Somatostatin-28 does not regulate islet function in the dog.

Abstract

Somatostatin-28 (S-28) is a naturally occurring N-terminally extended form of the tetradecapeptide somatostatin (S-14). The concept has arisen that S-28 is a gut hormone that regulates insulin secretion. This concept is based on 1) reports that S-28 is a more potent inhibitor of insulin secretion than S-14; 2) the finding that S-28 is present in D-cells of the intestine and is released after a meal; and 3) the demonstration of selective binding of S-28 to B-cells of the rat islet. To critically test this hypothesis we have 1) measured the circulating levels of somatostatin-like immunoreactivity (SLI) during infusions of S-28 and S-14 to accurately compare their potencies to inhibit insulin and glucagon secretion from the in vivo dog pancreas, and 2) measured the circulating levels of endogenous SLI released after a meal and compared these to the circulating levels of infused S-28 needed to inhibit insulin secretion. Infusion of S-28 at rates of 170 and 500 pmol/min raised arterial SLI levels by 282 +/- 26 and 885 +/- 98 fmol/ml, respectively. Immunoreactive insulin (IRI) output was inhibited by 20 +/- 11% (P less than 0.05) and 52 +/- 7% (P less than 0.0005), respectively. Immunoreactive glucagon (IRG) output was not significantly altered by either dose. Pancreatic SLI output was inhibited by 32 +/- 5% (P less than 0.0005) by the 500 pmol/min infusion. Infusion of S-28 at 50 pmol/min increased arterial SLI by 108 +/- 17 fmol/ml, but did not alter IRI output (+4 +/- 20%). In comparison, infusion of S-14 (100 pmol/min) raised arterial SLI levels by a similar amount (110 +/- 21 fmol/ml), but, unlike S-28, inhibited both IRI (-50 +/- 6%, P less than 0.0005) and IRG output (-17 +/- 8%; P less than 0.05). Thus, comparable increments in arterial S-28 failed to reproduce the inhibition of IRI secretion seen during the S-14 infusion, while similar inhibition was seen with an 8-fold increment. This suggests that S-28 is significantly less potent than S-14 in the dog. After a mixed meal, endogenous SLI levels increased by 35 +/- 3 fmol/ml in arterial plasma.(ABSTRACT TRUNCATED AT 400 WORDS)