Glucagon-Like Peptide-I Analogs: Effects on Insulin Secretion and Adenosine 3′,5′-Monophosphate Formation*

Abstract

Glucagon-like peptide 1-(7-37) [GLP-I-(7-37)] is a 31-amino acid hormone which may have an important role in the regulation of insulin secretion, It is processed from preproglucagon and found in the pancreas, brain, and, in highest quantity, intestine. In previous studies we found that GLP-I-(7-37) is a potent insulin secretagogue, and its effect was indistinguishable from that of GLP-I-(7-36) amide at concentrations of 10(-11) M. Herein we report insulinotropic effects of additional GLP-I analogs. GLP-I-(7-34) had no stimulatory effect on insulin release at 10(-10) M, but had a partial effect at 10(-9) M and was as active as GLP-I-(7-37) at 10(-8) M. GLP-I-(7-33) had no effect at any concentration tested. GLP-I-(8-37) caused no significant effect on insulin release at 10(-9) and 10(-8) M, but did have an effect at the high concentration of 10(-7) M. Similar results were found with cAMP formation in the beta TC1 line. In this system GLP-I-(7-34) was less potent than GLP-I-(7-37) at a concentration of 5 x 10(-9) M. GLP-I-(7-33) had only about 0.1% the potency of GLP-I-(7-37); thus, there is good agreement between cAMP formation in the beta-cell line and insulin secretion from the perfused pancreas experiments. We conclude that histidine in the 7 position in the N-terminus of GLP-I-(7-37) is crucial for cAMP formation and insulin secretion, and that removal of the last three C-terminus residues of GLP-I-(7-37) results in only partial loss of activity; the residue in the 34 position is, however, essential for the insulinotropic action.

Insulinotropic Glucagon-like Peptide-1-mediated Activation of Non-selective Cation Currents in Insulinoma Cells Is Mimicked by Maitotoxin

“…DISCUSSION We propose that MTX activates the same non-selective cation current as stimulation with the peptide hormones GLP-1 and PACAP (6,7,22). These hormones couple through GTPbinding proteins (G-proteins) to activate adenylyl cyclase and elevate intracellular cAMP in ␤-cells (3)(4)(5), and cAMP analogs can also activate these non-selective cation currents. However, activation of G-proteins, by dialysis of cells with KF, KF ϩ AlF 3 , or GTP␥S (compounds that stimulate G-protein mediated activation of non-selective cation currents in epithelial cells (33) and activate K ϩ ATP channels in RINm5F and HIT-T15 insulinoma cells (34,35)), neither activated nor inhibited the MTX-sensitive current.…”

“…The consensus model of glucose-stimulated insulin secretion is that closure of ATP-sensitive K ϩ channels (K [3][4][5]. One mechanism underlying this increased responsiveness is the enhanced closure of K ϩ ATP channels (2).…”

“…Alternatively, I Ca-NS may have a small component that is carried through separate divalent cationselective channels that are inhibited by staurosporine. I Ca-NS is neither activated nor inhibited by dialysis with KF, KF ؉ AlF 3 or GTP␥S (guanosine 5-O-(3-thiotriphosphate)), suggesting that GTP-binding proteins do not play a major role in the activation of this current.The consensus model of glucose-stimulated insulin secretion is that closure of ATP-sensitive K ϩ channels (K [3][4][5]. One mechanism underlying this increased responsiveness is the enhanced closure of K ϩ ATP channels (2).…”

“…2), that elevate intracellular cAMP levels (3)(4)(5). One mechanism underlying this increased responsiveness is the enhanced closure of K ϩ ATP channels (2).…”

Identification of the guanine binding domain peptide of the GTP‐binding site of glucagon

“…This was confirmed with proteolytic digests using either chymotrypsin or trypsin. According to other studies, the N-terminal region may be important for biological activity through a mechanism not involving receptor binding (Gefel et al, 1990;Mommsen & Moon, 1990;Plisetskaya, 1990). Some studies have suggested that the N-terminal region of the hormone forms an aliphatic alpha-helix, which inserts into the lipid bilayer (Oomen & Kaplan, 1990).…”