G-PROTEIN COUPLED RECEPTORS IN GASTROINTESTINAL PHYSIOLOGY - IV - NEURAL REGULATION OF GASTROINTESTINAL SMOOTH-MUSCLE

G protein-coupled receptors receive many of the neural, hormonal, and paracrine inputs to gastrointestinal (GI) smooth muscle cells. This ar ticle examines the major G protein-coupled receptors, G proteins, and effecters that mediate responses to enteric neuromuscular transmitters . Excitatory transmitters primarily couple through G(q/11) and G(i)/G( o) proteins and elicit responses via formation of inositol trisphospha te and diacylglycerol and inhibition of adenylyl cyclase. Several inhi bitory transmitters couple through G(s) and activation of adenylyl cyc lase. There are interesting examples, however, of inhibitory transmitt ers apparently using pathways regulated by G(q/11) to elicit responses by localized Ca2+ release and activation of Ca2+-dependent ion channe ls. G protein-coupled receptors may also be differentially expressed b y smooth muscle cells and interstitial cells of Cajal, which may incre ase the diversity of responses and allow specialized innervation of GI muscle tissues.