359-OR: a- or ß-Adrenergic Blockade Does Not Affect Transplanted Islet Responses to Hypoglycemia in Type 1 Diabetes (T1D)

Recipients of intrahepatic islet transplantation for T1D exhibit appropriate suppression of endogenous insulin and activation of glucagon secretion in response to insulin-induced hypoglycemia with restored glucose counterregulation and protection against clinically significant hypoglycemia. Whether sympathetic activation of adrenergic receptors on transplanted islets is required for these responses is not known. To evaluate the adrenergic contribution to post-transplant glucose counterregulation, we performed a randomized, double-blind crossover study of responses during a hyperinsulinemic (1 mU·kg^-1·min^-1) euglycemic (EU, ∼90 mg/dL)-hypoglycemic (HYPO, ∼50 mg/dL) clamp conducted under phentolamine (0.95 µg·kg^-1·min^-1; α-adrenergic blockage), propranolol (0.48 µg·kg^-1·min^-1; β-adrenergic blockage), or placebo infusion. Subjects were 5F/4M with median (range) age 53 (34 - 63) years, T1D duration 29 (18 - 56) years, post-transplant 7 (2 - 8) years, HbA_1c 5.8 (4.5 - 6.8) %, insulin in-/dependent 5/4, on tacrolimus-based immunosuppression, and spent 97 (76 - 99) % time in range 70 - 180 mg/dL and 1 (0 - 3) % time with hypoglycemia < 70 mg/dL by CGM. During the EU-HYPO clamp, blood pressure was lower with phentolamine and heart rate lower with propranolol vs. placebo (P < 0.05). There was no difference in suppression of C-peptide or activation of glucagon with phentolamine or propranolol vs. placebo. Pancreatic polypeptide was greater with phentolamine vs. placebo during EU (P < 0.05), and free fatty acids were lower and the glucose infusion rate higher with propranolol vs. placebo during HYPO (P < 0.05) with no differences seen for endogenous glucose production. These results indicate physiologic α- and β-adrenergic blockage had no effect on transplanted islet responses to hypoglycemia. Sympathetic re-innervation, as occurs with re-vascularization of intrahepatic islets, may not be necessary for post-transplant recovery of glucose counterregulation in T1D.