On the possible existence of multiple endocrine, paracrine and neurocrine messengers in secretory cell systems.

A multiplicity of regulatory molecules, messengers, are secreted by a large variety of neuronal, endocrine and paracrine cell populations. To some extent the anatomical locus of messenger production or the cytophysiological characteristics of cells producing a messenger determines its role as a hormone, transmitter, modulator or paracrine regulator. Thus, the same messenger may function as a hormone in one location and as a neurotransmitter in another. This versatility in functions makes it difficult to assign a definite physiological role to a given messenger. Our evidence for the occurrence of cell systems producing more than one type of messenger is presented. The practical and conceptual difficulties in determining definite functions of messengers leads to our present inability to examine critically well-known dogmas like the "one hormone-one cell" concept and "Dale's principle". Recognition of the fact that some cell systems do indeed produce multiple messengers provides us with valuable tools for investigating endocrine and neurocrine secretion and has far-reaching implications for studies of cell differentiation and cell pathology.