Fig. 2. Signal transduction pathways activated by vasoactive intestinal peptide (VIP) in the lacrimal gland. VIPR, VIP receptor; Gαs, alpha subunit of Gs G protein; AC, adenylate cyclase; cAMP, cyclic adenosine monophosphate; ATP, adenosine triphosphate; AMP, adenosine monophosphate; PKA, protein kinase A; cAMP PDE, cAMP phosphodiesterase. (Modified from Dartt DA: Regulation of lacrimal gland secretion by neurotransmitters and the EGF family of growth factors.Exp Eye Res 73:741, 2001, with permission.)