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Specific binding of [125I]glucagon-like peptide-1(7-36)amide ([125I]GLP-1(7-36)amide) to solubilized rat adipose tissue membranes was found to be dependent on temperature, time, and membrane protein concentration and readily dissociated. GLP-1(1-36)amide, GLP-2, or glucagon (10(-6) M) did not compete with [125I]GLP-1(7-36)amide binding. Half-maximal binding was achieved with 8 x 10(-10) M unlabeled GLP-1(7-36)amide, and the Scatchard plot revealed the presence of high and low affinity binding sites with Kd values of approximately 0.6 and 20 nM, respectively. The binding capacity of [125I]GLP-1(7-36)amide was about 3 times higher than that of [125I]glucagon, while the high affinity Kd and the half-maximal binding of the two peptides were similar. The presence and abundance of GLP-1(7-36)amide receptors in fat tissue together with the previous findings that the peptide stimulates glycerol and cAMP production in rat adipocytes and stimulates fatty acid synthesis in explants of rat adipose tissue open the possibility that this insulinotropic intestinal peptide may also be involved in the regulation of lipid metabolism in health and disease.

Specific receptors for glucagon and for glucagon-like peptide-1 (GLP-1) (7-36)amide have been found in solubilized human adipose membranes. The 50% inhibition dose of the corresponding unlabeled peptide was near to their physiological levels [ID50, 0.5 nmol/L for glucagon and 1.0 nmol/L for GLP-1(7-36)amide;]. In both cases, the presence of high affinity receptors was evident [Kd, 0.5 and 0.7 nmol/L for glucagon and GLP-1(7-36)amide, respectively]; the high affinity maximal binding capacity for GLP-1(7-36)amide was higher than that for glucagon (893 and 117 fmol/mg solubilized fat membranes, respectively). Glucagon at 10(-6) mol/L did not compete with the [125I]GLP-1(7-36)amide binding, nor did GLP-1(7-36)amide (10(-6) mol/L) compete with that of [125I]glucagon. The relative abundance of GLP-1(7-36)amide receptors in human adipose tissue is further support for a direct and probably important action of this peptide in the metabolism of the fat cell.

A higher specific binding of GLP-1(7-36)amide is found in skeletal muscle plasma membranes from adult streptozotocin (STZ)-treated rats (insulin-dependent diabetes mellitus model) and from neonatal STZ-treated rats (non insulin-dependent diabetes mellitus model), as compared to that in normal controls; no apparent change in the affinity was observed, that indicating the presence in both diabetic models of an increased number of high affinity binding sites for the peptide. The maximal specific GLP-1(7-16)amide binding in the non insulin-dependent diabetes mellitus model was found to be significantly higher than that in the insulin-dependent diabetes mellitus model. As GLP-1(7-36)amide exerts a glycogenic effect in the rat skeletal muscle, the present data suggest that the action of the peptide in the muscle glucose metabolism may be increased in states of insulin deficiency accompanied or not by insulin resistance.

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