What type of passive transport allows molecules to move through transport proteins?

Passive Transport

Simple diffusion always occurs down an electrochemical gradient, which is a composite of the concentration and electrical gradients (electrochemical gradient). With an undissociated molecule, only the concentration gradient is relevant; for a charged ion the electrical gradient also must be considered. Simple diffusion does not require a direct energy source, although active transport is usually necessary to establish the initial concentration and electrical gradients.

Facilitated diffusion (coupled or carrier-mediated diffusion) depends on an interaction of the molecule or ion with a specific membrane carrier protein that facilitates its passage across the cell membrane's lipid bilayer. In almost all cases of carrier-mediated transport in the kidney, two or more ions or molecules share the carrier: one moiety moves down its electrochemical gradient, and the other(s) moves against the gradient and is transported “uphill.”

Diffusion through a membrane channel (or pore) formed by specific integral membrane proteins is also a form of facilitated diffusion, because it allows charged, polar, and lipophobic molecules to pass through the membrane at a high rate.

Active and Passive Transport Processes can be Evaluated by Considering Direction of Electrochemical Potential Difference (Driving Force)

As stated above, passive transport is energetically downhill, driven by the pre-existing driving force; this force depends on the chemical or electrochemical gradient, for uncharged and charged solutes, respectively. Under isothermal conditions the driving force encompasses differences in concentration, electrical potential, and/or pressure across the membrane. Under these conditions, the electrochemical potential difference (Δμ¯j) for the jth ion is given by Eq. (2.1):

(2.1)Δμ¯j=zjVmF+RTln(CjiCjo)+ΔPV¯j

where z is the valence, Vm is the membrane voltage, F is the Faraday constant, R is the gas constant, T is the absolute temperature, C is the concentration, i and o refer to the two sides of the membrane (inside and outside, respectively), ΔP is the transmembrane hydrostatic pressure difference, and V¯j is the ion’s partial molar volume. The electrochemical potential has the three components defined above, given by the three terms on the right side of the equation. Across animal cell membranes, steady-state hydrostatic or osmotic pressure differences are small or nil (see Chapter 4), and therefore the third term of Eq. (2.1) is eliminated, yielding:

(2.2)Δμ¯j=z jVmF+RTln(CjiCjo)

This equation is used to evaluate the driving force for ion transport under isobaric conditions. In the case of nonelectrolytes, z=0 and the first term of Eq. (2.2) can also be eliminated, yielding:

(2.3)Δμj=RTln(C jiCjo)

where Δμj denotes the chemical potential difference. This equation describes the driving force for nonelectrolyte transport.

From Eq. (2.2) (under isobaric and isothermal conditions), Ussing111 derived the flux-ratio equation, a fundamental expression which provides a thermodynamic test for active or passive transport:

(2.4)Jin/Jout=(Ci/Co)exp(zVm F/RT)

where J is flux (the subscripts denote influx and efflux, respectively). The test proceeds as follows: the ratio of unidirectional fluxes (Jin/Jout) is determined experimentally, and the driving forces are measured; if the ratio deviates from the prediction given by Eq. (2.4), which evaluates the passive driving forces, then active transport is suspected. Deviations from the flux-ratio equation can also result from the presence of exchange diffusion and single-file diffusion, as discussed by Schultz.90

Passive Transport

The passive transfer of molecules across a membrane depends on (1) concentration and electrochemical differences across the membrane, (2) molecular weight, (3) lipid solubility, (4) degree of ionization, and (5) membrane surface area and thickness. Passive transfer is driven principally by a concentration gradient and occurs through the lipid membrane (e.g., lipophilic molecules and water) or within protein channels that traverse the lipid bilayer (e.g., charged substances such as ions).33 Drugs with a molecular weight less than 600 Da cross the placenta by passive diffusion.34

