Carbachol stimulates granule exocytosis, phospholipase C (PLC), and phospholipase D (PLD) in RBL-2H3hm1 mast cells by a mechanism that involves Galpha(q), However, mastoparan stimulates the same responses through G(i) protein. Both G(i) and Galpha(q) pathways are suppressed by Clostridium difficile toxin B, suggesting that Rac and Cdc42 small GTPases are also involved. Over-expression of beta(1)Pix, a guanine nucleotide exchange factor for Rae and Cdc42, enhances mastoparan-but not carbachol-induced hexosaminidase secretion and PLC and PLD activation. Furthermore, cells expressing beta(1)Pix exhibit elevated levels of mastoparan-stimulated IP3 production. Taken together, these findings implicate beta(1)Pix in regulating hexoasaminidase secretion and IP3 production in early stage upon mastoparan stimulation. (C) 2004 Elsevier Inc. All rights reserved.