The Metabolic Effects of Insulin and Glucagon

Even while at rest or sleeping, the body is continually using energy to drive the vital processes that keep us alive. Physical activity increases energy requirements above those in the basal (resting) state.

However, although energy use is continuous, the intake of energy in the form of food is intermittent. Thus, excess fuels taken in with a meal must be stored for subsequent use in between meals. Insulin and glucagon are the primary hormones that coordinate and regulate the storage and release of the body's fuel.

The Metabolic Effects of Insulin

Insulin is a polypeptide hormone that travels around the bloodstream. Most of the cells in the body carry receptors for the molecule in their cell membranes. Once the hormone has become bound to one of these receptors, the receptor gives a signal to the cell's interior. This signal leads to many enzyme controlled reactions which, in turn lead to changes in the metabolism of the cell.

Many of the effects of insulin depend on the particular cell type in which it stimulates. However, in nearly all of the cells that have insulin receptors in their cell membrane, the binding of insulin to the receptors leads to increased glucose uptake of the cell.

The two types of cells that are the main exceptions are the brain and the liver. However, this is only due to the fact that these cells are readily permeable to glucose, even in the absence of insulin. Liver cell membranes do contain insulin and glucagon receptors, but binding of the hormone to them affects cellular processes other than glucose permeability.

The animation below illustrates the way insulin brings about the increase in glucose uptake.

Glucose enters the cells of the body through glucose transporter (GLUT) proteins which are embedded within the cell membrane. This is a process called facilitated diffusion. When insulin binds to it's receptor, the intracellular domain of the receptor changes shape slightly. This sets off a chain of reactions. These reactions serve to activate certain enzymes. As a result, more glucose transporter proteins are released from intracellular stores and move to the plasma membrane and become embedded within it.

What do you think will be the effect of increased numbers of glucose transporter proteins within the plasma membrane?

That's not all. You may be wondering what happens to the all this glucose that is now entering the cells.

Another of the actions of insulin is to stimulate the rate at which glucose is used up in cellular respiration. This occurs due to the fact that insulin stimulates the activity of the some of the enzymes which carry out glycolysis.

The most important body tissues in terms of insulin (and glucagon) action are:

Below is a summary of the way insulin influences the physiological processes within the cells of each of these tissues. The most important ones are emphasised by the pointing fingers.

A indicates that the process stated is stimulated within the cells of that tissue by the hormone.

			Liver		Muscle		Adipose
	Stimulation of glucose uptake
	Stimulation of cellular respiration
	Stimulation of glycogenesis
	Inhibition of glycogenolysis
	Stimulation of amino acid uptake
	Stimulation of protein synthesis
	Inhibition of protein degradation
	Stimulation of fatty acid and triglyceride synthesis
	Inhibition of lipolysis
	Stimulation of lipoproteins uptake

Take a look at the metabolic map which summarises nutrient flow after a meal.

The Metabolic Effects of Glucagon

Like insulin, glucagon is a polypeptide hormone. However, in contrast to insulin, receptors for glucagon are not as abundant in cells throughout the body. The action of glucagon to increase blood glucose concentrations is largely as a result of the effects it has on cells of the liver after binding to membrane receptors.

			The Liver		Muscle		Adipose
	Stimulation of glycogenolysis
	Inhibition of glycogenesis
	Stimulation of gluconeogenesis
	Stimulation of lipolysis
	Stimulation of ketone formation
	Stimulation of amino acid uptake