[Impact of islet alpha cell loss on insulin secretion]

Abstract

Objective: To investigate the impact of islet alpha cell loss on insulin secretion.

Methods: Human pancreatic tissues underwent over-digestion of collagenase. Digestion was terminated in half of the tissue suspension, and half of the suspension was put into solution of collagenase to undergo over-digestion so as to cause loss of islet alpha cells. The tissues of digested human pancreas and over-digested human pancreas were fixed and immunohistochemical staining was made to observe the loss of islet alpha cells. The contents of insulin and glucagon in solutions of normal human pancreas, human islet of pancreas with normal enzyme secretion, and over-digested human islet were measured by radioimmunoassay. The ratios of insulin/glucagon in these three solutions were calculated so as to analyze the loss of alpha cells. Different human pancreatic tissues were inoculated in 24-well plate. Glucose solutions of different concentrations were added into the suspensions. The concentration of insulin in the supernatant was measured after incubation. C57/BL Normal human pancreas and human pancreas with islet alpha cell loss were transplanted into the renal capsules of two groups of 5 C57/BL mice with experimental diabetes mellitus respectively. The blood insulin was measured for two days. On the 3(rd) day, the left kidneys of the mice were taken to undergo insulin/glucagon immunohistochemical staining to observe the loss of alpha cells. Human islets with alpha cell loss were transplanted into the renal capsules of another 10 mice. Then 5 of them received injection of normal saline and the other 5 received glucagon for 7 days, then glucagon was injected into the mice of normal saline group, and vise versa. The blood sugar was measured every day.

Results: Glucagon immunohistochemistry showed alpha cell loss from islet periphery in over-digested human pancreas. The insulin/glucagons ratios were 100:2.34 and 100:2.05 in the normal pancreas and isolated normal islets respectively, and was 100:0.314 in the over-digested islets, showing a loss of > 80% of alpha cells. As compared with normal islets, glucose-induced insulin secretion was significantly decreased. The insulin concentration was 222.7 micro U/ml +/- 32.1 micro U/ml in the suspension of cultured normal pancreatic cells and was 124.6 micro U/ml +/- 12.6 micro U/ml in the supernatant of suspension of pancreatic islets with alpha cell loss (P < 0.01). The average blood sugar was 8.9 micro U/ml +/- 1.98 micro U/ml in mice grafted with normal islets and was 21.3 micro U/ml +/- 2.2 micro U/ml in mice grafted with alpha cell deficient islets (P < 0.01). However, insulin secretion in islets with alpha cell loss was significantly improved by exogenous glucagon in vitro and in vivo.

Conclusion: The function of insulin secretion in islets with alpha cell loss is significantly decreased. Insulin secretion of islets with alpha cell loss can be improved by exogenous glucagon.