Peptide secretion: what do we know?

Understanding factors that regulate peptide release became an issue when the presence and possible role of these compounds as transmitter/modulators in various systems were realized. Many studies measuring the levels of peptides in various tissues and fluids have been performed using radioimmunoassay. However, because these peptides are measured in postmortem tissues, or perfusates that are collected at time intervals that do not approach the time scale used for exocytosis, limited information can be derived from these data. Recently the quantitative use of Northern analysis, RNase protection assays, and in situ hybridization has led to a large literature reporting on changes in peptide mRNA levels as a consequence of a variety of treatments. The assumptions involved in using radioimmunoassay measurements of peptide levels and the various methods used to measure peptide mRNAs are different, but data obtained from experiments using both methods are nonetheless used as an indication of the regulation of peptidergic neurons, and ultimately of peptide release. The mechanisms dedicated to translating cellular input into alterations in secretion have begun to be appreciated at a molecular level. Herein we will discuss the cell biology of regulated secretion and consider some levels in this pathway at which peptide release may be controlled.