GLUCAGON-LIKE PEPTIDE-1 (GLP-1) RELEASES THYROTROPIN (TSH) - CHARACTERIZATION OF BINDING-SITES FOR GLP-1 ON ALPHA-TSH CELLS

We have demonstrated specific binding sites for [I-125]glucagon-like p eptide 1 (GLP-1) on membranes from the rodent thyrotrope cell line, al pha-TSH. Specific [I-125]GLP-1 binding was saturable and time dependen t. Equilibrium saturation binding analysis was consistent with the pre sence of a single class of binding site (binding capacity, 85 +/- 7 fm ol/mg protein) with a dissociation constant (K-d) of 28 +/- 13 pM. The specific GLP-1 receptor agonists, exendin-4 and exendin-3, and the an tagonist, exendin-(9-39), bound to the receptor sites with high affini ty (K-i = 190 +/- 70 pm: 130 +/- 50 and 1200 +/- 470 pM, respectively) . Chemical cross-linking of [I-125]GLP-1-receptor complexes revealed a single band of 64,300 +/- 100 M(r) in alpha-TSH membranes. In additio n, specific PCR studies demonstrated the presence of GLP-1 receptor me ssenger RNA. Binding of the peptide to alpha-TSH cell membranes result ed in increased intracellular cAMP concentrations (10 nM GLP-1, 1010 /- 83 pmol/10(6) cells . h; control, 175 +/- 60 pmol/10(6) cells . h; P < 0.002), indicating that the receptor is linked to stimulation of a denylyl cyclase. GLP-1-mediated increases in cAMP were inhibited by ex endin-(9-39) in a dose-dependent manner. Furthermore, GLP-1 stimulates basal TSH release from dispersed anterior pituitary cells in a concen tration-dependent manner(100 nM GLP-1, 63 +/- 3 fmol/10 cells . h; con trol, 35 +/- 1 fmol/10(6) cells . h; P < 0.0005), but had no effect on basal PRL, GH, or LH release.