alpha-Melanocyte-stimulating hormone (MSH) antagonizes the binding of interleukin-1 beta (IL-1) to its type I receptors on T cells. IL-1 modulates insulin secretion in the insulin-secreting tumor cell line, HIT-T 15 cells. In the present studies, we examined a hypothesis that alpha-MSH may modulate insulin secretion in HIT-T 15 cells. The addition of alpha-MSH over the concentrations of 10(-10) M inhibited insulin secretion for 4 h. In addition, the carboxyl-terminal tripeptide of alpha-MSH [alpha-MSH(11-13)], which is supposed to be an important sequence of alpha-MSH on its antipyretic effects, significantly inhibited insulin secretion at the concentration of 10(-9) M. In contrast, acetyl-[Nle4,D-Phe7] alpha-MSH(4-10)-amide (10(-11) to 10(-9) M) failed to inhibit insulin secretion. The maximal effect of alpha-MSH on insulin secretion was obtained at the concentration of 10(-9) M. The addition of alpha-MSH from 10(-11) to 10(-9) M significantly inhibited 7.5 mM glucose-stimulated insulin secretion at each period of 0-5, 5-15, and 15-30 min. Radioreceptor assay suggested that HIT-T 15 cells possess the specific binding sites for 125I-labeled alpha-MSH. The present studies indicated that alpha-MSH inhibits insulin secretion through the specific binding site in HIT-T 15 cells.