Regulation of Calcium Homeostasis

Calcium absorption from the intestines is promoted by active and passive transport mechanisms (Hoenderop et al., 2005; Christakos et al., 2014). Active calcium absorption occurs via the intercellular pathway in the duodenum, while passive calcium absorption occurs via the paracellular pathway in the jejunum and ileum. Transient receptor potential vanilloid 6 (TRPV6), a six-transmembrane selective calcium channel expressed in the apical (luminal) side of intestinal epithelial cells, has been identified as an essential calcium channel regulated by 1α,25(OH)2D3 (Peng et al., 1999; Suzuki et al., 2008) (Fig. 2). Ca2 + entering the cell via the TRPV 6 channel is captured by calbindin-D9K, a calcium binding protein in the cytosol. Captured Ca2 + is extruded to the basolateral side by plasma membrane Ca2 +-ATPase 1b (PMCA1b), which is expressed in the basolateral membranes facing to the intestinal blood capillaries. Although Na/Ca-exchanger 1 (NCX1) is abundantly expressed in the basolateral membranes of duodenum epithelial cells, PMCA1b is considered to play a major role in the 1α,25(OH)2D3-induced basolateral Ca2 + extrusion to the intestinal blood capillaries. The expression of TRPV6 and calbindin-D9K is tightly regulated by 1α,25(OH)2D3. However, the vitamin D-induced promotion of intestinal calcium absorption is normal in calbindin-D9K-deficient mice as well as TRPV6-deficient mice. However, increase of duodenal Ca2 + absorption after 1α,25(OH)2D3 treatment in double calbindin-D9K and TRPV6-deficient mice was only 40% as compared to wild-type mice (Benn et al., 2008). Therefore, vitamin D also appears to regulate calcium passive transport. Claudin is a major protein of tight junctions. In VDR KO mice, the expression of claudin-2 (Cld 2) and Cld 12 in the jejunum and ileum was found to be markedly decreased. 1α,25(OH)2D3 upregulated the expression of Cld 2 and Cld 12, and enhanced the passive transport of calcium (Fujita et al., 2008). Thus, 1α,25(OH)2D3 is considered to promote the active and passive intestinal absorption of calcium.

The kidneys play an important role in maintaining calcium homeostasis (Quarles, 2012; Veldurthy et al., 2016) (Fig. 2). Approximately 50% of free calcium in the blood is filtered through the glomerulus. Eighty five percent of free calcium is passively transported, while the remaining 15% is reabsorbed by active transport. This active transport occurs in the tubular distal part and is regulated by 1α,25(OH)2D3. Active calcium reabsorption in the kidneys occurs through a similar mechanism to intestinal calcium absorption (Hoenderop et al., 2003). In the case of the kidneys, TRPV5 functions as a luminal side calcium channel, calbindin-D28k as an intracellular calcium transport protein, and NCX1 and PMCA1b for extrusion to the basolateral side. The expression of TRPV5 and cabindin-D28K in the kidneys is promoted by 1α,25(OH)2D3. Thus, 1α,25(OH)2D3 plays an important role in calcium reabsorption in the kidneys.

1α,25(OH)2D3 stimulates calcium mobilization from bone (Raisz et al., 1972). 1α,25(OH)2D3 enhances RANKL expression in osteoblastic cells and induces calcium mobilization via osteoclastic bone resorption (Suda et al., 1999). 1α,25(OH)2D3 also stimulates the secretion of FGF23 from osteocytes (Lanske et al., 2014). In the parathyroid gland, 1α,25(OH)2D3 suppresses the secretion of PTH (Slatopolsky and Brown, 2002). The suppression of 1α,25(OH)2D3 secretion as well as PTH secretion is considered to be a type of negative feedback reaction. Thus, 1α,25(OH)2D3 plays roles in the regulation of calcium homeostasis.

Sugars are transported by carriers

Several solutes are transported by solute-selective simple carriers. A good example is theglucose carrier that mediates glucose transport across the human red blood cell (RBC) PM. This protein, GLUT-1, belongs to a family of sugar transporters (Box 10.2). GLUT-1 has 12 membrane-spanning helices, 5 of which areamphiphilic: each has a hydrophobic and a hydrophilic surface. The hydrophobic regions interact with the surrounding lipids in the bilayer. In contrast, the hydrophilic surfaces of the five helices face one another and form a central, water-filled transmembranechannel or pore (Fig. 10.2). This is the general structure of many transporter molecules.

The need for carrier-mediated glucose transport is exemplified by the consequences of mutations in the human GLUT-1 gene. GLUT-1 also is expressed in the brain, where it mediates glucose transport across the blood-brain barrier and glucose uptake by glial cells. Individuals with a mutant GLUT-1 gene have a cerebrospinal fluid glucose concentration that is approximately 50% of normal despite a normal blood glucose level. The low brain glucose causes a devastating neurological syndrome (Box 10.3).

