Hormonal mechanisms include the following:

1. Enzyme activation or inhibition via second messengers, as noted for epinephrine and glucagon;

2. Stimulation of the synthesis of particular proteins, through activation of specific genes; and

3. Selective increases in the cellular uptake of certain metabolites. Among this last category are some receptors that serve directly as ion channels, with hormone binding causing a conformational change that opens the channel, and other receptors that stimulate uptake by still-unknown mechanisms, such as the effects of insulin upon glucose uptake.

Most hormones interacting with intracellular receptors exert their effects by controlling rates of transcription of specific genes. In this case, the hormone binds to a receptor and the complex migrates to the nucleus, where it interacts with specific DNA sites. Hormones in this class include steroids, thyroid hormones (see here), and the hormonal forms of vitamin D. In addition, retinoids, derived from retinoic acid (related to vitamin A), exert regulatory effects in embryonic development through interactions with intracellular receptors.

Mechanisms of hormones that act through membrane-bound receptors that interact with G proteins (see here) Figure 12.13 or that have catalytic domains on the cytoplasmic side, are summarized in Figure 23.7. The end result of most interactions between a hormone and a membrane receptor is activation of one or more protein kinases, whether or not a second messenger is involved. More than 100 different protein kinases have been described in vertebrate cells, all of them related, as determined by amino acid sequence homologies. More recent work is uncovering a host of specific protein phosphatases, also subject to control by cell signaling mechanisms.