Amplification of pulsatile glucagon counterregulation by switch-off of α-cell-suppressing signals in streptozotocin-treated rats

Abstract

Glucagon counterregulation (GCR) is a key protection against hypoglycemia that is compromised in diabetes via an unknown mechanism. To test the hypothesis that alpha-cell-inhibiting signals that are switched off during hypoglycemia amplify GCR, we studied streptozotocin (STZ)-treated male Wistar rats and estimated the effect on GCR of intrapancreatic infusion and termination during hypoglycemia of saline, insulin, and somatostatin. Times 10 min before and 45 min after the switch-off were analyzed. Insulin and somatostatin, but not saline, switch-off significantly increased the glucagon levels (P = 0.03), and the fold increases relative to baseline were significantly higher (P < 0.05) in the insulin and somatostatin groups vs. the saline group. The peak concentrations were also higher in the insulin (368 pg/ml) and somatostatin (228 pg/ml) groups vs. the saline (114 pg/ml) group (P < 0.05). GCR was pulsatile in most animals, indicating a feedback regulation. After the switch-off, the number of secretory events and the total pulsatile production were lower in the saline group vs. the insulin and somatostatin groups (P < 0.05), indicating enhancement of glucagon pulsatile activity by insulin and somatostatin compared with saline. Network modeling analysis demonstrates that reciprocal interactions between alpha- and delta-cells can explain the amplification by interpreting the GCR as a rebound response to the switch-off. The model justifies experimental designs to further study the intrapancreatic network in relation to the switch-off phenomenon. The results of this proof-of-concept interdisciplinary study support the hypothesis that GCR develops as a rebound pulsatile response of the intrapancreatic endocrine feedback network to switch-off of alpha-cell-inhibiting islet signals.

Models of Glucagon Secretion, Their Application to the Analysis of the Defects in Glucagon Counterregulation and Potential Extension to Approximate Glucagon Action

“…8 The postulated network 5 included relationships between the α cells, δ cells, BG, and switchoff signals (intrapancreatically infused α-cell inhibitors that are terminated during hypoglycemia). This network explained the repair of GCR in diabetic rats by switch-off signals by interpreting the GCR as a rebound or disinhibition effect.…”

“…57 We have shown that a network based on the relationships mentioned earlier explains key experimental findings and have presented experimental evidence to support the proposed construct. 5,6 In particular, we have used differential equations-based modeling to show that the proposed network explains each of the following findings in diabetic rats: 6. Higher GCR response to insulin vs somatostatin switch-off and stronger glucagon suppression by somatostatin than by insulin before a switch-off.…”

“…Higher GCR response to insulin vs somatostatin switch-off and stronger glucagon suppression by somatostatin than by insulin before a switch-off. 5 Next, we have simplified the network in such a way that somatostatin is no longer explicitly involved but is incorporated implicitly. 7 This new MCN is shown in Figure 1.…”

“…In the in vivo experiments, 5 BG was first reduced to 100-110 mg/dl, followed by a constant intrapancreatic infusion of a switch-off signal (saline, insulin, or somatostatin); blood samples were collected every 5 minutes. Ten minutes after the start of the infusion of the switch-off signal, an insulin bolus (12 U/kg) was given intravenously to induce hypoglycemia.…”

“…We first developed 5,6 a model of the endocrine pancreas suitable for the study of rodent physiology. Later, a new simplified construct 7 (in which δ-cell somatostatin was not explicitly involved) was also shown to closely approximate the GCR control mechanisms.…”