Mechanisms of Proximal Tubule Calcium Transport

Calcium absorption by proximal tubules may be mediated by a combination of passive and active transport mechanisms. The transepithelial absorption of calcium can be conceptually described by the following relation:

calcium absorption=passive transport+active transport

where passive absorption is the sum of diffusion and solvent drag. These relations can be expressed formally as:44

J Ca2+=PCa2 +(ΔCCa2++ZiRTC¯Ca2+Δ ψ)diffusion+(1−σCa2+)C¯Ca2+Jvsolventdrag+JCa2+activeactivetransport

where, PCa2+ is the apparent calcium permeability, ΔCCa2+ is the transepithelial calcium concentration difference ([lumen-to-bath] – [bath-to-lumen]), zi is the valence, R the gas constant, T the absolute temperature, CCa2+ is the average transmural calcium concentration across the tubule ([lumen-to-bath+bath-to-lumen]/2), ΔΨ is the transepithelial voltage, σCa2+is the reflection coefficient for calcium, Jv is the net fluid absorption, and J{Ca2+active} is the metabolically active, transcellular calcium transport.

Most evidence suggests that proximal tubule calcium transport is passive, i.e., energetically independent, and occurs primarily by diffusion and solvent drag. These mechanisms imply that calcium absorption proceeds primarily by the paracellular route (Fig. 64.4) through the lateral intercellular space between adjoining cells. By contrast, active transport is a two-step process, wherein calcium enters the cell across apical plasma membranes and is then extruded across basolateral plasma membranes. Basolateral efflux occurs against a steep electrochemical gradient and is powdered by the hydrolysis of ATP by the Na-K-ATPase. A general schematic representation of these processes in proximal tubules is shown in Fig. 64.4.

It should be noted that, although small by comparison with paracellular calcium absorption, active cellular absorption by proximal tubules amounts to some 20 µmol/min,45 which, in fact, is approximately twice that of the distal nephron, where calcium absorption is entirely cellular.

The mechanism of calcium transport by the S2 segment of the proximal straight tubule resembles that of the proximal convoluted tubule. Studies using isolated perfused rabbit S2 proximal tubules, under experimental conditions designed to minimize net fluid movement and the electrochemical gradient for Ca2+, generally are consistent with the idea that passive driving forces are the major determinant of calcium absorption.42,46 However, evidence for a significant amount of active calcium transport has been reported.43 Sacks and Bourdeau47 showed that when passive driving forces across isolated S2 segments of rabbit proximal straight tubules were experimentally manipulated, the direction and rate of net calcium flux were predicted by the magnitude of the imposed electrochemical gradient. Thus, passive diffusion appears to be the major mechanism of transport in proximal straight tubules.

The presence of active, transcellular calcium absorption by proximal tubules, no matter how slight its magnitude, necessitates specific transport proteins in apical plasma membranes to admit calcium and others in basolateral membranes to mediate its extrusion. As far as is presently known, cellular calcium entry is mediated by calcium channels. Support for the presence of such channels takes the form of electrophysiological48–51 and pharmacological51–60 evidence. The molecular identity of such proximal tubule calcium channels is unknown but appears not to be TrpV5 or TrpV6.61 Basolateral calcium efflux in energetically dependent. Two proteins capable of mediating such transport are the plasma membrane Ca2+-ATPase (PMCA) or the NCX Na+/Ca2+ exchanger. Proximal tubule cells express PMCA1 and PMCA4 isoforms, which may serve as the primary mechanism of cellular Ca2+ efflux.62 Proximal tubules also express the NCX1 Na+/Ca2+ exchanger.63 The relative contribution of NCX1 and PMCA1/4 to cellular calcium absorption is not known.

In summary, proximal tubules exhibit high calcium permeability and low transepithelial electrical resistance. Most calcium absorption proceeds through passive mechanisms and traverses the paracellular pathway (Fig. 64.4). The majority of passive absorption is diffusive, with an additional slight contribution by solvent drag. The active component constitutes 20% of the total calcium absorption and proceeds through a transcellular pathway that involve entry through apical membrane calcium channels and exit across basolateral membranes that is mediated by isoforms of the plasma membrane Ca2+-ATPase and/or the Na+/Ca2+ exchanger.

