Transepithelial potential difference

Transepithelial Potential Difference (TEPD) is the voltage difference measured across a cellular epithelium. This electrical potential difference arises due to the active and passive transport of ions across the epithelial layer, which separates two bodily compartments. The TEPD is a fundamental physiological parameter that plays a critical role in various bodily functions, including the regulation of water and electrolyte balance, and the maintenance of the acid-base balance.

Overview

The epithelium is a layer of cells that lines the surfaces and cavities of the body’s organs. It serves as a barrier and a gateway for the movement of substances between the body’s internal and external environments. The transepithelial potential difference is generated by the movement of ions, primarily sodium (Na+), potassium (K+), chloride (Cl-), and bicarbonate (HCO3-), across the epithelium. This movement is mediated by various ion channels, ion pumps, and transporters located on the apical (facing the lumen) and basolateral (facing the blood) sides of the epithelial cells.

Mechanism

The generation of TEPD involves several key mechanisms:

• Active Transport: The Na+/K+-ATPase pump on the basolateral membrane actively transports Na+ out of the cell and K+ into the cell, creating a concentration gradient.
• Passive Transport: This gradient allows for the passive movement of ions through channels in the membranes. For example, Na+ may enter the cell from the lumen through specific channels on the apical membrane.
• Electrochemical Gradient: The movement of ions creates an electrochemical gradient across the epithelium, leading to the generation of a potential difference.

Physiological Significance

The TEPD is crucial for several physiological processes:

• Absorption and Secretion: The potential difference drives the absorption of nutrients and water from the lumen into the blood and the secretion of waste products in the opposite direction.
• Electrolyte Balance: It plays a role in maintaining the balance of electrolytes in the body, which is vital for many cellular functions.
• Acid-Base Balance: The transport of ions like bicarbonate and hydrogen ions across the epithelium helps in regulating the body’s pH level.

Clinical Relevance

Alterations in the transepithelial potential difference can indicate or contribute to various diseases and conditions:

• Cystic Fibrosis: A reduced TEPD is observed in the sweat glands of individuals with cystic fibrosis, due to a defect in the CFTR chloride channel.
• Diarrhea and Dehydration: Abnormal ion transport can lead to excessive secretion or poor absorption in the intestines, resulting in diarrhea and potential dehydration.
• Electrolyte Imbalances: Disorders affecting ion transport can lead to imbalances in electrolytes, affecting heart, muscle, and nerve function.

Measurement

The transepithelial potential difference is measured using electrodes placed on either side of the epithelial layer. This non-invasive technique provides valuable information about the ion transport properties of the epithelium and its integrity.