Mechanisms of Disease
F R A N K L I N H . E P S T E I N , M. D. , Editor
ION CHANNELS — BASIC SCIENCE AND CLINICAL DISEASE
MICHAEL J. ACKERMAN, M.D., PH.D., DAVID E. CLAPHAM, M.D., PH.D.
ON channels constitute a class of proteins that is ultimately responsible for generating and orchestrating the electrical signals passing through the thinking brain, the beating heart, and the contracting muscle. Using the methods of molecular biology and patch-clamp electrophysiology, investigators have recently cloned, expressed, and characterized the genes encoding many of these proteins. Ion-channel proteins are under intense scrutiny in an effort to determine their roles in pathophysiology and as potential targets for drugs. Defective ion-channel proteins are responsible for cystic fibrosis,1 the long-QT syndrome,2 heritable hypertension (Liddle’s syndrome),3,4 familial persistent hyperinsulinemic hypoglycemia of infancy,5,6 hereditary nephrolithiasis (Dent’s disease), and a variety of hereditary myopathies,7-9 including generalized myotonia (Becker’s disease), myotonia congenita (Thomsen’s disease), periodic paralyses, malignant hyperthermia, and central core storage disease (Table 1). Elucidating the mechanisms of these diseases will benefit medicine as a whole, not just patients with a particular disease. For instance, although the inherited long-QT syndrome is not common, identifying the underlying defects in the KVLQT1 and HERG potassium channels and the SCN5A sodium channels may benefit the study of ventricular arrhythmias, which are responsible for 50,000 sudden deaths each year in the United States. Likewise, al-
though a defect in the recently cloned epithelial sodium channel (ENaC) is the basis of a very rare form of inherited hypertension (Liddle’s syndrome, or pseudoaldosteronism), normal ENaC may serve as an alternative target in attempts to correct the physiologic defects created by the cystic fibrosis transmembrane regulator (CFTR), which is mutated in patients with cystic fibrosis, and work with ENaC may provide insight into the mechanism of essential hypertension. This review focuses on ion channels as functioning physiologic proteins, sources of disease, and targets for therapy. We will discuss two prominent diseases caused by defects in ion-channel proteins, as well as two specific ion channels whose recent molecular identification raises new prospects for pharmacologic manipulation. PHYSIOLOGY OF ION CHANNELS
From the Department of Pediatrics and Adolescent Medicine, Mayo Foundation, Rochester, Minn. (M.J.A.); and the Department of Cardiology, Children’s Hospital Medical Center, Department of Neurobiology, Harvard Medical School, Boston (D.E.C.). Address reprint requests to Dr. Ackerman at the Department of Pediatrics and Adolescent Medicine, Mayo Eugenio Litta Children’s Hospital, Mayo Foundation, Rochester, MN 55905. ©1997, Massachusetts Medical Society.
Ion channels are macromolecular protein tunnels that span the lipid bilayer of the cell membrane. Approximately 30 percent of the energy expended by cells is used to maintain the gradient of sodium and potassium ions across the cell membrane. Ion channels use this stored energy much as a switch releases the electrical energy of a battery. They are more efficient than enzymes; small conformational changes change (gate) a single channel from closed to open, allowing up to 10 million ions to flow into or out of the cell each second. A few picoamps (10 12 A) of current are generated by the flow of highly selected ions each time the channel opens. Since ion channels are efficient, their numbers per cell are relatively low; a few thousand of a given type are usually sufficient. Ion channels are usually classified according to the type of ion they allow to pass — sodium, potassium, calcium, or chloride — although some are less selective. They may be gated by extracellular ligands, changes in...