Mechanism Of Hormonal Action

A. Hormone Entry into Target Ceils

1. Peptide-Internalized Hormones

Most peptide hormones do not require entry into the cytoplasm of their target cells to initiate a biological response since the ligand-binding domain of the receptor exists on the outer surface of the target cell (see Figure 1-22). As discussed in relation to Figure 1-20, some peptide hormones (e.g., insulin and nerve growth factor), when bound to their receptor, are subjected to endocytosis and are moved into the interior of the cell.

2. Steroid Hormones

As discussed in Chapter 2, most steroid hormones have limited solubility in plasma due to their intrinsic hydrophobic character; accordingly, each steroid hormone is carried through the plasma compartment by specific plasma transport proteins (PTP). Each transport protein has a specific ligand-binding domain for

TABLE 1-7 Signal Transduction by G Proteins

Product transducer




Adenylate cyclase Phospholipase C cGMP phosphodiesterase Ca2+ channel K+ channel cAMP, Gs (aßy) IP3, DAG Gp (avßy)" 5'-GMP

" The stimulatory G proteins that activate the phospholipase, Ca2+ channel, and K+ channel are designated Gp, Gc, and Gk, respectively.

Receptor Effector


Receptor Effector

Extracellular side

First messenger

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