Surface-active properties of vasoactive intestinal peptide*

The purpose of this study was to determine whether human vasoactive intestinal peptide (VIP) aggregates in aqueous solution and, if so, whether the peptide interacts with a biomimetic phospholipid monolayer and increases surface pressure. Using a custom-made Teflon trough containing HEPES buffer (pH 7.4) at room temperature and a surface tensiometer, we found that the critical micellar concentration (CMC) of VIP is 0.4 microM. Surface pressure of a dipalmitoylphosphatidylcholine (DPPC) monolayer spread over the HEPES buffer declined significantly over 120 min because of phospholipid decomposition. However, injection of VIP at concentrations above CMC into the subphase of the monolayer elicited a significant concentration-dependent increase in surface pressure that persisted for 120 min (P < 0.05). Unlike VIP, injection of [(8)Arg]-vasopressin at an equimolar concentration only prevented the time-dependent decline in DPPC monolayer surface pressure. Taken together, these data indicate that human VIP aggregates in aqueous solution and expresses surface-active properties at physiological concentrations in vitro. We suggest that these attributes could have a role in modulating the bioactive effects of the peptide in vivo.