Counterregulatory responses for the defense of blood glucose
Glucagon secretion increases rapidly in response to hypoglycemia. Indeed many studies have shown that the glucagon response is the primary essential defense mechanism utilized by the body to restore blood glucose to normal. Patients with diabetes frequently develop defective counterregulatory responses to hypoglycemia associated with reduced or absent glucagon responses. This is an important clinical problem, as current diabetes management with intensive insulin regimens usually increases the risk and frequency of hypoglycemic events. Although the mechanisms regulating the sensing and response to hypoglycemia remain incompletely understood, glucose sensors in the brain (hypothalamus and brainstem) and portal system play important roles in this counterregulatory system.
What is the molecular identity of these glucose sensors and what goes wrong with the α cell in the setting of repeated hypoglycemia?
Studies of mice with a genetic mutation in the KATP channel have shown that that the Kir6.2 gene is essential for glucose-responsiveness in the ventromedial hypothalamus, as these mice exhibit a severe defect in the glucagon response to systemic hypoglycemia. Glucose-responsive neurons utilize several mechanisms to sense blood glucose and neuroglycopenia. Mice with genetic activation of the KATP channel exhibit lack glucose-sensitive neurons in the VMH and exhibit a severe defect in glucagon secretion in response to either systemic hypoglycemia or neuroglycopenia. See ATP-sensitive K+ channels in the hypothalamus are essential for the maintenance of glucose homeostasis. Nat Neurosci. 2001 May;4(5):507-512. Hence, the KATP channel is clearly an essential component of the CNS glucose sensor in mice.
The glucose transporter GLUT2 is also an essential component of the peripheral sensors coupling detection of hypoglycemia to appropriate glucagon secretion from the islet A cell. Glucagon secretion in response to low or high glucose is impaired in GLUT2-/- mice, and abnormal autonomic nervous system tone may contribute to this defect. See Evidence that extrapancreatic glut2-dependent glucose sensors control glucagon secretion. Diabetes. 2001 Jun;50(6):1282-9
Although considerable evidence implicates a role for glucose-sensing neurons in the counterregulatory response to hypoglycemia, glucose-sensing astrocytes appear essential for the α-cell secretory response. Surprisingly, transgenic restoration of GLUT2 in astrocytes, but not in neurons, restores an appropriate glucagon response to hypoglycemia. See Regulation of glucagon secretion by glucose transporter type 2 (glut2) and astrocyte-dependent glucose sensors. J Clin Invest. 2005 Dec;115(12):3545-53.
Similar to observations made in humans, mice exhibit a sexually dimorphic glucagon response to hypoglycemia, with levels of circulating glucagon increasing to a greater extent in female mice after administration of 2-deoxyglucose or following insulin-induced hypoglycemia. Female mice also exhibit a greater glucagon response to carbachol or clonidine. See Gender difference in the glucagon response to glucopenic stress in mice. Am J Physiol Regul Integr Comp Physiol. 2002 282(1):R281-R288
The PAC1 receptor also plays an
essential role in the murine response to hypoglycemia as PAC1R-/- mice exhibit a
markedly reduced glucagon response to
insulin-induced hypoglycemia, as described in The
neuropeptide PACAP contributes to the glucagon response to insulin-induced
hypoglycaemia in mice. Acta Physiol Scand. 2002 May;175(1):25-8.
Given the increasing tendency for tight control in Type 1 and Type 2 diabetes, understanding the pathophysiology of hypoglycemia and defective counterregulation is essential. For the importance of antecedent hypoglycemia in the human counterregulatory response, see Diabetes 2000 Jan;49(1):73-81 Effects of antecedent hypoglycemia on subsequent counterregulatory responses to exercise.
Understanding the defective counterregulatory response requires detailed knowledge of how glucose regulates the a cell. The direct effect of glucose on glucagon secretion and gene expression in the a cell is likely modest and indirect, as illustrated in Endocrinology 2000 Jan;141(1):174-80 Glucose regulates proinsulin and prosomatostatin but not proglucagon messenger ribonucleic acid levels in rat pancreatic islets.
Proopiomelanocortin (and POMC-derived peptides) is critical for an appropriate glucagon response to hypoglycemia in mice. The POMC-null mouse lacks central and peripheral MSH signaling, fails to develop the adrenal gland properly, and is devoid of corticosterone and epinephrine. POMC mutants have normal serum levels of insulin, and fasting glucose, yet a profound inability to recover from insulin-induced hypoglycemia. This defect was in the Altered glucose homeostasis in proopiomelanocortin-null mouse mutants lacking central and peripheral melanocortin. Endocrinology. 2003 Dec;144(12):5194-202.
Neural inputs have been recognized as important contributors to normal islet function, and are thought to be involved in the islet A cell response to hypoglycemia. Studies of dogs subjected to pancreatic denervation followed by exposure to mild non-insulin-induced hypoglycemia (5 mM) using a phosphorylase inhibitor demonstrated that glucagon responses are normal in this experimental setting. See Pancreatic response to mild non-insulin-induced hypoglycemia does not involve extrinsic neural input. Diabetes. 2001Nov;50 (11): 2487-96.
