Diversity and regulation of amiloride-sensitive Na⁺ channels

Amiloride-sensitive Na⁺ channels play a vital role in many important physiological processes such as delineation of the final urine composition, sensory transduction, and whole-body Na⁺ homeostasis. These channels display a wide range of biophysical properties, and are regulated by cAMP-mediated second messenger systems. The first of these channels has recently been cloned. This cloned amiloride-sensitive Na⁺ channel is termined ENaC (Epithelial Na⁺ Channel) and, in heterologous cellular expression systems, displays a single channel conductance of 4 to 7 pS, a high P(Na)/P(K) (> 10), a high amiloride sensitivity (K(i)(amil) = 150 nM), and relatively long open and closed times. ENaC may form the core conduction element of many of these functionally diverse forms of Na⁺ channel. The kinetic and regulatory differences between these channels may be due, in large measure, to unique polypeptides that associate with the core element, forming a functional channel unit.