Hypoglycaemia is an important complication of insulin therapy in patients with diabetes. At the onset of the disease most patients recognize the symptoms (Fig. 42d) and are able to take remedial action, but ‘hypoglycaemia awareness’ decreases with the duration of insulin treatment so that after 20 years of diabetes up to a half of patients may have lost their awareness of the symptoms. Severe hypoglycaemia, requiring the assistance of another person for treatment, is an important cause of morbidity and mortality in insulin treated diabetics. Family members, friends or school staff should be educated in recognition of the symp- toms and how to treat it. Early symptoms can be treated with oral carbohydrate; if the patient is unable to swallow intramuscular glucagon is helpful as is buccal glucose gel. Patients and their relatives can be trained to administer intramuscular glucagon. If there is evidence of impaired consciousness medical advice should be sought and the patient treated with intravenous glucose.
Biosynthesis, storage and secretion Glucagon is synthesized principally in the pancreatic α-cell, and is cleaved from a much larger precursor molecule, preproglucagon (179 amino acids in humans). The preproglucagon gene in humans is located on chromosome 2. Preproglucagon yields proglucagon (Fig. 42a). The N-terminal fragment of proglucagon is termed glicentin-related polypeptide fragment (GRPP), so-called because it contains glicentin (glucagon-like immunoreactivity-1), an intestinal glucagon sequence-containing polypeptide. GRPP and glucagon are stored together in the cell in granules, and released together in approximately equimolar quantities.
Both these peptides are also stored and released from cells in the gut, and glucagon and GRPP form part of a larger family of gut hormones (see Chapter 43). The glucagon content of a healthy human adult pancreas ranges from about 3–5 μg/g of net pancreas weight.
Chemically, glucagon is a polypeptide of molecular weight of about 3.5 kDa, consisting of 29 amino acids. The amino acid sequence of glucagon has been well conserved throughout evolution, and the whole amino acid sequence is required for full biological activity. If the N-terminal histidine is replaced, the molecule loses biological activity. Insulin, on the other hand, depends for its action more on the integrity of its three-dimensional structure, rather than an absolute dependence on the amino acid sequence. Unlike insulin, glucagon does not have a stable three-dimensional structure in physiological solutions, but may acquire this when it binds to its receptor.
Secretion of glucagon (Fig. 42b). Glucagon is rapidly secreted when plasma glucose concentrations fall, and secretion is inhibited when glucose concentrations rise. Secretion is inhibited also by other energy substrates, such as ketone bodies and fatty acids. Amino acids, particularly arginine, stimulate glucagon secretion (as they do insulin). In this situation, where both insulin and glucagon are released simultaneously, the effect may be to allow insulin to promote protein synthesis without a disturbance of normal glucose homeostasis.
Insulin inhibits glucagon secretion (Fig. 42b), perhaps through a paracrine reciprocal interaction between the pancreatic α and β cells. Glucagon secretion is affected by gut hormones (see Chapter 43), being stimulated by cholecystokinin (CCK) and vasointestinal peptide (VIP). Somatostatin, another hormone secreted by the pancreas, among many other tissues, inhibits the secretion of both glucagon and insulin.
The nervous system mediates glucagon release, which is effected by cholinergic and β-adrenergic stimulation. Electrical stimulation of the ventromedial hypothalamus in experimental animals increases glucagon release.
Once released into the circulation, glucagon circulates unbound to any plasma protein, and exists in several forms. The hormone has a short half-life of about 5 minutes, being rapidly degraded, especially in the kidney and liver. In the liver, glucagon binds to a specific membrane receptor, after which it is degraded, a degradation process apparently peculiar to glucagon.
Mechanism of action
Glucagon binds to a membrane receptor on the target cell and activates the adenylate cyclase second messenger system (Fig. 42c). It was through the study of glucagon action on gluconeogenesis that the second messenger system of cellular response was first discovered. Glucose up-regulates glucagon receptor expression while glucagon and agents that increase intracellular cAMP down-regulate glucagon receptor expression. Glucagon receptor antagonists have now been identified and may become available for controlling circulating glucose in patients.
Effects of glucagon
Glucagon has the opposite effects to those exerted by insulin. In the liver, the hormone promotes the formation of glucose from the breakdown of glycogen. Glucagon, through cAMP, blocks the enzyme cascade leading to glycogen synthesis at the level of the enzyme activities between fructose-6-phosphate and fructose-1,6-diphosphate, and between pyruvate and phosphoenolpyruvate. The glycolytic action of glucagon is essential for maintaining short-term glucose blood levels, especially in the fed state, when glycogen stores are high. In the liver, glucagon promotes the conversion of amino acids to glucose. The hormone also promotes the conversion of free fatty acids to ketone bodies.
Within the hepatocyte, glucagon is lipolytic, liberating free fatty acids and glycerol, but its actions on the hepatocyte may only be significant when insulin concentrations are low, since insulin is a potent inhibitor of hepatocyte lipolysis.
Glucagon receptor mutations
Like mutations in the insulin receptor, mutations in the glucagon receptor gene have been reported to be linked to Type 2 diabetes. A single heterozygous missense mutation in exon 2 of the glucagon receptor gene, which changes a glycine to a serine (Gly40Ser), was associated with diabetes in a population of patients with Type 2 diabetes. The mutated receptor was studied in vitro and the mutant receptor bound glucagon with an approximately three-fold lower affinity compared with the wild type receptor. Furthermore, the production of cAMP in response to glucagon was decreased in cells expressing the mutant receptor compared with those expressing the wild type.