POSTNATAL MATURATION OF POTASSIUM-TRANSPORT IN RABBIT CORTICAL COLLECTING DUCT

Clearance studies in newborns demonstrate low rates of urinary excretion of potassium, suggesting that the neonatal kidney contributes to the conservation of potassium necessary for growth. Because the cortical collecting duct (CCD) is a primary site for potassium secretion in the adult, we sought to examine the transport capacity of this segment for potassium during postnatal maturation. CCDs isolated from rabbits of various ages (5-6 animals/age group) were microperfused in vitro with solutions simulating plasma. The concentrations of potassium in samples of collected fluid, measured by helium glow photometry, were used to calculate net transport. At a flow rate of similar to 1.6 nl.min(-1).mm(-1) net potassium secretion was absent at birth, first became evident at 4 wk of age (-11.08 +/- 2.39 pmol.min(-1).mm(-1)), and increased sharply thereafter to reach mature rates (-23.08 +/- 3.47 pmol.min(-1).mm(-1); P < 0.05) by 6 wk of age. To determine whether low distal tubular flow rates limit net potassium secretion in the neonate, we perfused CCDs at two or more flow rates in the 0.5-5 nl.min(-1).mm(-1) range. In CCDs taken from animals greater than or equal to 6 wk of age, potassium secretion showed a significant linear correlation with flow rate (y = -10.0x - 7.45; r = 0.87; n = 12). In CCDs of neonates, net potassium secretion was minimally enhanced only at supraphysiological flow rates (-0.57 +/- 0.30 pmol.min(-1).mm(-1) at 1.6 +/- 0.4 nl.min(-1).mm(-1) to -2.31 +/- 0.59 pmol.min(-1).mm(-1) at 3.9 c 1.3 nl.min(-1).mm(-1); P < 0.05); even at these high flow rates potassium secretion was substantially less than observed at the slowest flow rates used in the adult (-13.2 +/- 1.62 pmol.min(-1).mm(-1) at 0.60 +/- 0.07 nl.min(-1).mm(-1)). In contrast to the absence of net potassium secretion observed at 2 wk of age, the rate of net sodium absorption was similar to 50% of that measured in the adult (15.3 +/- 3.3 vs. 24.8 +/- 5.8 pmol.min(-1).mm(-1)). The postnatal increase in transepithelial voltage paralleled that of net sodium absorption. We conclude that the low rates of potassium excretion characteristic of the newborn kidney are due, at least in part, to a low secretory capacity of the CCD; functional immaturity of the paracellular pathway in the neonatal segment may additionally limit the generation of transepithelial cation gradients. The discrepancy between sodium absorption and potassium secretion suggests that maturation of the luminal sodium channels and/or basolateral Na+-K+ pump precedes functional development of apical potassium channels in the CCD.