Fig. 3. Signal transduction pathways activated by α1-adrenergic agonists in the lacrimal gland. Norepi, norephinephrine; Gαq, alpha subunit of Gq G protein; DAG, diacylglycerol; PKC, protein kinase C; ErbB1, the EGF receptor; ErbB, the EGF family of growth factor receptor; P, phosphorylated tyrosine residues on the ErbBs; SOS, Grb2 and Shc, adaptor proteins, raf, mitogen-activated protein kinase kinase kinase; MEK, mitogen-activated protein kinase kinase; MAPK, p44/p42 mitogen-activated protein kinase; ER, endoplasmic reticulum. (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.)