What factors control the counterregulatory response to hypoglycemia in human subjects? Two studies suggest that intraportal glucose and thiazolidinedione treatment modify the magnitude and threshold of the counterregulatory glucagon response to hypoglycemia. See Oral Glucose Augments the Counterregulatory Hormone Response during Insulin-Induced Hypoglycemia in Humans. J Clin Endocrinol Metab. 2001 Feb 1;86(2):645-648 and Troglitazone Amplifies Counterregulatory Responses to Hypoglycemia in Nondiabetic Subjects. J Clin Endocrinol Metab. 2001 Feb 1;86(2):521-528
Intriguingly, the symptoms and relative deficiency of counterregulatory responses evoked during insulin-induced hypoglycemia in Type 1 diabetics may be less severe in the postprandial, compared to the fasting state, as outlined in Counterregulatory hormone and symptom responses to insulin-induced hypoglycemia in the postprandial state in humans. Diabetes. 2003 Nov;52(11):2774-83
Adverse consequences of sulfonylurea therapy for the glucagon response to hypoglycemia
In a study of patients with type 2 diabetes, oral glibenclamide suppressed the glucagon response to hypoglycemia in patients receiving insulin JCEM 1999 84:3140-3145. The peripheral insulin levels were the same in the glibenclamide plus insulin versus the insulin alone groups. These findings suggest that intraislet insulin, or another action of glibenclamide, suppresses glucagon release from the a cell, inappropriately in the presence of hypoglycemia.
Although the major focus on the role of glucagon and epinephrine as counterregulatory hormones in the response to insulin-induced hypoglycemia has been primarily in subjects with Type 1 diabetes. The term Hypoglycemia-Associated Autonomic Failure (HAAF) is well recognized as an often self-perpetuating problem in patients with Type 1 diabetes that can be reversed by the avoidance of repeated hypoglycemia. As Type 2 diabetes progresses, b cell failure and insulin administration becomes increasingly common. The glucagon response to hypoglycemia may also be markedly attenuated or absent in insulin-treated patients with Type 2 diabetes, but not in subjects treated with oral hypoglycemia agents who may retain appropriate sensing of and responsivity to hypoglycemia. Hence, as patients with Type 2 diabetes become insulin-dependent, they may also be at increased risk for hypoglycemia and HAAF. See Hypoglycemia-associated autonomic failure in advanced type 2 diabetes. Diabetes. 2002 Mar;51(3):724-33.
Nevertheless, in the correct experimental setting such as a euglycemic stepped hyperinsulinemic progressively hypoglycemic clamp, generation of additional intraislet hyperinsulinemia with the sulfonylurea tolbutamide results in further attenuation of the glucagon response, but not the catecholamine response to hypoglycemia in healthy young adults. See Intraislet hyperinsulinemia prevents the glucagon response to hypoglycemia despite an intact autonomic response. Diabetes. 2002 Apr;51(4):958-65
Similarly, a series of studies using either perfusion of the intact rat pancreas in vivo with sequential insulin, or incubation of rodent or human islets with medium with or without insulin, have established a key role for the presence or absence of a cell exposure to insulin as a determinant of whether the a cell will secrete glucose in response to hypoglycemia. Infusion of low dose insulin into the superior pancreaticoduodenal artery (SPDa) of STZ-administered rats followed by cessation of insulin administration was associated with restoration of the glucagonresponse to hypoglycemia. See Regulation of alpha-Cell Function by the beta-Cell During Hypoglycemia in Wistar Rats: the "Switch-off" Hypothesis. Diabetes. 2004 Jun;53(6):1482-7.
More recent studies implicate a role for zinc, co-secreted together with insulin from β-cells, as a key modulator of glucagon secretion. Zinc free insulin did not significantly modulate alpha cell glucagon secretion in experimental rodents during hypoglycemia, whereas reducing exposure of the pancreas to zinc chloride markedly improved the glucagon response to hypoglycemia-See Zinc, not insulin, regulates the rat alpha-cell response to hypoglycemia in vivo. Diabetes. 2007 Apr;56(4):1107-12. Epub 2007 Feb 22.
A related series of experiments with perifused rat and human islets also demonstrates a key relationship between sequential diminution of insulin exposure and an improved glucagon secretory response, as outlined in Regulation of alpha-Cell Function by the beta-Cell in Isolated Human and Rat Islets Deprived of Glucose: the "Switch-off" Hypothesis. Diabetes. 2004 Jun;53(6):1488-95
Additional experimental data from studies done in the BB rat further implicates hyperinsulinemia as an important determinant of defective glucagon secretion in response to hypoglycemia. If blood glucose was lowered with phlorizin or AICAR, the glucagon response to hypoglycemia was significantly greater than if glucose was lowered with insulin. See AICAR and phlorizin reverse the hypoglycemia-specific defect in glucagon secretion in the diabetic BB rat. Am J Physiol Endocrinol Metab. 2002 Nov;283(5):E1076-83
Defective counterregulation in patients following intrahepatic islet transplantation
Transplantation of human islets into the liver has provided marked improvement in glucose control in several dozen patients. These patients were often experiencing hypoglycemia unawareness and numerous hypoglycemic episodes prior to transplantation. Although islet transplantation provides prolonged independence and reduction in hypoglycemia episodes, it is not associated with normalization of the counterregulatory response to hypoglycemia. Transplanted subjects exhibited defects in the glucagon and epinephrine responses to graded hypoglycemia. In contrast, the glucagon response to arginine remains robust in the same subjects. These findings differ from previous studies in individuals with whole pancreas transplants Pancreas transplantation restores epinephrine response and symptom recognition during hypoglycemia in patients with long-standing type I diabetes and autonomic neuropathy. Diabetes. 1997 Feb;46(2):249-57. The physiological mechanisms accounting for these observations remain unclear, as discussed in Intrahepatic islet transplantation in type 1 diabetic patients does not restore hypoglycemic hormonal counterregulation or symptom recognition after insulin independence. Diabetes. 2002 Dec;51(12):3428-34. and Glycemic Thresholds for Activation of Counterregulatory Hormone and Symptom Responses in Islet Transplant Recipients. J Clin Endocrinol Metab. 2006 Dec 27; [Epub ahead of print]