Tubular function

The proximal convoluted tubules are lined with cuboidal cells, which possess an inner brush border with microvilli, and possess a high mitochondrial content. The distal convoluted tubules are lined with cuboidal cells, which possess many mitochondria but no microvilli. The tubular cells demonstrate polarity, whereby the apical or luminal membrane and the basolateral or peritubular membrane

» are structurally and functionally different. The luminal and basolateral aspects of l the tubular cell membrane are separated by the tight junction, which is com-

y posed primarily of the zona occludens.

l The plasma ultrafiltrate is modified in the tubules by the following processes:

g Active transport:

Primary active transport: directly coupled to ATP hydrolysis; Secondary active transport (co-transport): sodium with glucose, amino acids or carboxylic acids.

Simple diffusion using transcellular (through the basolateral or luminal membrane) or paracellular (across tight junctions and lateral intercellular spaces) routes.

Movement via ion channels.

Co-transport (symport): carrier-mediated transport. Counter-transport (antiport): carrier-mediated transport. Endocytosis.

The transport maximum describes the maximum rate at which a system is able to transport a solute.

The active transport of sodium across the tubular epithelial cells is achieved by the influence of the Na+/K+-ATPase, located in the basolateral membrane. This transport is coupled with specific membrane carrier proteins in order to enable the influx (symport) or efflux (antiport) of other molecules. The latter are examples of secondary active transport. Amino acids, glucose, phosphate and chloride can all be co-transported ( symport) with sodium entry. Hydrogen and calcium ions can be counter-transported (antiport) against sodium entry.

Mechanisms for reabsorption of sodium

Sodium is the major osmotically active ion in the body and has a major influence on water balance. More than 99% of the filtered sodium load is reabsorbed. There are multiple luminal systems that draw sodium from tubular lumen into tubular cell, acting in conjunction with basolateral systems that convey sodium from the tubular cell into the interstitium and thereby into blood vessels. The basolateral sodium pump produces a favourable electrochemical gradient that facilitates luminal entry of sodium via a variety of mechanisms.

Proximal convoluted tubule (65%-75% )

Luminal systems that draw sodium from the tubular lumen into the cell: Sodium symport solute co-transport system Sodium glucose co-transport system

Sodium amino acid co-transport system r o

Sodium-phosphate co-transport system S

Na+/H+ exchange system (counter-transporter) s

Cl driven sodium reabsorption V

A basolateral system that actively pumps sodium from the cell into the ?

interstitium: Ï

Thick ascending limb of the loop of Henle (20%)

Luminal system:

Na+/K+/2Cl~co-transport system: frusemide-sensitive carrier. This is the site of action of loop diuretics. Chloride ion leaves the cell via the basolateral Cl channel or KCl co-transporter. Basolateral system that actively pumps sodium into the interstitium: Na+/K+-ATPase.

Distal convoluted tubule and cortical collecting duct (10%)

Luminal systems: Na+/Cl~co-transport system: thiazide-sensitive carrier Selective sodium conductive channels, regulated by aldosterone: amiloride-sensitive channel Basolateral system: Na+K+-ATPase

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