Na(+)-K(+)-ATPase-mediated basolateral rubidium uptake in the maturing rabbit cortical collecting duct

An unexpected journey: conceptual evolution of mechanoregulated potassium transport in the distal nephron

Flow-induced K secretion (FIKS) in the aldosterone-sensitive distal nephron (ASDN) is mediated by large-conductance, Ca(2+)/stretch-activated BK channels composed of pore-forming α-subunits (BKα) and accessory β-subunits. This channel also plays a critical role in the renal adaptation to dietary K loading. Within the ASDN, the cortical collecting duct (CCD) is a major site for the final renal regulation of K homeostasis. Principal cells in the ASDN possess a single apical cilium whereas the surfaces of adjacent intercalated cells, devoid of cilia, are decorated with abundant microvilli and microplicae. Increases in tubular (urinary) flow rate, induced by volume expansion, diuretics, or a high K diet, subject CCD cells to hydrodynamic forces (fluid shear stress, circumferential stretch, and drag/torque on apical cilia and presumably microvilli/microplicae) that are transduced into increases in principal (PC) and intercalated (IC) cell cytoplasmic Ca(2+) concentration that activate apical voltage-, stretch- and Ca(2+)-activated BK channels, which mediate FIKS. This review summarizes studies by ourselves and others that have led to the evolving picture that the BK channel is localized in a macromolecular complex at the apical membrane, composed of mechanosensitive apical Ca(2+) channels and a variety of kinases/phosphatases as well as other signaling molecules anchored to the cytoskeleton, and that an increase in tubular fluid flow rate leads to IC- and PC-specific responses determined, in large part, by the cell-specific composition of the BK channels.

Potassium transport in the maturing kidney

The distal nephron and colon are the primary sites of regulation of potassium (K(+)) homeostasis, responsible for maintaining a zero balance in adults and net positive balance in growing infants and children. Distal nephron segments can either secrete or reabsorb K(+) depending on the metabolic needs of the organism. In the healthy adult kidney, K(+) secretion predominates over K(+) absorption. Baseline K(+) secretion occurs via the apical low-conductance secretory K(+) (SK) channel, whereas the maxi-K channel mediates flow-stimulated net urinary K(+) secretion. The K(+) retention characteristic of the neonatal kidney appears to be due not only to the absence of apical secretory K(+) channels in the distal nephron but also to a predominance of apical H-K-adenosine triphosphatase (ATPase), which presumably mediates K(+) absorption. Both luminal and peritubular factors regulate the balance between K(+) secretion and absorption. Perturbation in any of these factors can lead to K(+) imbalance. In turn, these factors may serve as effective targets for the treatment of both hyper-and hypokalemia. The purpose of this review is to present an overview of recent advances in our understanding of mechanisms of K(+) transport in the maturing kidney.

Lack of a role of membrane-protein interactions in flow-dependent activation of ENaC

Rates of Na(+) absorption in the distal nephron increase proportionally with the rates of tubular flow. We tested the hypothesis that the deformation or tension generated in the plasma membrane in response to flow activates the epithelial sodium channel (ENaC). We modified the physical properties of the membrane by changing the temperature and the content of cholesterol. Rates of net Na(+) absorption measured in cortical collecting ducts (CCDs) perfused at room temperature at slow (approximately 1) and fast (approximately 5 nl.min(-1).mm(-1)) flow rates were less than those measured at 37 degrees C at the same flow rates, although increases in tubular fluid flow rates led to comparable relative increases in net Na(+) absorption at both temperatures. Xenopus laevis oocytes expressing ENaC responded to an increase in shear stress at 22-25 degrees C with a discrete delay followed by a monoexponential increase in whole-cell Na(+) currents. We observed that temperature affected 1) basal currents, 2) delay times, 3) kinetics of activation, and 4) fold-increase in macroscopic currents in response to flow. The magnitude of the response to flow displayed biphasic behavior as a function of temperature, with a minimal value at 25 degrees C. Steady-state fluorescence anisotropic measurements of purified plasma membranes did not show any obvious phase transition behavior over a temperature range from 8.3 degrees C to 36.5 degrees C. Modification of the content of membrane cholesterol did not affect the response to flow. Our results suggest that the flow-dependent activation of ENaC is not influenced by modifications in the intrinsic properties of the plasma membrane.

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.

Ontogeny of renal sodium transport

One of the main functions of the adult kidney is to maintain a constant extracellular fluid balance. The adult kidney does this, by and large, by filtering a massive quantity of fluid and reabsorbing the solutes needed to maintain volume and electrolyte homeostasis, while leaving the waste products to be excreted in the urine. One of the most precisely regulated functions of the adult kidney is to maintain sodium balance. The challenge of the neonatal kidney is even greater. It must maintain a positive salt balance for growth while the neonate is fed a diet that is very low in sodium. This review focuses on how the neonatal kidney reabsorbs NaCl with a special emphasis on the differences between the neonatal and adult kidney.

Maturational changes in renal tubular transport

PURPOSE OF REVIEW

This review examines the maturational changes that occur in renal tubules during postnatal development.

RECENT FINDINGS

The ability to study transport in neonatal tubules and the use of molecular techniques have allowed studies that not only examine the mechanism of solute and water transport in neonates but also what causes the maturational changes in transport at a molecular and cellular level.

SUMMARY

This review demonstrates that there are significant quantitative and qualitative differences in transport during postnatal maturation in every nephron segment. In some segments the maturational changes involve simply a change in abundance of transporters, while in others the difference in transport is due to changes in transporter isoforms, changes in paracellular permeability or changes in intracellular signaling that regulate the transporter. This review focuses on these changes and what is known about what causes the maturational changes in transport.