The Defective Glucagon Response From Transplanted Intrahepatic Pancreatic Islets During Hypoglycemia Is Transplantation Site–Determined

Abstract

The optimal site for pancreatic islet cell transplantation is presently unclear, although the liver has been the most commonly used. However, glucagon secretion from islets that have been autotransplanted in liver has been reported to be unresponsive to hypoglycemia yet responsive to arginine. To determine whether this selective glucagon secretory defect is related to the intrahepatic site of islet implantation or to the process of transplantation per se, we studied counterregulatory responses to hypoglycemia in dogs with pancreatic islet autotransplantation in the hepatic parenchyma (the intrahepatic [IH] group, n = 9) or the peritoneal cavity (the intraperitoneal [IP] group, n = 9), following total pancreatectomy, and compared them with the responses in normal controls (n = 10). Dogs were subjected to a hypoglycemic hyperinsulinemic (5 mU · kg⁻¹ min⁻¹) clamp for 90 min under general anesthesia. Arterial glucose concentrations were clamped at 2.7 mmol/l for the final 45 min of the clamp. Immediately following the clamp, glucagon responses to IV arginine (5 g) were also assessed. During hypoglycemia, glucagon responses in the IH group (maximal incremental glucagon = 33 ± 21 ng/l; glucagon area under curve [AUC] = 713 ± 1,022 ng · 1⁻¹ · min⁻¹) were significantly lower than either the IP (maximal incremental glucagon = 92 ± 32 ng/l; glucagon AUC = 4,090 ± 1,600 ng · 1⁻¹ · min⁻¹) or control (maximal incremental glucagon = 154 ± 71 ng/l; glucagon AUC = 6,943 ± 2,842 ng · 1⁻¹ · min⁻¹) group (IH vs. IP group, P < 0.05; control vs. IH group, P < 0.01). Glucagon responses in the IP group did not differ significantly from the control group. Epinephrine responses to hypoglycemia were similar in all groups, whereas neither of the transplanted groups (IH and IP) had pancreatic polypeptide responses. There was a prompt rise in plasma glucagon after intravenous arginine in all groups. These data indicate that glucagon unresponsiveness to hypoglycemia is specific to intrahepatically transplanted islets, rendering the liver a disadvantageous site for optimal α-cell function.