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

Two chimeric peptides, consisting of the linear vasopressin receptor V-1 an tagonist PhAc-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-T in the N-terminus and mas toparan in the C-terminus connected directly (M375) or via 6-aminohexanoic acid (M391), have been synthesised. At 10 mu M concentration, these novel p eptides increased insulin secretion from isolated rat pancreatic islet cell s 18-26-fold at 3.3 mM glucose and 3.5-5-fold at 16.7 mM glucose. PTX pretr eatment of the islets decreased the peptide-induced insulin release. M375 a nd M391 bind to V-1a vasopressin receptors with affinities lower than the u nmodified vasopressin antagonist, but with K-D values of 3.76 nM and 9.02 n M, respectively, both chimeras are high affinity ligands. The GTPase activi ty and GTP gamma S binding in the presence of these peptides has been chara cterised in Rin m5F cells. Comparison of the influence of the peptides M375 and M391 on GTPase activity in native and pertussis toxin-treated cells su ggests a selective activation of G alpha(i)/G alpha(o) subunits, combined w ith a suppression of other GTPases, primarily G alpha(s). However, the GTP gamma S binding data show that the peptides retain some of the activating p roperty even in PTX-treated cell membranes. In conclusion, the conjugation of mastoparan with the V-1a receptor antagonists produce peptides with prop erties different from the parent peptides that could be used to elucidate t he role of different G proteins in insulin release. (C) 1999 Elsevier Scien ce B.V. All rights reserved.