Hormone Effects on the Target Cell

Surface Membrane Receptors. These receptors are located on the outer surface of the plasma membrane (see Fig. 28—2).6 Surface receptors tend to recognize the peptide hormones and some amino acid derivatives (e.g., pituitary hormones, catecholamines). They are typically linked to specific intracellular enzymes. When stimulated by a peptidelike hormone, 

An example of a hormone that exerts its effects through a surface receptor-second messenger system is ACTH.36 ACTH is a polypeptide that binds to a surface receptor on adrenal cortex cells. The surface receptor then stimulates the adenylate cyclase enzyme to increase production of cAMP, which acts as a second messenger (the hormone was the first messenger), and increases the activity of other enzymes within the cell to synthesize adrenal steroids such as cortisol. For a more detailed description of surface receptor-second messenger systems, see Chapter 4. 

FIGURE 28-2 Primary cellular locations of hormone receptors. Peptide hormones tend to bind to surface membrane receptors (site 0 steroid hormones bind to cytosolic receptors [site 10 and thyroid hormones bind to receptors in the cell nucleus [site III). 

FIGURE 28-3 Sequence of events of steroid hormone action. [1] Steroid hormone enters the cell, binds to a oytosolio receptor, and creates an activated steroid-receptor complex CS-FT). C2] S-R complex travels to the cell s nucleus, where it binds to specific gene segments on nuclear chromatin. [3] DNA undergoes transcription into messenger RNA (mRNA) units. (4) mRNA undergoes translation in the cytosol into specific proteins that alter cell function. 

steroids exert their primary effects by acting on receptors located within the cell. As discussed in Chapter 4, it is apparent that these substances exert some of their effects by binding to a second set of receptors located on the cell surface.1318 Studies continue to define the exact role of steroidal surface receptors and their contribution to the effects of each type of steroid 

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