Somatostatin receptor type 2 antagonism improves glucagon and corticosterone counterregulatory responses to hypoglycemia in streptozotocin-induced diabetic rats

Abstract

Diminished responsiveness to hypoglycemia contributes to defective counterregulation in diabetes. Pancreatic and/or circulating somatostatin are elevated in diabetes, which may inhibit counterregulatory hormone release during hypoglycemia. Thus, a selective somatostatin receptor type 2 antagonist (SSTR2a) should improve hormone counterregulation to hypoglycemia. Nondiabetic (N) and streptozotocin-induced diabetic (D) rats underwent 4-h infusion of saline or SSTR2a with insulin-induced hypoglycemia clamped at 2.5 ± 0.5 mmol/L. To evaluate the effect of the SSTR2a in the absence of hypoglycemia, rats underwent a 4-h infusion of saline (Ctrl:N, Ctrl:D) or SSTR2a (Ctrl:D+SSTR2a) only. The attenuated glucagon response to hypoglycemia in D (P < 0.0002) was fully restored by SSTR2a (P < 0.0001). Furthermore, the attenuated corticosterone response in D (P < 0.002) was also enhanced by SSTR2a (P < 0.05). In the absence of hypoglycemia, SSTR2a did not alter basal blood glucose levels. D exhibited 62% more pancreatic somatostatin than N after hypoglycemia. In N rats, SSTR2a did not augment the glucagon or corticosterone response to hypoglycemia. Thus, somatostatin may contribute to impaired glucagon responsiveness to hypoglycemia in diabetes. We demonstrate that SSTR2 antagonism enhances hypoglycemia-stimulated glucagon and corticosterone release in D but not in N rats. SSTR2 antagonism does not affect basal glycemia in D rats.

FIG. 1.

Blood glucose levels during (A) hypoglycemia clamp (10 units/kg insulin, 3,000 nmol/kg/h SSTR2a), (B) hypoglycemia clamp (5 units/kg insulin, 3,000 nmol/kg/h SSTR2a), and in (C) control experiments in the absence of insulin (0 units/kg insulin, 3,000 nmol/kg/h SSTR2a). D: Hypoglycemia clamp in N rats (10 units/kg insulin, 3,000 nmol/kg/h SSTR2a). E: Hypoglycemia clamp in N rats (5 units/kg insulin, 3,000 nmol/kg/h SSTR2a). Bracketed numbers beside group names indicate doses of insulin (units/kg) and SSTR2a (nmol/kg/h). Hypoglycemia experiments using 10 units/kg insulin and 3,000 nmol/kg/h SSTR2a: N, n = 14; D, n = 14; D+SSTR2a, n = 18. Hypoglycemia experiments using 5 units/kg insulin and 3,000 nmol/kg/h SSTR2a: N, n = 5; D, n = 4; D+SSTR2a, n = 4. Control experiments using 0 units/kg insulin and 3,000 nmol/kg/h SSTR2a: N, n = 7; D, n = 7; D+SSTR2a, n = 7. Data are represented as means ± SEM.

FIG. 2.

Plasma hormone levels and glucose infusion during hypoglycemia clamp (10 units/kg insulin, 3,000 nmol/kg/h SSTR2 antagonist). A: Glucagon levels. B: Corticosterone responses. C: Glucose infusion rate and total glucose infused. D: Effect of SSTR2a on plasma glucagon levels in N rats. E: Effect of SSTR2a on plasma corticosterone levels in N rats. N, n = 14; D, n = 14; D+SSTR2a, n = 18; N+SSTR2a, n = 4. Bracketed numbers beside group names indicate doses of insulin (units/kg) and SSTR2a (nmol/kg/h). Areas under the curve from baseline (∆AUC) were calculated using Prism software (GraphPad Software, San Diego, CA). White bars, N; black bars, D; gray bars, D+SSTR2a. Data are represented as means ± SEM. ★P < 0.002 D vs. N; †P < 0.05 D vs. D+SSTR2a; ‡P < 0.05 N vs. N+SSTR2a.

FIG. 3.

Plasma hormone levels and glucose infusion during hypoglycemia clamp (5 units/kg insulin, 3,000 nmol/kg/h SSTR2a). A: Glucagon levels. B: Corticosterone responses. C: Glucose infusion rate and total glucose infused. D: Effect of SSTR2a on plasma glucagon levels in N rats. E: Effect of SSTR2a on plasma corticosterone levels in N rats. N, n = 5; D, n = 4; D+SSTR2a, n = 4; N+SSTR2a, n = 10. Bracketed numbers beside group names indicate doses of insulin (U/kg) and SSTR2a (nmol/kg/h). Areas under the curve from baseline (∆AUC) were calculated using Prism software (GraphPad Software, San Diego, CA). White bars, N; black bars, D; gray bars, D+SSTR2a. Data are represented as means ± SEM. ★P < 0.002 D vs. N; †P < 0.05 D vs. D+SSTR2a; ‡P < 0.05 N vs. N+SSTR2a.

FIG. 4.

Pancreatic protein content of glucagon (A) and somatostatin (B) and plasma somatostatin (C) after hypoglycemia clamp (10 units/kg insulin, 3,000 nmol/kg/h SSTR2a) and control experiments in the absence of insulin (0 units/kg, 3,000 nmol/kg/h). Solid bars in the left panels represent hypoglycemia experiments. Hashed bars in the right panels represent control experiments. Bracketed numbers beside group names indicate doses of insulin (U/kg) and SSTR2a (nmol/kg/h). Hypoglycemia experiments using 10 units/kg insulin and 3,000 nmol/kg/h SSTR2a: N, n = 14; D, n = 14; D+SSTR2a, n = 18. Control experiments using 0 units/kg insulin and 3,000 nmol/kg/h SSTR2a: N, n = 7; D, n = 7; D+SSTR2a, n = 7. Data are represented as means ± SEM. ★P < 0.05 D, D+SSTR2a vs. N.