countertransport

Countertransport, also known as antiport, is a membrane transport mechanism in which a transporter protein moves two or more substrates in opposite directions across a lipid bilayer. Unlike symporters that carry substrates in the same direction, countertransport exchanges one species for another as the transporter alternates access to each side of the membrane.

Most countertransport is a form of secondary active transport. The movement of one substrate down its electrochemical gradient provides the energy to move a second substrate against its gradient. Commonly, ions such as Na+, H+, or Cl− are exchanged for Ca2+, HCO3−, or other ions, enabling rapid adjustments of intracellular composition without direct ATP expenditure.

Representative examples include the Na+/Ca2+ exchanger, which typically extrudes Ca2+ from cells in exchange for Na+ to help regulate calcium homeostasis; the Na+/H+ exchanger, which swaps extracellular Na+ for intracellular H+ to influence intracellular pH; and the Cl−/HCO3− exchanger, which mediates bicarbonate and chloride movement to help maintain acid–base balance.

Physiological roles of countertransport encompass regulation of intracellular calcium concentration and pH, maintenance of cell volume, and stabilization of membrane potential in certain tissues. The activity of these transporters is shaped by ionic gradients created by primary active transporters, membrane potential, phosphorylation, and interacting regulatory proteins.

Terminology varies; antiport is a common term in biochemistry, while countertransport emphasizes the exchange aspect. Dysfunctions in countertransport can contribute to disease processes, such as impaired calcium handling in cardiac cells or disturbed acid–base homeostasis.