[How do hormones act?].

Hormones act via receptors, i.e. structures which contain specific binding sites for any given hormone. The transmission of information is accomplished by reversible binding of the hormone to the receptor. Steroid hormones and triiodothyronine act via receptors which are located in the nucleus and regulate the biosynthesis of specific proteins. Peptide hormones and catecholamines combine with receptors within the cell membrane, thereby regulating the membrane-bound enzyme adenyle cyclase. The product of this enzyme, cyclic AMP, acts as a transmitter of the action of these hormones in the interior of the cell.