| Cell-surface receptors: polypeptide hormone receptors
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| Polypeptide hormones act through membrane receptors
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In contrast to the steroid hormones, polypeptide hormones cannot cross cell membranes and must initiate their effects on their target cells via specific cell surface receptors (see Fig. 38.1). As they do not themselves enter the target cell, they are termed 'first messengers' and their intracellular effects are mediated by low-molecular-weight signaling molecules, such as cyclic adenosine monophosphate (cAMP) or calcium, which are called 'second messengers'. In fact, the term 'polypeptide hormones' encompasses a wide range of families of hormones, growth factors, and cytokines that use transmembrane signal transduction cassettes to elicit their biological effects.
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