Nucleotides and ion channel regulation.

Abstract

Signal transduction by hormones, neurotransmitters, and growth factors is almost invariably associated with changes in the activity of one or more ion channels in the plasma membrane of cells. Alterations in receptors and channels are implicated in the pathophysiology of many disease states. At least three hereditary diseases (cystic fibrosis, hyperkalemic periodic paralysis, and malignant hyperthermia) have been linked to defects in channels for Cl, Na-, and Ca2+, respectively (1-3). Therefore, knowledge of channel structure and regulation is required for understanding ion channel dysfunction in disease as well as in normal states. Further, this knowledge is also essential for devising strategies for pharmacologic and/or immunologic intervention to help curb the extent of disease processes. The mechanisms by which extracellular signals regulate the opening and closing of various ion channels are diverse. Three broad classes of ion channels have been described: (1) voltage-gated channels, regulated by changes in transmembrane potential; (2) ligand-gated channels, regulated by binding of an extracellular ligand to the channel; and (3) second messenger-gated channels, regulated by changes in intracellular ions or co-factors stemming from activation of cell surface receptors. A wide variety of regulatory mechanisms has been described for channels falling into the last group, including stimulation by G proteins, phosphorylation by protein kinases, and binding by arachidonic acid, inositol phosphates, intracellular Ca2+, and nucleotides (4-7). To understand the molecular mechanisms underlying selective ion transport by channels, it is essential to elucidate the primary structure of the constituent protein(s) and to understand the control pathways involved in regulating channel function. Several approaches have been used to study the structure and functional regulation of ion channels. Electrophysiologic methods and the use of isotopic ion flux measurements have the advantage that functional studies can be carried out in vivo, without removing the channel from its native environment. However, it is impossible to identify from these studies alone the protein(s) involved in the observed activity. Biochemical methods directed at purification