The pancreatic beta-cell expresses an imidazoline-binding site that is involved in the regulation of insulin secretion. This site is pharmacologically atypical in comparison with the I(1) and I(2) sites described in other tissues, and it has been classified as I(3). The structural requirements for binding of ligands to the I(3) site have not been fully defined, although a range of synthetic I(3) ligands have been characterized in functional terms. Evidence has been presented that an endogenous I(3) ligand may exist, because extracts of brain contain an active principle that stimulates insulin secretion in a manner consistent with the involvement of I(3) sites. The active component has not been identified but has been equated with the long-sought clonidine displacing substance (CDS) that is proposed as the endogenous ligand for imidazoline-binding sites. Recent evidence has indicated that one active component of CDS may be a beta-carboline, but it is not known whether beta-carbolines can stimulate insulin secretion. Thus, we have studied the effects of beta-carbolines on insulin secretion and cytosolic Ca(2+) levels in rodent and human islet cells. The results reveal that harmane, pinoline, and norharmane cause a dose- and glucose-dependent increase in insulin secretion but show that this response differs in a number of ways from that elicited by the well-characterized I(3)-agonist, efaroxan. Thus, beta-carbolines represent a new class of insulin secretagogues, although it remains unclear whether their action is mediated solely by I(3) sites in the beta cell.