The reabsorption of NaCl from the apical compartment reduces the osmotic pressure in that compartment, while the addition of NaCl to the basolateral compartment increases the osmotic pressure in that compartment. Consequently, a transepithelial osmotic pressure gradient is established, which facilitates the movement of water from the apical to the basolateral compartment (reabsorption). Conversely, in NaCl-secreting epithelia, the transepithelial secretion of NaCl creates a transepithelial osmotic pressure gradient that drives water secretion.

In certain epithelia, such as the proximal tubule of the renal nephron, the movement of water across the epithelium via the paracellular pathway can lead to the movement of additional solute. This process is known as solvent drag and occurs because solutes dissolved in the water can pass through the tight junction along with the water.

Like the establishment of transepithelial concentration and voltage gradients, the establishment of transepithelial osmotic pressure gradients requires the transcellular transport of solutes by the epithelial cells. Different aquaporin isoforms are often expressed in the apical and basolateral membrane, and multiple isoforms may be expressed in one or more of the membrane domains.

Epithelial transport regulation is necessary to maintain homeostasis. This regulation involves neural or hormonal mechanisms, or both, depending on the epithelium. For example, solute and water transport by the epithelial cells lining the intestine and colon are regulated by the enteric nervous system of the gastrointestinal tract. Transport by the epithelial cells of the renal nephron is regulated by the sympathetic nervous system. Aldosterone, a steroid hormone produced by the adrenal cortex, stimulates NaCl transport by the epithelial cells of the colon, renal nephron, and sweat ducts. Paracrine regulation, which involves locally produced and locally acting substances, can also regulate epithelial cell transport. An example of this is the stimulation of HCl secretion in the stomach by histamine. Histamine released by cells near the stomach's epithelial cells acts on the HCl-secreting cells (parietal cells) and stimulates them to secrete HCl.

When a regulatory signal affects an epithelial cell, it can respond in different ways, including the retrieval of transporters from the membrane through endocytosis or the insertion of transporters into the membrane from an intracellular vesicular pool (via a process called Change in activity of membrane transporters (e.g., channel gating), and the synthesis of specific transporters, which are then inserted into the membrane. The first two mechanisms can occur rapidly (within seconds to minutes), while transporter synthesis takes additional time (minutes to days).
Fig. 2.13A , the reabsorption of NaCl from the apical compartment lowers the osmotic pressure in that compartment, whereas the addition of NaCl to the basolateral compartment raises the osmotic pressure in that compartment. As a result, a transepithelial osmotic pressure gradient is established that drives the movement of water from the apical to the basolateral compartment (i.e., reabsorption). The opposite occurs with NaCl-secreting epithelia (see
Fig. 2.13B ), in which the transepithelial secretion of NaCl establishes a transepithelial osmotic pressure gradient that drives water secretion.
In some epithelia (e.g., proximal tubule of the renal nephron), the movement of water across the epithelium via the paracellular pathway can drive the movement of additional solute. This process is termed solvent drag and reflects the fact that solutes dissolved in the water will traverse the tight junction with the water.
As is the case with the establishment of transepithelial concentration and voltage gradients, the establishment of transepithelial osmotic pressure gradients requires transcellular transport of solutes by the epithelial cells.
iDifferent aquaporin isoforms are often expressed in the apical and basolateral membrane. In addition, multiple isoforms may be expressed in one or more of the membrane domains.
CHAPTER 2 Homeostasis: Volume and Composition of Body Fluid Compartments
NaCl reabsorption •Fig. 2.13 The Role of the Paracellular Pathway in Epithelial
Transport. A, Na+ transportthroughthecellgeneratesatransepithelial voltagethatthendrivesthepassivemovementofCl− throughthetightjunction.NaClreabsorptionresults.B, Cl− transportthroughthecellgeneratesatransepithelialvoltagethatthendrivesthepassivetransportofNa+ throughthetightjunction.NaClsecretionresults.
Regulation of Epithelial Transport
Epithelial transport must be regulated to meet the homeostatic needs of the individual. Depending on the epithelium, this regulation involves neural or hormonal mechanisms, or both. For example, the enteric nervous system of the gastrointestinal tract regulates solute and water transport by the epithelial cells that line the intestine and colon. Similarly, the sympathetic nervous system regulates transport by the epithelial cells of the renal nephron. Aldosterone, a steroid hormone produced by the adrenal cortex (see ), is an example of a hormone that stimulates NaCl transport by the epithelial cells of the colon, renal nephron, and sweat ducts. Epithelial cell transport can also be regulated by locally produced and locally acting substances, a process termed paracrine regulation. The stimulation of HCl secretion in the stomach by histamine is an example of this process. Cells that are located near the epithelial cells of the stomach release histamine, which acts on the HCl-secreting cells of the stomach (parietal cells) and stimulates them to secrete HCl.
When acted upon by a regulatory signal, the epithelial cell may respond in several different ways, including: • Retrieval of transporters from the membrane, by endocytosis, or insertion of transporters into the membrane from an intracellular vesicular pool, by a process called
Change in activity of membrane transporters (e.g., channel gating)
Synthesis of specific transporters, and their insertion into the membrane
Na+ The first two mechanisms can occur quite rapidly (seconds 2Cl– to minutes), but the synthesis of transporters takes addi-K+ tional time (minutes to days).