Distal convoluted tubule

Abstract

The distal convoluted tubule (DCT) is a short nephron segment, interposed between the macula densa and collecting duct. Even though it is short, it plays a key role in regulating extracellular fluid volume and electrolyte homeostasis. DCT cells are rich in mitochondria, and possess the highest density of Na+/K+-ATPase along the nephron, where it is expressed on the highly amplified basolateral membranes. DCT cells are largely water impermeable, and reabsorb sodium and chloride across the apical membrane via electroneurtral pathways. Prominent among this is the thiazide-sensitive sodium chloride cotransporter, target of widely used diuretic drugs. These cells also play a key role in magnesium reabsorption, which occurs predominantly, via a transient receptor potential channel (TRPM6). Human genetic diseases in which DCT function is perturbed have provided critical insights into the physiological role of the DCT, and how transport is regulated. These include Familial Hyperkalemic Hypertension, the salt-wasting diseases Gitelman syndrome and EAST syndrome, and hereditary hypomagnesemias. The DCT is also established as an important target for the hormones angiotensin II and aldosterone; it also appears to respond to sympathetic-nerve stimulation and changes in plasma potassium. Here, we discuss what is currently known about DCT physiology. Early studies that determined transport rates of ions by the DCT are described, as are the channels and transporters expressed along the DCT with the advent of molecular cloning. Regulation of expression and activity of these channels and transporters is also described; particular emphasis is placed on the contribution of genetic forms of DCT dysregulation to our understanding.

Figure 1

Panel A: Structure of mammalian nephron: organization of nephron including distal tubule. Two types of nephrons are shown. A superficial nephron, at left, contains proximal and distal tubules that ascend to kidney surface and possess short loops of Henle. Loops and collecting ducts of superficial nephrons are located within medullary rays (MR). In superficial nephrons, distal tubule comprises distal convoluted tubule (DCT, shown in white), a short connecting tubule (CNT, shown as hatched), and a portion of collecting duct epithelium (shown in white) that begins proximal to junction to form collecting duct. A juxtamedullary nephron (such as that shown at right) has long loops of Henle that descend into renal medulla. Distal tubules comprise only a DCT and CNT segment. In juxtamedullary nephrons, CNT join to form branched arcades that ascend through cortical labyrinth (CL) before emptying into collecting duct (CCT). [Modified from Kriz and Kaissling (230)]. Panel B: Models of DCT and CNT cells. Note the extensive basolateral amplification and multiple mitochondria of DCT cells.

Figure 2

Left: Transepithelial voltage (VT, in mV) along rat distal tubule. Data obtained during in vivo micropuncture are indicated by solid symbols and plotted as a percentage of total distal tubule length [×(564); λ(313); ν(22) τ(183); E(89)]. Data obtained by microperfusing cortical collecting ducts in vitro are presented as open or gray symbols within 2 ovals. Data collected without mineralocorticoid hormone treatment are shown in the grey oval [–MA; ▼(431); μ(399); θ(431)]. Data collected in presence of mineralocorticoid hormone treatment are shown in the hatched oval [+MA; ■, (498); ●, (399)]. Location of DCT, CNT; and CCT is inferred from percentage length along distal tubule (104). Right: transepithelial resistance (RT) along rat renal distal tubule. Data obtained during in vivo micropuncture are indicated by solid symbols (references as in top panel). Data obtained by microperfusion in vitro are indicated by open symbols (references as in top panel).

Figure 3

Ion transport pathways in the DCT1 and DCT2. Panel A: Each channel or transporter is shown schematically and is numbered, according to the scheme in Table 1. The estimated solute fluxes (in pmol·min⁻¹·mm⁻¹) through each pathway are adapted from Weinstein (554). In the present scheme, however, ENaC (9) and ROMK (10) are restricted to the DCT2, and the luminal KCl cotransport has been omitted. Additionally, Weinstein models OH⁻ traversing Cl⁻ channels. Big potassium (BK) channels are also likely to be present in this segment, but are not shown. The solute concentrations, pH, and membrane voltage of modeled DCT cells are shown in the box. Concentrations are in mmol/L. Net solute fluxes are not given for the DCT2, as this segment has not been modeled, but are likely to be lower, at baseline. Panel B: Schematic of predominant transported ions, cell types (intercalated cells are indicated by gray circles), and predominant transport proteins. Asterisk indicates that, although ROMK has been detected in the DCT1 by immunohistochemical techniques, it has not been detected in apical patches along this segment.

Figure 4

Regulation of sodium and chloride transport along The DCT. Panel A: Major factors known to regulate NCC along the DCT are shown. Note that regulation of ENaC and ROMK is omitted, as it is similar to regulation along the CNT and CD, Various peptide (Angll, insulin, and AVP) and steroid (aldosterone and estrogen) hormones, as well as norepinephrine (NE), stimulate NaCl transport primarily by enhancing NCC activity, although effects on other transporters are also likely. NCC is stimulated by low paracellular [K⁺], perhaps directly via Kir4.1/5.1 channels, but also perhaps by stimulating cell chloride depletion via KCC4. Transporters are identified by numbers shown in Table 1. Panel B: highly simplified scheme of NCC regulation by WNK kinases (WNK4 is shown]. NCC is activated, via phosphorylation (red arrow), by SPAK, at conserved sites as shown. The specific SPAK phosphorylation sites are given according to rodent residue number. Other sites are also shown, which may be phosphorylated by other kinases. Two N-linked glycosylation sites are shown. SPAK In turn is activated by WNKs. Both WNK1 and WNK4 can activate SPAK. Sites of protein-protein interaction are shown by black lines. The conserved C terminus (CCT) of SPAK binds to an RFXT motif on NCC, shown as a vertical grey bar. In WNK4, there are autoinhibitory (AI) and coiled coil (CC) domains. Panel C: shows a highly simplified diagram of NCC activation by various stimuli, acting via a WNK and SPAK or OSR1. Note that MO25 can facilitate the action of SPAK on NCC. NCC is dephosphorylated by PP1 and PP4. After synthesis, NCC can be degraded by endoplasmic reticulum mediated decay (ERAD). Panel D: simplified scheme of current views of NCC regulation by WNKs and KLHL3 and cullin 3. As noted, WNKs interact to exert complex stimulatory and inhibitory effects. As KS-WNK1 lacks kinase activity, it appears to be predominantly an inhibitory WNK. WNK4, which is a less potent kinase at baseline, can bind to, and inhibit, WNK1. In the presence of angiotensin II, however, WNK4 becomes more active, stimulating SPAK directly. WNK4, and likely WNK1, are targeted for degradation by KLHL3 and cullin 3, as discussed in the text. Much of this scheme remains speculative and incomplete. Note that WNK kinases also have effects on NCC, ROMK, and ENaC that appear related to degradative activity (as shown in Panel C). These are omitted here for clarity.

Figure 5

Calcium and magnesium reabsorptive pathways in the DCT1 and DCT2 (and connecting tubule). The majority of magnesium traverses the DCT1, as shown, whereas the majority of calcium traverses pathways in the DCT2/CNT. Each channel or transporter is numbered according to the scheme in Table 1. The solute fluxes of calcium are adapted from Bonny and Edwards (40), as are the intracellular concentrations of calcium and Calbindin D28K (CaBP). Note that the presence of this buffer keeps intracellular calcium very low, permitting appropriate signal transduction, despite ongoing transepithelial flux. The intracellular magnesium concentration is from Glaudemans (162), and is much higher. For this reason, intracellular buffering does not appear necessary. Additional details are given in the text.