One reason for this activity is that peptide hormones appear to relate to more and more areas of physiology. We also discussed the roles of prostaglandins and other eicosanoids in regulating basal fluid secretion rates in mosquito Malpighian tubules and mediating cellular immune reactions to bacterial infections. At the cellular level, the physiological actions of prostaglandins are mediated in a manner similar to the mediation of peptide hormones. One of the most contemporary arenas of insect and mammalian endocrinology is cellular signal transduction mechanisms. Just to get a grip on terminology, we want to recognize four forms of signaling by secreted molecules.
Endocrine signaling refers to the secretion of a molecule into general circulation.
The secreted species will eventually interact with target cells in one or more tissues quite distant from the endocrine gland. We also recognize paracrine signaling , in which a secreted molecule is very rapidly taken into or inactivated by neighboring cells. In this case the secreted molecule influences just a few cells in the immediate neighborhood of the secreting cells. Paracrine signaling is related to autocoid signaling . In this case, a cell may release a molecule which interacts with receptors on the surface of the same cell.