Could cytoplasmic concentration gradients for sodium and ATP exist in intact renal cells?
In renal cells, the Na+ pump maintains a transmembrane concentration gradient for sodium ensuring the net reabsorption of sodium with or without cotransported species. This process requires a significant fraction of the ATP turnover of proximal tubules and thick ascending limbs. To understand the potential regulatory influences of Na+ and ATP on the activity of the Na+ pump in these nephron segments, the apparent kinetics of the membrane-bound Na+-K+ ATPase and of the cellular Na+ pump were studied in different preparations of dog proximal tubules and thick ascending limbs (tubular suspensions, tissue homogenates, and basolateral membrane vesicles) obtained from dog kidney cortex and red medulla. Two determinant kinetic parameters, i.e., the apparent Michaelis constant (Km) and the saturating concentrations for sodium and ATP, were compared with the intracellular concentrations of Na+ and ATP measured under physiological conditions. In both types of tubules, the apparent Km value for N
