Beta-cells are found in the majority of the inner area of pancreatic islets. They are the most common cell in the pancreatic islet. Beta-cells produce both insulin and C-peptide, and are primarily active in the fed state. The gamma cells of the pancreatic islets secrete somatostatin.

C-peptide is produced in equal amounts as insulin, but has a longer half-life and thus is a better indicator of insulin release. Insulin is produced by beta-cells in the pancreas during a fed state. Insulin is involved in translocation of the GLUT-4 receptor.

Glucagon is released in a fasted or high-stress state (including increased concentration of blood cortisol or epinephrine). It is also induced when blood insulin levels are decreased. Recall that glucagon and insulin have antagonistic functions, and are thus secreted in opposite temporal patterns.

Tryptophan is a precursor for serotonin. Phenylalanine is a precursor for dopamine, norepinephrine, and epinephrine. Histamine acts both as a mediator of the inflammatory response and as a neurotransmitter in the central nervous system. Tyrosine is a precursor for dopamine. Serine is not a precursor for any neurotransmitter.

Glucagon receptors and beta-adrenoreceptors (for epinephrine) on cells trigger the release of cAMP, starting a phosphorylation cascade which ultimately activates glycogen phosphorylase and inhibits glycogen synthase. In liver cells, alpha-adrenoreceptors (also for epinephrine) releases calcium ions, which also begins a phosphorylation cascade ultimately leading to glycogen degradation. Glycogen is broken down into glucose which can undergo glycolysis for the production of ATP.

One must know the phosphorylation system in order to fully understand this conclusion, but logically, an increase of glucose in a cell (or insulin, which is released when blood glucose levels are high) shouldn't trigger a cell to make more glucose, as this implies there is an abundance of glucose in the cell.

Epinephrine, released by adrenal glands, is a neurotransmitter which is responsible for the "fight or flight" response, in which an organism needs energy fast. Therefore, an increase of glucose is needed for glycolysis.

Glucagon, released by the pancreas, is directly released when blood glucose levels are low, and therefore it is logical that it must signal for an increase of glucose production.

A hormone that has its receptor located on the surface of a cell will be a peptide hormone, not a steroid hormone. A steroid hormone can diffuse through the cell membrane and will find its receptor inside of the cell. The only peptide hormone listed as an answer choice is oxytocin.

Peptide hormones are hydrophilic and polar, and therefore cannot diffuse across the cell membrane to find a receptor within the cell. That answer choice actually describes how non-polar, hydrophobic steroid hormones exhibit their effects.

While it is certainly possible that a peptide hormone could have the effect of feedback inhibition on another hormone, that does not describe how most peptide hormones initially exert their effects. Likewise, a peptide hormone could eventually result in a change in plasma osmolality of the blood, but that does not describe how most exert their effects.

When insulin acts on its receptor, it has the overarching function to begin storing energy. Insulin is released when the level of glucose in the blood is high. Thus, more glucose is taken into cells, as is fat. So breakdown of glycogen and breakdown into fatty acids would not occur in the presence of insulin - these processes imply that the body is in need of energy. Moreover, the GLUT4 transporters are the main method by which glucose is taken into cells when insulin is active, so there would be increased activity of these transporters.

In this question, we're asked to determine a set of metabolic pathways that would be activated by insulin.

Firstly, let's recall what the primary function of insulin is. After consuming a meal, digestion and absorption allows for the increase in blood glucose levels and fatty acid levels. To help regulate this, insulin is secreted from the pancreas.

Insulin's function is to reduce blood sugar levels by allowing cells to absorb glucose through their plasma membranes via glucose transporters. Because cells are now taking up more glucose and thus now have a surplus of it, some of that glucose is used to produce energy via glycolysis. Additionally, excess glucose absorbed in the liver can be converted into glycogen via glycogenesis.

As for fats, insulin helps in the formation of fatty acids as opposed to their degradation. This is because after consuming a meal, a large amount of energy is available in the form of macromolecules absorbed via the breakdown of food. So rather than using stored fatty acids to supply energy, the body takes this opportunity to synthesize new fatty acids for storage.

Dipeptidyl peptidase-4 (DPP4) curbs the amount of insulin that is released in response to increased glucose. Activating DPP4 would not increase the insulin secretion. Increased ATP concentration would increase the amount of insulin released because it would activate active transporters. Inhibiting potassium channels would slow the termination of the action potential, allowing insulin to be secreted for longer. Increasing glucose in the bloodstream would activate more pancreatic cells to release insulin.