Renal potassium handling in healthy and sick newborns.

Growing infants must maintain a state of positive K+ balance, a task accomplished, in large part, by the kidney. The distal nephron is uniquely adapted to retain total body K+ early in life. The magnitude and direction of net K+ transport in the cortical collecting duct (CCD), the segment responsible for the final renal regulation of K+ balance in the adult, reflect the balance of opposing fluxes of K+ secretion and K+ absorption. Evidence now indicates that the low capacity of the neonatal CCD for K+ secretion is due, at least in part, to a relative paucity of conducting K+ channels in the urinary membrane. A relative excess of K+ absorption in this nephron segment may further reduce net urinary K+ secretion. Under conditions prevailing in vivo, the balance of fluxes in the CCD likely contributes to the relative K+ retention characteristic of the neonatal kidney.