Chapter 5 Polyphosphoinositides, phosphoproteins, and receptor function in rabbit iris smooth muscles

Abstract

Publisher Summary This chapter discusses the role of polyphosphoinositides and protein phosphorylation in muscarinic cholinergic and α 1 -adrenergic receptor function in the iris of the eye. The iris contains sphincter and dilator muscles and these are innervated by excitatory cholinergic and adrenergic nerves, respectively. The presence of muscarinic cholinergic and α 1 -adrenergic receptors has also been demonstrated through radioligand binding studies. Since early observations on the link between phosphatidylinositol 4,5-bisphosphate (PIP 2 ) breakdown and the activation of muscarinic cholinergic and α 1 -adrenergic receptors in the iris muscle, experiments have reported extensively on the molecular mechanism and physiological significance of the receptor-mediated phosphatidylinositol 4-phosphate (PIP), breakdown into myo -inositol 1-phosphate (IP), and diacylglycerol (DG) in this tissue. In addition, the demonstration of a close correlation between agonist-stimulated PIP 2 breakdown, and agonist-induced muscle contraction conclude that this phenomenon is an early event in the pathway, which leads from receptor activation to muscle response. The chapter focuses on the concept that the agonist-stimulated breakdown of PIP in the iris muscle is a primary (initial) event that could couple activated muscarinic cholinergic and α 1 -adrenergic receptors through the mobilization of cellular Ca 2+ to muscle contraction.