A membrane protein that effects the active transport of a substance across a cell membrane while transporting ions in the opposite direction. An antiporter is a type of cotransporter; the ions, typically hydrogen ions (H⁺) or sodium ions (Na⁺), flow down their concentration gradient, and in so doing provide the energy for the transport of another substance in the other direction. For example, heart-muscle cells have a Na⁺/Ca⁺ antiporter, which is driven by the inward flow of sodium ions to pump calcium ions (Ca⁺) out of the cell. Hence, the energy for antiporters derives ultimately from the energy-consuming mechanisms that establish the concentration gradient of the driving ions. Compare symporter; uniporter.