Effects of somatostatin receptor type 2 antagonism during insulin-induced hypoglycaemia in male rats with prediabetes.

AIMS: To examine if glucagon counterregulatory defects exist in a rat model of prediabetes (pre-T2D) and to assess if a selective somatostatin receptor 2 antagonist (SSTR2a), ZT-01, enhances the glucagon response to insulin-induced hypoglycaemia. MATERIALS AND METHODS: Hyperglycaemia was induced in 8- to 9-week-old male, Sprague-Dawley rats via 7 weeks of high-fat diet followed by a single, low-dose intraperitoneal injection of streptozotocin (30 mg/kg). After 2 weeks of basal insulin therapy (0-4 U/d insulin glargine, administered subcutaneously [SC]) to facilitate partial glycaemic recovery and a pre-T2D phenotype, n = 17 pre-T2D and n = 10 normal chow-fed control rats underwent the first of two hypoglycaemic treatment-crossover experiments, separated by a 1-week washout period. On each experimental day, SSTR2a (3 mg/kg ZT-01, SC) or vehicle was administered 1 hour prior to insulin-induced hypoglycaemia (insulin aspart, 6 U/kg, SC). RESULTS: Glucagon counterregulation was marginally reduced with the induction of pre-T2D. Treatment with SSTR2a raised peak plasma glucagon levels and glucagon area under the curve before and after insulin overdose in both and pre-T2D rats. Blood glucose concentration was elevated by 30 minutes after SSTR2a treatment in pre-T2D rats, and hypoglycaemia onset (≤3.9 mmol/L) was delayed by 15 ± 12 minutes compared with vehicle (P < 0.001), despite similar glucose nadirs in the two treatment groups (1.4 ± 0.3 mmol/L). SSTR2a treatment had no effect on blood glucose levels in the control group or on the hypoglycaemia-induced decline in plasma C-peptide levels in either group. CONCLUSIONS: Treatment with an SSTR2a increases glucagon responsiveness and delays the onset of insulin-induced hypoglycaemia in this rat model of pre-T2D where only a modest deficiency in glucagon counterregulation exists.

Evaluating the effectiveness of a novel somatostatin receptor 2 antagonist, ZT-01, for hypoglycemia prevention in a rodent model of type 2 diabetes.

Background: Elevated levels of somatostatin blunt glucagon counterregulation during hypoglycemia in type 1 diabetes (T1D) and this can be improved using somatostatin receptor 2 (SSTR2) antagonists. Hypoglycemia also occurs in late-stage type 2 diabetes (T2D), particularly when insulin therapy is initiated, but the utility of SSTR2 antagonists in ameliorating hypoglycemia in this disease state is unknown. We examined the efficacy of a single-dose of SSTR2 antagonists in a rodent model of T2D. Methods: High-fat fed (HFF), low dose streptozotocin (STZ, 35 mg/kg)-induced T2D and HFF only, nondiabetic (controls-no STZ) rats were treated with the SSTR2 antagonists ZT-01/PRL-2903 or vehicle (n = 9-11/group) 60 min before an insulin tolerance test (ITT; 2-12 U/kg insulin aspart) or an oral glucose tolerance test (OGTT; 2 g/kg glucose via oral gavage) on separate days. Results: This rodent model of T2D is characterized by higher baseline glucose and HbA1c levels relative to HFF controls. T2D rats also had lower c-peptide levels at baseline and a blunted glucagon counterregulatory response to hypoglycemia when subjected to the ITT. SSTR2 antagonists increased the glucagon response and reduced incidence of hypoglycemia, which was more pronounced with ZT-01 than PRL-2903. ZT-01 treatment in the T2D rats increased glucagon levels above the control response within 60 min of dosing, and values remained elevated during the ITT (glucagon Cmax: 156 ± 50 vs. 77 ± 46 pg/mL, p < 0.01). Hypoglycemia incidence was attenuated with ZT-01 vs. controls (63% vs. 100%) and average time to hypoglycemia onset was also delayed (103.1 ± 24.6 vs. 66.1 ± 23.6 min, p < 0.05). ZT-01 administration at the OGTT onset increased the glucagon response without exacerbating hyperglycemia (2877 ± 806 vs. 2982 ± 781), potentially due to the corresponding increase in c-peptide levels (6251 ± 5463 vs. 14008 ± 5495, p = 0.013). Conclusion: Treatment with SSTR2 antagonists increases glucagon responses in a rat model of T2D and results in less hypoglycemia exposure. Future studies are required to determine the best dosing periods for chronic SSTR2 antagonism treatment in T2D.

Pharmacologic inhibition of somatostatin receptor 2 to restore glucagon counterregulation in diabetes.

Glucose homeostasis is primarily maintained by pancreatic hormones, insulin and glucagon, with an emerging role for a third islet hormone, somatostatin, in regulating insulin and glucagon responses. Under healthy conditions, somatostatin secreted from pancreatic islet δ-cells inhibits both insulin and glucagon release through somatostatin receptor- induced cAMP-mediated downregulation and paracrine inhibition of ß- and α-cells, respectively. Since glucagon is the body's most important anti-hypoglycemic hormone, and because glucagon counterregulation to hypoglycemia is lost in diabetes, the study of somatostatin biology has led to new investigational medications now in development that may help to restore glucagon counterregulation in type 1 diabetes. This review highlights the normal regulatory role of pancreatic somatostatin signaling in healthy islet function and how the inhibition of somatostatin receptor signaling in pancreatic α-cells may restore normal glucagon counterregulation in diabetes mellitus.

Somatostatin Receptor Antagonism Reverses Glucagon Counterregulatory Failure in Recurrently Hypoglycemic Male Rats.

Recent antecedent hypoglycemia is a known source of defective glucose counter-regulation in diabetes; the mechanisms perpetuating the cycle of progressive α-cell failure and recurrent hypoglycemia remain unknown. Somatostatin has been shown to suppress the glucagon response to acute hypoglycemia in rodent models of type 1 diabetes. We hypothesized that somatostatin receptor 2 antagonism (SSTR2a) would restore glucagon counterregulation and delay the onset of insulin-induced hypoglycemia in recurrently hypoglycemic, nondiabetic male rats. Healthy, male, Sprague-Dawley rats (n = 39) received bolus injections of insulin (10 U/kg, 8 U/kg, 5 U/kg) on 3 consecutive days to induce hypoglycemia. On day 4, animals were then treated with SSTR2a (10 mg/kg; n = 17) or vehicle (n = 12) 1 hour prior to the induction of hypoglycemia using insulin (5 U/kg). Plasma glucagon level during hypoglycemia was ~30% lower on day 3 (150 ± 75 pg/mL; P < .01), and 68% lower on day 4 in the vehicle group (70 ± 52 pg/mL; P < .001) compared with day 1 (219 ± 99 pg/mL). On day 4, SSTR2a prolonged euglycemia by 25 ± 5 minutes (P < .05) and restored the plasma glucagon response to hypoglycemia. Hepatic glycogen content of SSTR2a-treated rats was 35% lower than vehicle controls after hypoglycemia induction on day 4 (vehicle: 20 ± 7.0 vs SSTR2a: 13 ± 4.4 µmol/g; P < .01). SSTR2a treatment reverses the cumulative glucagon deficit resulting from 3 days of antecedent hypoglycemia in healthy rats. This reversal is associated with decreased hepatic glycogen content and delayed time to hypoglycemic onset. We conclude that recurrent hypoglycemia produces glucagon counterregulatory deficiency in healthy male rats, which can be improved by SSTR2a.