Growing subjects are in a state of positive K balance and have a limited ability to excrete K. This suggests that the immature kidney has a limited capacity for K secretion and/or enhanced ability to reabsorb K. The major regulatory site of K secretion in the kidney is the cortical collecting duct (CCD). In contrast to the high rates of net K secretion observed in CCDs isolated from adult animals and microperfused in vitro, segments from neonatal animals show no significant K transport. Yet, these same neonatal segments absorb Na at a rate half that measured in the adult, suggesting there exists a fundamental difference in cation transport mechanisms between the neonate and adult. K secretion in the CCD, mediated by principal cells, is determined by a two step process: active K uptake into the cell by Na-K-ATPase and passive diffusion down a favorable electrochemical gradient through apical K channels. To determine whether a paucity of apical K secretory channels limits net K secretion early in life, we will use patch clamp analysis to compare the apical K conductances of neonatal and mature principal cells. To examine whether the absence of conducting K secretory channels is due to a low open probability (P-O) of existing channels and/or a low channel number (N), we will next determine whether exposure of the neonatal CCD to factors known to increase P-O or N induces net K secretion. The discrepancy between onset of Na and K transport in the neonatal CCD suggests that qualitative changes in the transepithelial Na absorptive pathway occur during postnatal differentiation. To test this, we will compare the apical Na conductances (patch clamp analysis), membrane transporters active in Na reabsorption (helium glow photometry), and Na-K-ATPase activity (ouabain- sensitive basolateral 86Rb uptake) in neonatal and mature principal cells; the latter findings will be correlated with measurements of basolateral membrane surface area (electron microscopy). Because clearance studies in the neonate indicate significant K retention, we will also test the hypothesis that enhanced K absorption in the neonate reduces net urinary K excretion. To assess the K absorptive capacity of the CCD and outer medullary collecting duct (OMCD), we will determine the contribution of K absorption, measured as unidirectional lumen-to-bath 86Rb fluxes, to net K transport in segments isolated from maturing animals. Should we document significant K absorption early in life, we will test whether K absorption is coupled to H secretion by H-K-ATPase, a transporter immunocytochemically identified in intercalated cells. That population of intercalated cells possessing functional H-K-ATPase will then be identified and the polarity and activity of H-K exchange measured to determine if there is a maturational change in activity of this pump. The studies proposed in this application should help us to understand the physiologic basis for the limited K secretory capacity of the neonatal CCD and provide broad insight into the regulation of the K secretory process and its interrelationship with the mechanism of Na absorption.