Human calcitonin gene–related peptide (hCGRP-1) and human amylin (hA) have been reported to increase hepatic glucose output in vivo and to bind with high affinity to rat liver plasma membranes, resulting in increased cAMP production. These observations have led to the hypothesis that CGRP or amylin may be physiological regulators of liver glucose metabolism. Liver plasma membranes are derived from several cell types, including parenchymal (hepatocyte), Kupffer, endothelial, lipid storage, and smooth muscle cells. Because the parenchymal cell is responsible for the contribution of the liver to whole-body glucose homeostasis, it is important to verify the location and activity of the CGRP/amylin receptor to this cell. These studies separate liver cells prepared by collagenase digestion into parenchymal and nonparenchymal fractions by metrizamide gradient and differential centrifugation. 125I-labeled [Tyr-0]hCGRP-1 bound with high affinity to nonparenchymal cell fraction and was displaced by both hCGRP-1 and hA. hCGRP-1 bound with greater affinity than hA (Kd = 2.1 ± 1.6 × 10−11 vs. 2.6 ± 1.2 × 10−8 M) in a manner similar to the binding reported for liver plasma membrane fraction. Linear regression of receptor concentration against nonparenchymal cell count per milliliter was significant (r = 0.999, P = 0.026), leading to an estimate of 3000 receptors/cell. The parenchymal cell fraction bound very little 125I-[Tyr-0]hCGRP-1, and regression of receptor concentration against parenchymal cell count per milliliter was not significant (r = −0.708, P = 0.29), suggesting that binding was not due to parenchymal cells but instead to contamination by nonparenchymal cells. 125I-labeled rat amylin (125I-rA) binding gave similar results for the distribution of receptors but higher apparent affinity for hA (Kd = 2.0 ± 0.9 × 109 M). The following studies were then performed to determine the direct effect of CGRP and amylin on liver glucose metabolism. Of hCGRP-1, hA, human deamidated amylin, and rA, none alter glycogen synthesis (basal or insulin stimulated) or net glucose production (basal or glucagon stimulated) by isolated rat liver parenchymal cells. Glucagon, but not hCGRP-1 or hA, stimulated perfused liver glucose output. The lack of effect by hCGRP-1 and hA on isolated hepatocyte glucose metabolism is therefore not merely a result of the collagenase digestion procedure. These data indicate that CGRP/amylin receptors occur only on the nonparenchymal cells of rat liver and do not directly regulate liver glucose metabolism.

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