MODE OF HORMONE ACTION THROUGH INTRACELLULAR RECEPTORS -

• Steroid hormones are lipid-soluble and easily pass through the cell membrane of a target cell into the cytoplasm.
• In the cytoplasm they bind to specific intracellular receptors (proteins) to form a hormone receptor complex that enters the nucleus.
• In the nucleus, the hormone receptor complex binds to a specific regulatory site on the chromosome and activates certain genes (DNA).
• The activated gene transcribes mRNA which directs the synthesis of proteins and usually enzymes in the cyto- plasm. The enzymes promote the metabolic reactions in the cell.
• The actions of lipid soluble hormones are slower and last longer than the action of water-soluble hormones.

Hormones as Regulators (Feed Back Control). Hormones help in maintaining internal environment of the body. When the secretion of hormones is under the control of factors or other hormones it is called feed back control. The regulation of secretion of thyroxine from the thyroid gland is an example of such feed back control mechanism.

Feed back control is of two types.

(i) Positive Feed Back Control. If the level of thyroxine is less than normal limits in the blood, thyroxine level stimulates the hypothalamus to secrete more of TRH which results in increased secretion of TSH which in turn stimulates increased secretion of thyroxine. Such regulatory effect is called positive feed back control.

(ii) Negative Feed Back Control. The thyrotropin releasing hormone (TRH) horn the hypothalamus stimulates the anterior lobe of the pituitary gland to secrete the thyroid stimulating hormone (TSH). The TSH in turn stimu- lates the thyroid gland to secrete thyroxine. A high amount of thyroxine in the blood exerts an inhibitory effect on hypothalamus in such a way that less of TRHand TSH is produced respectively. This eventually results a decrease in thyroxine. This is called negative feed back control.