Amplification of pulsatile glucagon counterregulation by switch-off of alpha-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.

Fig. 1.

Minimal control network assumed to provide intrapancreatic regulation of glucagon counterregulation (GCR) in normal physiology.

Fig. 2.

Fold increase at each time point (t = −10, … , 45 min) relative to the pre-switch-off levels in the Saline (n = 7), Somatostatin (n = 6), and Insulin (n = 6) groups. Data are means ± SE averaged across the 3 groups.

Fig. 3.

Glucagon concentration from t = −10 to t = 0 min before the switch-off point (Pre) and from t = 25 to t = 45 min after the switch-off (Post) in the Saline (n = 7), Somatostatin (n = 6), and Insulin (n = 6) groups. Data are means ± SE. *Significantly different, P < 0.05.

Fig. 4.

Bottom: model-predicted glucagon rebound response to a switch-off signal (94:30–95:00) in the face of low-glucose conditions in streptozotocin (STZ)-treated rats. Top: GCR response in a representative animal from the Somatostatin switch-off group aligned in a way that the times of the switch-off in the 2 plots coincide (t = 95:00). Vertical dotted lines mark the time period studied in the experiment. BG, blood glucose.

Fig. 5.

Model-predicted restriction of the rebound response to a switch-off signal by high-glucose conditions in STZ-treated rats.

Fig. 6.

Bottom: model-predicted lack of glucagon response to glucose decline alone in STZ-treated rats. Top: GCR response in a representative animal from the Saline switch-off group aligned in a way that the times of the switch-off in the 2 plots coincide (t = 95:00). Vertical dotted lines mark the time period studied in the experiment.

Fig. 7.

Model-predicted attenuation of the switch-off effect due to disruption of glucagon-somatostatin interactions via constant infusion of glucagon (top), somatostatin (middle), and somatostatin antibody (bottom).