Paracrine agents are chemical messengers that participate in local communications between cells. Paracrine agents are synthesized by cells and released, once given the appropriate stimulus, into the extracellular fluid. They then diffuse to neighboring cells, some of which are their target cells. Given this broad definition, neurotransmitters theoretically could be classified as a subgroup of paracrine agents, but by convention they are not. Paracrine agents are generally inactivated rapidly by locally existing enzymes so that they do not enter the bloodstream in large quantities. There is one category of local chemical messengers that are not intercellular messengers—that is, they do not communicate between cells. Rather, the chemical is secreted by a cell into the extracellular fluid and then acts upon the very cell that secreted it. Such messengers are termed autocrine agents. Frequently a messenger may serve both paracrine and autocrine functions simultaneously—that is, molecules of the messenger released by a cell may act locally on adjacent cells as well as on the same cell that released the messenger.
One of the most exciting developments in physiology today is the identification of a growing number of paracrine /autocrine agents and the extremely diverse effects they exert. Their structures span the gamut from a simple gas (nitric oxide) to fatty acid derivatives to peptides and amino acid derivatives. They tend to be secreted by multiple cell types in many tissues and organs. According to their structures and functions, they can be gathered into families; for example, one such family constitutes the "growth factors," encompassing more than 50 distinct molecules, each of which is highly effective in stimulating certain cells to divide and/or differentiate.