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extensive inhibition of the drug-metabolizing enzymes may be observed because of the high concentration of the inhibitor in the vicinity of the drug-metabolizing enzymes.
membrane are in the SLC22 family and termed novel organic cation transporters (OCTNs) OCTN1 (SLC22A4) and OCTN2 (SLC22A5). These bifunctional transporters mediate both organic cation secretion and carnitine reabsorption. In the reuptake mode, the transporters function as Na+ cotransporters, relying on the inwardly driven Na+gradient created by Na+,K+-ATPase to move carnitine from tubular lumen into the cell. In the secretory mode, the transporters function as proton–organic cation exchangers: protons move from tubular lumen to cell interior in exchange for organic cations, which move from cytosol to tubular lumen.
OCT1 has four splice variants, one of which is functionally active, OCT1G/L554. OCT1 is expressed primarily in the liver, with some expression in heart, intestine, and skeletal muscle. In humans, very modest levels of OCT1 transcripts are detected in the kidney. The transport mecha-nism of OCT1 is electrogenic and saturable for transport of small-molecular-weight organic cations including tetraethylammonium (TEA) and dopamine. OCT1 also can mediate organic cation–organic cation exchange. Organic cations can trans-inhibit OCT1. When present on the cytosolic side of a membrane, the hydrophobic organic cations quinine and quinidine, which are poor substrates of OCT1, can trans-inhibit influx of organic cations via OCT1.
Human OCT1 (SLC22A1) accepts a wide array of monovalent organic cations with molecu-lar weights of <400, including many drugs (e.g., procainamide, metformin, and pindolol).
Inhibitors of OCT1 are generally more hydrophobic. Since OCT1 mammalian orthologs have
>80% amino acid identity, evolutionarily nonconserved residues among mammalian species clearly are involved in specificity differences.
OCT2 is located adjacent to OCT1 on chromosome 6 (6q26). A single splice variant of human OCT2, termed OCT2-A, in the kidney is a truncated form of OCT2 that appears to have a lower Kmfor substrates than OCT2. In the kidney, OCT2 is localized to the proximal tubule, distal tubules, and collecting ducts. In the proximal tubule, OCT2 is restricted to the basolateral mem-brane. The transport mechanism of OCT2 is similar to that of OCT1.
Like OCT1, OCT2 generally accepts a wide array of monovalent organic cations with molec-ular weights of <400. OCT2 is also present in neuronal tissues and may play a housekeeping role in neurons, taking up excess concentrations of neurotransmitters and recycling neurotransmitters by taking up breakdown products that then reenter monoamine synthetic pathways.
Human OCT3 is expressed in the liver, kidney (weakly), intestine, and placenta. Like OCT1 and OCT2, OCT3 appears to support electrogenic potential-sensitive organic cation transport. Some studies have suggested that OCT3 is the extraneuronal monoamine transporter based on its sub-strate specificity and potency of interaction with monoamine neurotransmitters. Because of its rel-atively low abundance in the kidney, OCT3 may play only a limited role in renal drug elimination.
OCTN1 (SLC22A4) is expressed in the kidney, trachea, and bone marrow and operates as an organic cation–proton exchanger. OCTN1 likely functions as a bidirectional pH- and ATP-dependent transporter at the apical membrane in renal tubular epithelial cells.
OCTN2 (SLC22A5) is expressed predominantly in the renal cortex, with very little expression in the medulla, and is localized to the apical membrane of the proximal tubule. OCTN2 transports
L-carnitine with high affinity in a Na+-dependent manner, whereas, Na+ does not influence OCTN2-mediated transport of organic cations. Thus, OCTN2 is thought to function as both a Na+ -dependent carnitine transporter and a Na+-independent organic cation transporter. Mutations in OCTN2 cause primary systemic carnitine deficiency.
ORGANIC ANION TRANSPORT Structurally diverse organic anions are secreted in the proximal tubule. The primary function of organic anion secretion appears to be the removal from the body of xenobiotics, including many weakly acidic drugs (e.g., pravastatin, captopril, p-amino-hippurate [PAH], and penicillins) and toxins (e.g., ochratoxin).
Two primary transporters on the basolateral membrane (Figure 2–8) mediate the flux of organic anions from interstitial fluid to tubule cells: OAT1 (SLC22A6) and OAT3 (SLC22A8).
Hydrophilic organic anions are transported across the basolateral membrane against an electro-chemical gradient in exchange with intracellular a-ketoglutarate, which moves down its concen-tration gradient from cytosol to blood. The outwardly directed gradient of a-ketoglutarate is maintained by a basolateral Na+-dicarboxylate transporter (NaDC3). The Na+gradient that drives NaDC3 is maintained by Na+,K+-ATPase.