Mechanism

In rats, micropuncture studies have revealed that about 25% of filtered chloride is reabsorbed by passive transport through “leaky” epithelium, and another 20% by active transport through chloride channels.11 The remaining 40% to 50% of chloride reabsorption is mediated by apical chloride-anion (formate, oxalate, bicarbonate) exchangers that work in parallel with NHE3 (Table 112-2).

In brief, apical chloride-anion recycling loads proximal tubule cells with chloride. Chloride-formate exchange, Na+-H+ exchange, and H+-formate symport occur in parallel with chloride-oxalate exchange, oxalate-sulfate exchange, and Na+-sulfate cotransport. Chloride exits the opposite side of the cell via basolateral chloride-bicarbonate exchangers and/or chloride channels.11–15

ARE IMMUNOHISTOCHEMISTRY-POSITIVE CELLS METASTASES OR DISPLACEMENT ARTIFACTS?

Rosser52 has proposed that IHC-detected “micrometastases” are not metastases at all but rather the passive transport to the ALN of tumor cells and epithelial debris dislodged by manipulation of the breast. In a small series, Carter and associates53 have found that subcapsular epithelial cells in the ALN of previously biopsied breast cancer patients were associated with macrophages, foreign body giant cells, and blood cells, suggesting traumatic displacement rather than biologic metastasis. Bleiweiss54 has reported 25 cases in which benign epithelial cells were found in the SLN of patients with proven breast cancers; in 88%, the epithelial cells were concordant with benign intraductal papillomas within the breast tissue and had presumably been dislodged by a prior biopsy procedure. Finally, Moore and coworkers55 have reviewed 4016 cases of SLN biopsy to determine if prior manipulation of the tumor site was related to the frequency of SLN metastasis. Comparing patients who had no biopsy, fine needle aspiration, core biopsy, or surgical biopsy, IHC-positive SLNs were found in 1%, 3%, 3.8%, and 4.6% of cases, respectively (P = 0.002). In contrast, the frequency of H&E-positive SLN was unrelated to method of biopsy. Although some proportion of SLN metastases may be artifactual, there is at present no reliable way for pathologists to distinguish passive and active spread, and decisions regarding systemic therapy in this setting must be individualized.

BILE ACID MALABSORPTION

In vitro studies of the absorption of bile acids from the ileum after restorative proctocolectomy imply that both active and passive transport mechanisms are normal (Hosie et al, 1990a,b, 1992b). However, in vivo studies indicate that the enterohepatic circulation of bile acids is impaired (Bain et al, 1995). Harvey et al (1991) report cholesterol supersaturation of bile after colectomy. Hylander et al (1991) showed that J-pouch construction produced increased stool mass, moderate bile acid deconjugation and malabsorption in nearly a half of the patients. However, these findings were not substantiated in studies from Japan (Natori et al, 1992).

Thin limb of the loop of Henle

The thin limb of the loop of Henle is composed of the thin descending limb (DLH) and the thin ascending limb (ALH). The DLH and ALH contribute importantly to the process of urinary concentration and dilution, primarily by passive transport of water and NaCl, respectively. The remarkable separation of passive water reabsorption in the DLH and passive NaCl reabsorption in the ALH is attributed to the abrupt change of the passive permeability properties of these segments at the tip of the loop of Henle.

The thin descending limb (DLH) begins at the end of the proximal tubule at the junction of the outer and inner stripes of the outer medulla. The thin descending limb is permeable to water but impermeable to Na+ and Cl−. This leads to water exit into a hypertonic interstitium and gradual concentration of the luminal solutes as water is progressively reabsorbed. The ascending thin limb (ALH) begins at the bend of the loop in long-looped nephrons. Short-looped nephrons have no ALH. The ALH is similar to the DLH in that it has a flat endothelial-like epithelium with minimal Na+/K+-ATPase activity. Therefore, it does not transport solutes actively. In contrast to the DLH, the ALH is water impermeable, moderately urea permeable, and highly NaCl permeable. As a result of these differences in permeability, the osmolarity of the tubular fluid can be passively decreased as the fluid moves from the bend of the loop of Henle towards the medullary thick ascending limb.