CATION-TRANSPORT SYSTEMS IN MITOCHONDRIA - NA+ AND K+ UNIPORTS AND EXCHANGERS

It is now well established that mitochondria contain three antiporters that transport monovalent cations. A latent, allosterically regulated K+/H+ antiport appears to serve as a cation-extruding device that hel ps maintain mitochondrial volume homeostasis. An apparently unregulate d Na+/H+ antiport keeps matrix [Na+] low and the Na+-gradient equal to the H+-gradient. A Na+/Ca2+ antiport provides a Ca2+-extruding mechan ism that permits the mitochondrion to regulate matrix [Ca2+] by balanc ing Ca2+ efflux against influx on the Ca2+ uniport. All three antiport s have well-defined physiological roles and their molecular properties and regulatory features are now being determined. Mitochondria also c ontain monovalent cation uniports, such as the recently described ATP- and glibenclamide-sensitive K+ channel and ruthenium red-sensitive un iports for Na+ and K+. A physiological role of such uniports has not b een established and their properties are just beginning to be defined.