Active Transport

Active transport is a critical biological process that allows cells to move solutes against an electrochemical gradient. This process requires direct energy input and is characterized by its selectivity, saturability, and susceptibility to competitive inhibition.

Primary active transporters, like Na⁺, K⁺ and -ATPase, directly utilize ATP to move ions across the membrane. These transporters play significant roles in various physiological processes. For instance, Na⁺, K⁺ and -ATPase maintain cellular ion homeostasis and are targeted by digoxin for heart failure treatment. Digoxin inhibits the Na⁺/K⁺ -ATPase pump in cardiac cells, leading to increased intracellular sodium. This, in turn, enhances calcium availability in the heart muscle and improves contractility, which helps in managing heart failure. Another group of primary active transporters, the ABC family, hydrolyzes ATP to export substrates, such as hormones and drugs, across membranes. P-glycoprotein (P-gp), a member of the ABC family, limits the absorption of orally administered drugs by exporting them into the GI tract lumen.

Secondary active transporters, part of the SLC superfamily, use stored electrochemical energy (usually a Na⁺ gradient) established by a primary active transporter (like the Na⁺/K⁺ pump) to translocate solutes across membranes. The Na⁺-Ca²⁺ exchange protein (SLC8), an antiporter, uses the inward flow of Na⁺ to drive an outward flow of Ca²⁺, maintaining low basal cytosolic Ca²⁺ levels. Other SLC cotransporters are symporters, where the driving force ion and solute move in the same direction. For instance, the CNT1 transporter, driven by the Na⁺ gradient, moves pyrimidine nucleosides and specific chemotherapeutic agents into cells. DAT, NET, and SERT symporters also harmonize the movement of Na⁺ and neurotransmitters like dopamine, norepinephrine, and serotonin in the same direction. These transporters have clinical relevance as targets for CNS-active agents in depression therapy.

In summary, through primary and secondary mechanisms, active transport plays a vital role in maintaining cellular function, drug transport, and therapeutic applications.