Role of Hormones as Messengers and Regulators:


Ø  Secretion of endocrine glands is called hormone. Hormones were first discovered by Starling and Bayliss in 1902.

Ø  Hormone can be defined as a chemical substance produced in one part of the body and carried by blood to a target organ where it regulates certain processes.

Ø  Hormones are also known as chemical messengers (by Huxley) or autocoids because they carry information and transmit it to the target organ and regulate different biological processes of the body.

Ø  Hormones are informational molecule which is secreted inside or outside the body in response to changes in the environment.

Ø  They are released into the Extracellular fluid, where they are diffused into the blood stream. The latter carries them from the site of production to the site of action. They act on specific organs called target organs.

Ø  Hormones are low molecular weight substances which are effective in low concentration. They catalyze physiological processes in both direction i.e. they can accelerate or inhibit a physiological processes.

Ø  The blood contains all the hormones but the cells of target organ can pick up the specific required hormones only and ignore all the others.

Ø  Hormones may stimulate or inhibit specific biological processes in the target organs to modify their activities, so they act as regulators. There is coordination between nerves and hormones. Nerve regulates synthesis and release of some hormones. Thus, hormonal coordination plays an important role in regulating body functions. For example, Calcitonin secreted by thyroid gland regulates the concentration of Ca and P in the blood.


Hormones as Messengers:

On the ventral side of diencephalon hypothalamus is present; it consists of number of scattered masses of grey matter in the white matter. Masses of grey matter containing neurons form hypothalamus nuclei. The neurons of hypothalamus nuclei secrete several hormones called neuro-hormones into the blood. Blood carries these neuro-hormones to the anterior pituitary where the neuro-hormones stimulate pituitary to release various hormones. Thus, neuro-hormones act as messengers. It is of two types:

         i.            Positive feedback control

       ii.            Negative feedback control

         i.            Positive feedback control: Thyroxine hormones are secreted by thyroid gland. The thyrotropin releasing hormone (TRH) from the hypothalamus stimulates the anterior pituitary to secrete thyroid stimulating hormone (TSH). TSH stimulates the thyroid gland to secrete thyroxine. If the level of thyroxine in blood is less than normal, this low thyroxine level stimulates hypothalamus to secrete more thyroid releasing hormone (TRH). This results in increased secretion of TSH which in turn stimulates increased secretion of thyroxine.

       ii.            Negative feedback control: If the level of thyroxine in blood is more than the normal this high thyroxine level produces an inhibitory effect on hypothalamus. As a result less TRH and less TSH are produced by hypothalamus and pituitary respectively. This results in decrease in thyroxine. This is called negative feedback control.


Hormones as Regulator:

Hormones regulate metabolic activities in various tissues. They are one kind of mechanism for signalling among cells and tissues. Hormones are distinguished from communication mechanism that depends on direct cell-cell contact through gap junctions. Hormones act by binding to receptors which are usually protein molecule. Receptors have two functions; first they bind the hormone and secondly they transducer the signal to affect the metabolism of the recipient cell. The ability of a cell to respond to hormones depends on two properties of the receptor molecule; how many of them are on a particular cell and how well they bind the hormone. The first property is called receptor number and second is called the affinity of the receptor for the hormone. The biochemical responsiveness of a cell to a hormone depends on the number of occupied receptors on the responsive cell.

Suppose that a hormone binds to a receptor with a dissociation constant given by the following equation:

              Kd=[R] [H]/ [RH]

In the receptor, R is the receptor, H is the hormone and RH is the receptor hormone complex. If 50 occupied receptors trigger the appropriate metabolic response, you can achieve the responsibility having 55 receptors on a cell with 90% of them occupied. How can this be achieved? If the second set of receptors had a tenfold greater affinity for the hormone, the same concentration of hormone would result in 50 bound receptors.

Rearranging the previous equation to solve for [H], the levels of circulating hormone yields

           [H] =K [RH] / [R]

If two receptors exist, type 1 and type2, each of which is responding to a constant concentration of hormone [H], then

           [H]=K₁ [RH₁] [R₁]= K₂ [RH₂] /[R₂]


So, higher Kd means lower affinity.

If you set the number of occupied receptors [RH₁] = [RH₂] = 50, we can solve the number of unoccupied receptors of each type [R₁] =50 and [R₂]= 50. Suppose the hormone concentration increased by 50%. In this case, structure receptor system R₁ would be more responsive R₂ would be close to saturation, complete saturation of R₂ would yield only 5 more occupied receptors. This means that the concentration of the occupied receptors can change when the receptor is about half occupied. The previous equation shows that the maximum responsiveness to a change in hormone concentration is possible when the associate constant of the receptor for a hormone is near the physiological concentration of the hormone.

The compounds that bind to the receptor can modulate the action of that receptor. Agonists act to reinforce the activity of a receptor by binding to it and mimicking the action of the receptor. Drugs can be either agonists or antagonists; for example, Isoproteranol is agonists for a receptor that increases blood pressure, while propanoid- a commonly used drug to decrease blood pressure and is an antagonist for another class of recipients. Both of these compounds are structurally related to the natural hormone epinephrine.

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