peptide. The purpose of these studies was to determine the biological activity of this somatostatin homolog. Inhibition of 125I-labelled tyr1-SRIF binding to bovine pituitary plasma membranes by catfish pancreatic somatostatin was approximately 33% that of SRIF. Pancreatic somatostatin had full biological activity measured by inhibition of growth hormone release from isolated rat pituitary cells, but 0.01-0.1% the potency of SRIF. Pancreatic somatostatin at 100 ng/ml produced a 50-60% inhibition of insulin and glucagon secretion from perfused rat pancreas, while SRIF produced comparable inhibition at 10 ng/ml. This report demonstrates that a larger molecular form and natural homolog of SRIF, isolated from fish pancreas, has the same (but reduced) biological activities in rat assay systems as somatostatin originally isolated from sheep hypothalamus.