USE OF I-125 [Y-39]EXENDIN-4 TO CHARACTERIZE EXENDIN RECEPTORS ON DISPERSED PANCREATIC ACINI AND GASTRIC CHIEF CELLS FROM GUINEA-PIG

We synthesized and iodinated an exendin-4, analogue, [Y-39]exendin-4 ( 700 Ci/mmol), for use as a radioligand to characterize exendin recepto rs on dispersed pancreatic acini and gastric chief cells from guinea p ig. Binding of this bioactive radioligand was rapid, temperature-depen dent and specific (not inhibited by other pancreatic or gastric secret agogues). Measurement of the ability of exendin-4 to inhibit the bindi ng of I-125-[Y-39]exendin-4 indicated the presence of two classes of r eceptors. Pancreatic acini had 12.5.10(10) binding sites/mg acinar pro tein of which 6% were high affinity (K-d = 0.5 nM) and 94% were low af finity (K-d = 0.1 mu M). Chief cells had 3370 binding sites/cell of wh ich 9% were high affinity (K-d = 0.3 nM) and 91% were low affinity (K- d = 0.2 mu M). Washing with 0.2 M acetic acid (pH 2.5), 0.2 M glycine (pH 10.5), or trypsin (100 mu g/ml) after 30 min incubation at 37 degr ees C, indicated that 63 and 49% of radioligand was internalized in ac ini and chief cells, respectively. Truncated glucagon-like peptide-1 ( tGLP-1), a mammalian peptide sharing 53% homology with exendin-4, inhi bited radioligand binding at the same concentrations that altered secr etion from acini and chief cells. Glucagon, GLP-1 and GLP-2 inhibited I-125-[Y-39]exendin-4 binding only at concentrations greater than or e qual to 100 nM. Exendin(9-39)NH2, a specific exendin-receptor antagoni st, potently inhibited I-125-[Y-39] exendin-4 binding (IC50 = 6.1 and 3.5 nM in acini and chief cells, respectively). In pancreatic acini an d gastric chief cells from guinea pig, exendin-3, exendin-4 and tGLP-1 increase cellular cAMP and modulate enzyme secretion by interacting w ith high-affinity exendin receptors. I-125-[Y-39] exendin-4 is a usefu l radioligand for studying exendin receptors.