Peptide hormones and catecholamine effects are very important as they influence ion channels, enzyme activity, and the cytoplasmic JAK kinases activity. The receptors for peptide hormones and the catecholamine hormones are located on the outer surface of the target cell's plasma membrane. This location is important, since these hormones are too large and hydrophilic to diffuse through the plasma membrane. When activated by hormone binding, the receptors trigger one or more of the signal transduction pathways. That is, the activated receptors directly influence: (1) ion channels that are part of the receptors; (2) enzyme activity that is part of the receptor; (3) activity of cytoplasmic JAK kinases associated with the receptor; or (4) G proteins coupled in the plasma membrane to effector proteins—ion channels and enzymes—that generate second messengers.
The opening or closing of ion channels changes the electrical potential across the membrane. When a calcium channel is involved, the cytosolic concentration of this important ionic second messenger is changed. The changes in enzyme activity are usually very rapid (for example, due to phosphorylation) and produce changes in the conformation and hence the activity of various cellular proteins. In some cases the signal transduction pathways also lead to activation (or inhibition) of particular genes, causing a change in the rate of synthesis of the proteins coded for by these genes. Thus, peptide hormones and catecholamines may exert both rapid and delayed (gene transcription) actions on the same target cell.