 Dear students, in this topic we shall discuss the mechanisms of hormone action. The action of hormone can be divided into two phases. First, the formation of a hormone receptor complex. In the second phase, the hormone binding to the receptor produces the effect and this phenomena and this series of chemical reactions, this produce the response, which produce the response from the second phase of hormonal action. First, we shall discuss the formation of hormone receptor complex, which is the first step of a hormone action. During this, the hormone binds to a specific receptor at the target cell. This binding results in the formation of hormone receptor complex. Dear students, the receptors of hormones are large proteins. Every target cell may have about 20,000 to 100,000 receptors. Cells, who do not have receptors like this, do not bind to them. To bind to the hormone, only a few cells have receptors like this and other cells do not have receptors like this, which is the reason for the specificity of the hormones. Dear students, now we shall discuss where the hormone receptors are located in the cell. The receptors for lipid insoluble hydrophilic water soluble hormones, for example, catecholamines, protein and peptide hormones are present in the plasma membrane. These hormones being lipid insoluble cannot penetrate the cell, so they bind to the surface receptors. The steroid hormones and lipid soluble hormones can enter in the cell, so their receptors are present inside the cell. The steroid hormones have two receptors in a cell, the primary receptors are present in the cytoplasm, while they have secondary receptors inside the nucleus. The steroid hormones being lipid soluble can easily enter into the cytoplasm and bind to the receptors which lie inside the cell. So lipid insoluble hormones cannot enter in the cell, so they have to bind to the plasma membrane because they cannot enter in the cell. But steroid hormones can enter in the cell, so they can bind with the cytoplasmic receptors. The steroid hormones and lipid soluble hormones are thyroid hormones, thyroxine and triadothyronine. These hormones are found in their receptors in the nucleus and are not present in cytoplasm. Dear students, now we shall discuss the phenomena that produces response. The formation of hormone receptor complex initiates a cascade of reactions in the cell. These intracellular reactions and mechanisms vary for the hormones which bind to cytoplasmic receptors or to the plasma membrane receptors or to the nuclear receptors. Now we shall discuss the mechanism of action of lipid soluble hormones. The lipid soluble hormones which bind to the cytoplasmic or nuclear receptors make the hormone receptor complexes which translocate finally into the nucleus and there they act directly on the DNA causing changes in gene expression. Such effects are long term and may last for hours to days. The mechanism of action of lipid insoluble hormones is different from those of lipid soluble hormones. Their binding to cell membrane receptors leads to the production of second messengers. The second messengers may be one or more than one in this series. The third messengers amplify the signal and the response produced is rapid but short lived and may last for a few minutes to a few hours. This effect is produced through the activation of different effector proteins. Dear students now we shall discuss the mechanism of action of prostaglandins. Prostaglandins are lipid soluble hormones however they bind to the cell surface receptors and produce a rapid and short lived response which is similar to that of lipid insoluble hormones.