Effects of vasopressin-mastoparan chimeric peptides on insulin release and G-protein activity

Two chimeric peptides, consisting of the linear vasopressin receptor V₁ antagonist PhAc-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr, in the N-terminus and mastoparan in the C-terminus connected directly (M375) or via 6-aminohexanoic acid (M391), have been synthesised. At 10 μM concentration, these novel peptides increased insulin secretion from isolated rat pancreatic islet cells 18-26-fold at 3.3 mM glucose and 3.5-5-fold at 16.7 mM glucose. PTX pretreatment of the islets decreased the peptide-induced insulin release. M375 and M391 bind to V(1a) vasopressin receptors with affinities lower than the unmodified vasopressin antagonist, but with K(D) values of 3.76 nM and 9.02 nM, respectively, both chimeras are high affinity ligands. The GTPase activity and GTPγS binding in the presence of these peptides has been characterised in Rin m5F cells. Comparison of the influence of the peptides M375 and M391 on GTPase activity in native and pertussis toxin-treated cells suggests a selective activation of Gα(i)/Gα(o) subunits, combined with a suppression of other GTPases, primarily Gα(s). However, the GTPγS binding data show that the peptides retain some of the activating property even in PTX-treated cell membranes. In conclusion, the conjugation of mastoparan with the V(1a) receptor antagonists produce peptides with properties different from the parent peptides that could be used to elucidate the role of different G proteins in insulin release.