L. M. Satlin

Organic anion transporting polypeptide mediates organic anion/HCO3/- exchange

Organic anion transporting polypeptide (oatp) is an integral membrane protein cloned from rat liver that mediates Na+-independent transport of organic anions such as sulfobromophthalein and taurocholic acid. Previous studies in rat hepatocytes suggested that organic anion uptake is associated with base exchange. To better characterize the mechanism of oatp-mediated organic anion uptake, we examined transport of taurocholate in a HeLa cell line stably transfected with oatp under the regulation of a zinc-inducible promoter (Shi, X., Bai, S., Ford, A. C., Burk, R. D., Jacquemin, E., Hagenbuch, B., Meier, P. J., and Wolkoff, A. W. (1995) J. Biol. Chem. 270, 25591–25595). Whereas noninduced transfected cells showed virtually no uptake of [3H]taurocholate, taurocholate uptake by induced cells was Na+-independent and saturable (K m = 19.4 ± 3.3 μm; V max = 62.2 ± 1.4 pmol/min/mg protein; n = 3). To test whether organic anion transport is coupled to HCO3 − extrusion, we compared the rates of taurocholate-dependent HCO3 − efflux from alkali-loaded noninduced and induced cells. Monolayers grown on glass coverslips were loaded with the pH-sensitive dye 2′,7′-bis(carboxyethyl)-5(6)-carboxyfluorescein; intracellular pH (pHi) was measured by excitation ratio fluorometry. Noninduced and induced cells were alkalinized to an equivalent pHi (∼7.7) by transient exposure to a 50 mmHCO3 −, Cl−-free solution. In the absence of extracellular Cl− and taurocholate, isohydric reduction of superfusate HCO3 −concentration from 50 to 25 mm resulted in an insignificant change in pHi over time (dpHi/dt) in both groups. Addition of 25 μm taurocholate to the superfusate led to a rapid fall in pHi in induced (−0.037 ± 0.011 pH units/min to pHi of 7.41 ± 0.14) but not in noninduced (0.003 ± 0.006 pH units/min to pHi of 7.61 ± 0.08) cells (p < 0.03). These data indicate that oatp-mediated taurocholate transport is Na+-independent, saturable, and accompanied by HCO3 −exchange. We conclude that organic anion/base exchange is an important, potentially regulatable component of oatp function.

Postnatal maturation of the rabbit cortical collecting duct.

The mature, fully differentiated cortical collecting duct plays a major role in the final renal regulation of Na+, K+ and H+ transport. To characterize the growth of this segment, we measured the outer diameter and the dry weight of cortical collecting ducts isolated from newborn, 1-month-old, and adult rabbits. During the 1st month of life no significant changes were observed; however, there was a 60% increase in both parameters after the 4th week of life. Growth-related accretion of K+ was demonstrated by showing tubular K+ content to increase by 60% with maturation. Concomitant with the increase in tubular size, total cell number per millimeter of tubular length rose by 30%. Approximately 50% of the observed increment in tubular size could be accounted for by cell hyperplasia, with the remaining increase resulting from cell hypertrophy. Hypertrophy of principal cells was confirmed by scanning electron microscopy, which demonstrated a doubling of the circumferential width without any change in longitudinal length. Hyperplasia was confirmed, using a fluorescent chromatin stain, by our finding of a mitotic frequency of 3/1000 cells in the neonatal mid-cortical collecting duct; the observed number of mitoses was 10-fold higher at the most cortical end (ampulla). The number of intercalated cells per millimeter of tubule length, identified by bright green fluorescence after cortical collecting ducts were stained with 6-carboxyfluorescein diacetate, was found to double during maturation, the increase being significant only after the 4th postnatal week. We conclude that maturation of the mid-cortical collecting duct results from both cellular hyperplasia and hypertrophy. It is unlikely that this segment plays a major role in regulating Na+, K+, and H+ transport in the neonatal kidney.

Maturation of renal potassium transport

SummaryThe studies outlined in this review suggest that the immaturity of distal nephron segments may hinder urinary excretion of potassium early in life. Among the factors that may limit potassium secretion by principal cells in the neonatal cortical collecting duct are an unfavorable electrochemical gradient (reduced Ki, Na+−K+-ATPase activity and/or Vte), limited membrane permeability to potassium and sodium, low tubular fluid flow rate, reduced luminal sodium concentration, or increased paracellular backleak. Alternatively, enhanced potassium absorption by other relatively well-differentiated distal nephron segments may contribute in part to a reduced net potassium excretory rate in the newborn.It should be kept in mind, however, that the limited potassium secretory capacity of the immature kidney becomes clinically relevant only under conditions of potassium excess. Under normal circumstances, the tendency of the newborn to retain potassium is an appropriate and necessary condition for growth.