| SLC22A3, also termed OCT3, is a widely expressed polyspecific organic cation transporter of the SLC22 family that mediates low-affinity, high-capacity uptake of endogenous monoamines and a range of cationic drugs, thereby contributing to neurotransmitter clearance, drug disposition, and tissue exposure in organs such as brain, heart, and liver. The protein is a typical solute carrier with multiple transmembrane helices forming a central aqueous pore that alternates access across the plasma membrane in an electrogenic, membrane potential–dependent manner, allowing facilitated diffusion of positively charged substrates including norepinephrine, epinephrine, dopamine, histamine, and neuromodulators such as agmatine and salsolinol, as well as drugs like metformin, lidocaine, quinidine, oxaliplatin, and lamivudine. OCT3 acts as a major extraneuronal monoamine transporter in the central nervous system, where its broad substrate selectivity and high capacity regulate interstitial levels of norepinephrine, serotonin, and other cationic signaling molecules, shaping the spillover of monoamines from synapses, the duration of receptor activation, and behavioral responses. OCT3 activity is linked to susceptibility to depression and other neuropsychiatric conditions, and several clinically used antidepressants inhibit OCT3-mediated uptake in vitro at micromolar concentrations, indicating that OCT3 can serve as an additional target influencing monoaminergic tone beyond high-affinity SLC6 transporters. Expression outside the nervous system extends OCT3 function to cardiovascular and metabolic contexts: strong expression in the human heart, particularly in vascular endothelial cells, supports uptake of catecholamines and cationic drugs into myocardium and contributes to local catecholamine clearance and cardiac drug handling. OCT3-dependent metformin transport into cardiac tissue is inhibited by commonly used cardiovascular drugs such as verapamil and carvedilol, highlighting a transporter-mediated mechanism for drug–drug interactions and potential modification of cardiac drug exposure. SLC22A3 variants are implicated in coronary artery disease and related cardiovascular phenotypes, and specific polymorphisms alter transporter expression, substrate affinity, or uptake capacity, which can change clearance of endogenous cations and OCT3-substrate drugs and thereby modify inflammatory responses, blood pressure regulation, or susceptibility to vascular disease. Across tissues, OCT3 is regulated post-transcriptionally and by signaling pathways such as Ca²⁺/calmodulin, which can acutely enhance cation uptake, providing a mechanism for dynamic adjustment of transporter activity in response to cellular state. These properties define SLC22A3/OCT3 as a structurally conserved, electrogenic organic cation transporter that links monoamine and drug handling in brain, heart, liver, and other organs to neuropsychiatric outcomes, cardiovascular risk, and variability in therapeutic response, making it a key target for studies on monoaminergic signaling, cardiotoxicity mitigation, and pharmacogenetic optimization of cationic drugs. |
