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Adrenaline, also called epinephrine, is an endogenous catecholamine messenger: the body makes it and can release it into blood as a hormone. The adrenal medulla, the inner region of an adrenal gland, secretes adrenaline in response to sympathetic nervous system stimulation. A circulating hormone broadcasts a signal, but each receiving biological cell interprets it through its own receptors and intracellular machinery. Adrenaline activates both alpha-adrenergic receptors and beta-adrenergic receptors; it does not command one identical response everywhere.^[ref][ref]

Fuel mobilization is a useful example. A beta-adrenergic receptor can increase adenylyl cyclase activity through a stimulatory G protein, increasing the intracellular second messenger cAMP. Protein kinase A responds to cAMP; a phosphorylation relay through phosphorylase kinase activates glycogen phosphorylase, favoring glycogenolysis. Here adrenaline is the extracellular message, cAMP is the intracellular message, and glycogen is the reserve being mobilized. These are three different roles.^{[ref][ref][ref]}

The two reserves provide different material. Glycogenolysis draws on glycogen; lipolysis breaks down stored triglycerides, releasing fatty acids and glycerol. A skeletal muscle biological cell's own reserve and fuel released by adipose tissue are different supplies. Increasing a signal cannot mobilize an empty reserve: receptor activation changes the use of available material rather than creating fuel. The adrenergic adipocyte assay below gives direct human evidence for the lipolysis branch.^[ref]

Receptor context can even oppose this branch. In the human adipocyte preparation below, beta-adrenergic receptors promoted cAMP accumulation, whereas alpha-2 adrenergic receptors opposed it. Alpha-1 adrenergic receptors were associated with a different phospholipid readout. The balance of these responses matters more than the name of the messenger alone.^[ref]

This separation is useful when explaining signal transduction: ask which receptor is present, which relay it controls, which reserve or target is available, and which response was actually measured. Receptor binding occupancy, second messenger production and final fuel release are distinct observations. A stronger cAMP signal need not imply an equally large change in every downstream process; substrate availability and the rest of the receiving biological cell still matter.

The extract-transfer experiment under second messenger supplies the earlier experimental separation between hormone reception and an intracellular intermediary. Its existing account includes the preparations and controls. The complementary adrenergic adipocyte assay asks a different question: can the same adrenaline stimulus engage opposing receptors within a human preparation?

In the 1981 study, investigators incubated human adipocytes or their cell membranes from fat near the abdominal lining with epinephrine. They combined detection of bound ligands by radioactive decay with biochemical readouts: adenylyl cyclase activity, cAMP and released glycerol. Blocking alpha-2 adrenergic receptors increased the epinephrine response; blocking alpha-1 adrenergic receptors did not. A separate labeled-phosphate incorporation readout in a phospholipid showed the reverse blocker sensitivity. Thus the comparisons separated response branches.^[ref]

The glycerol readout indicates lipolysis, rather than directly measuring whole-body fuel use. Isolated adipocytes omit circulation and nervous input; blocker selectivity also limits the inference. These comparisons support receptor-specific responses in that preparation, not identical behavior in all biological cells.

Glucagon is a peptide hormone made by alpha cells in the pancreatic islets. Its mature polypeptide has 29 amino acid residues. Alpha cells store glucagon in secretory granules and release it by exocytosis. Glucagon is a message about fuel availability; glucose is the sugar whose supply it helps regulate. A falling blood glucose concentration favors glucagon release, whereas a rise usually suppresses it. Amino acids can also stimulate release, so a meal containing protein need not produce the same hormone pattern as a meal dominated by carbohydrates. ^[ref]

The principal blood glucose-raising target is the liver. In a hepatocyte, the glucagon receptor is a G protein-coupled receptor: activation of a stimulatory G protein drives adenylyl cyclase, increasing the second messenger cAMP and activating protein kinase A. Downstream phosphorylation, including the phosphorylase kinase relay to glycogen phosphorylase, favors glycogenolysis. Glucagon also supports gluconeogenesis. These are different supplies: glycogenolysis draws on a finite glycogen reserve, whereas gluconeogenesis makes glucose from other substrates and depends on their availability. Longer fasting depletes the glycogen reserve; a stronger hormone signal cannot release material that is no longer there. ^{[ref] [ref]}

Do not copy the liver pathway into every target of insulin. Skeletal muscle uses its own glycogen reserve locally; its glucose uptake and the liver's glucose output are different flows. Nor is direct glucagon-stimulated lipolysis in human adipose tissue as firmly established as its action in the liver. Glucagon action depends on the tissue, other hormones, and available substrates. ^[ref]

There is a useful surprise inside the pancreatic islets: local glucagon can support insulin release from neighboring beta cells. The islet perifusion experiment distinguishes that interaction from glucagon's action on the liver. Thus the two hormones can oppose each other's effects on blood glucose while cooperating locally. Delta cells supply another local brake through somatostatin. ^{[ref] [ref]}

Insulin is a protein hormone. Its mature form has two polypeptide chains; identifying their sequences and protein disulfide bonds provided the motivating example in Sanger's insulin sequencing experiment. See Cooper: insulin as a two-chain hormone, figure 2.16.

A meal illustrates the insulin side of blood glucose homeostasis: rising glucose stimulates beta cells in the pancreatic islets, and released insulin favors uptake and storage of fuel. Alpha cells produce glucagon, while delta cells produce somatostatin, which inhibits release of both insulin and glucagon. The pancreas therefore contains interacting sensor and secretory biological cells, rather than one undifferentiated source of hormones. ^[ref]

Insulin and glucagon are peptide hormones, built from amino acid residues joined by peptide bonds; glucose is a small carbohydrate molecule. A hormone changes signal transduction, not the identity of the fuel it regulates. Human mature insulin has 51 amino acid residues in two polypeptide chains. Beta cells process proinsulin to insulin and C-peptide, storing the products in secretory granules for exocytosis. Increased glucose metabolism raises the ATP-to-ADP ratio, tending to close ATP-sensitive potassium channels; depolarization and entry of calcium ions then trigger exocytosis. This is an important triggering pathway, with additional amplification and modulatory inputs.^[ref]

Binding to the insulin receptor, a receptor tyrosine kinase, starts phosphorylation of the receptor and insulin receptor substrates. Recruitment of phosphoinositide 3-kinase and activation of Akt transmit a major metabolic branch. In skeletal muscle and adipose tissue, that branch helps move GLUT4 to the cell membrane, increasing glucose uptake by facilitated diffusion. GLUT4 is a membrane transport protein, not an insulin receptor. Insulin also favors glycogenesis in skeletal muscle and the liver, and inhibits lipolysis in adipose tissue. In the liver, suppression of glucose output involves enzyme regulation, gene expression, and indirect changes in substrate supply. Its relative direct and indirect contributions are more complex than a single Akt arrow. ^[ref]

Insulin is not a universal permission slip for glucose entry: red blood cells and much of the nervous system use uptake mechanisms that are not acutely dependent on insulin. Nor does the liver use the same GLUT4 switch as skeletal muscle. This is why reduced insulin action can change blood glucose dramatically without stopping all glucose uptake. ^[ref]

Diabetes can disrupt production, response, and counterregulation. Type 1 diabetes usually involves destruction of beta cells and severe loss of endogenous insulin; type 2 diabetes combines insulin resistance with insufficient beta cell compensation. High circulating insulin can therefore coexist with impaired insulin action. Inappropriate glucagon release can add to excessive liver glucose output, and the response to low blood glucose can also be impaired: "more glucagon" is not a complete description of diabetes. These distinctions explain mechanisms, rather than prescribe treatment. ^{[ref] [ref]}