TRANSPORTERS INVOLVED IN PHARMACODYNAMICS:
DRUG ACTION IN THE BRAIN
Neurotransmitters are packaged in vesicles in presynaptic neurons, released in the synapse by vesi-cle fusion with the plasma membrane, and—except for acetylcholine—are then taken back into the presynaptic neurons or postsynaptic cells (see Chapter 6). Transporters involved in the neuronal
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reuptake of neurotransmitters and the regulation of their levels in the synaptic cleft belong to two major superfamilies, SLC1 and SLC6. Transporters in both families play roles in reuptake of g-aminobutyric acid (GABA), glutamate, and the monoamine neurotransmitters norepinephrine, serotonin, and dopamine. These transporters may serve as pharmacologic targets for neuropsychi-atric drugs.
SLC6 family members localized in the brain and involved in neurotransmitter reuptake into presynaptic neurons include the norepinephrine transporter (NET, SLC6A2), the dopamine trans-porter (DAT, SLC6A3), the serotonin transtrans-porter (SERT, SLC6A4), and several GABA reuptake transporters (GAT1, GAT2, and GAT3). Each of these transporters appears to have 12 transmem-brane domains and a large extracellular loop with glycosylation sites between transmemtransmem-brane domains 3 and 4. Typically, these proteins are ∼600 amino acids in length. SLC6 family members depend on the Na+gradient to actively transport their substrates into cells. Cl–is also required, although to a variable extent depending on the family member.
Through reuptake mechanisms, the neurotransmitter transporters in the SLC6A family regulate the concentrations and persistence of neurotransmitters in the synaptic cleft; the extent of transmit-ter uptake also influences subsequent vesicular storage of transmittransmit-ters. Further, the transportransmit-ters can function in the reverse direction by exporting neurotransmitters in a Na+-independent fashion.
Many of these transporters also are present in other tissues (e.g., kidney and platelets), where they may serve other roles.
SLC6A1 (GAT1), SLC6A11 (GAT3), AND SLC6A13 (GAT2) GAT1 is the most important GABA transporter in the brain; it predominantly is expressed in presynaptic GABAergic neurons.
GAT1 is found in abundance in the neocortex, cerebellum, basal ganglia, brainstem, spinal cord, FIGURE 2–8 Model of organic anion secretory transporters in the proximal tubule. Rectangles depict transporters in the SLC22 family, OAT1 (SLC22A6) and OAT3 (SLC22A8), and hexagons depict transporters in the ABC super-family, MRP2 (ABCC2) and MRP4 (ABCC4). NPT1 (SLC17A1) is depicted as a circle. OA–, organic anion; a-KG, a-ketoglutarate.
retina, and olfactory bulb. GAT3 is found only in the brain, largely in glial cells. GAT2 is found in peripheral tissues, including the kidney and liver, and in the choroid plexus and meninges within the CNS. The presence of GAT2 in the choroid plexus and its absence in presynaptic neurons sug-gest that this transporter may play a primary role in maintaining GABA homeostasis in the CSF.
GAT1 is the target of the antiepileptic drug tiagabine, which presumably acts to increase GABA levels in the synaptic cleft of GABAergic neurons by inhibiting the reuptake of GABA. GAT3 is the target for the nipecotic acid derivatives that are anticonvulsants.
SLC6A2 (NET) NET is expressed in central and peripheral nervous tissues and adrenal chro-maffin cells. In the brain, NET colocalizes with neuronal markers, consistent with a role in reup-take of monoamine neurotransmitters. The transporter functions in the Na+-dependent reuptake of norepinephrine and dopamine and as a higher-capacity norepinephrine channel. A major role of NET is to limit the synaptic dwell time of norepinephrine and to terminate its actions, salvaging norepinephrine for subsequent repackaging. NET participates in the regulation of many neurologi-cal functions, including memory and mood. NET is a drug target for the antidepressant desipramine, other tricyclic antidepressants, and cocaine. Orthostatic intolerance, a rare familial disorder characterized by an abnormal blood pressure and heart rate response to postural changes, has been associated with a mutation in NET.
SLC6A3 (DAT) DAT is located primarily in the brain in dopaminergic neurons. The primary function of DAT is the reuptake of dopamine, terminating its actions. Although present on presy-naptic neurons at the sypresy-naptic junction, DAT is also present in abundance away from the sypresy-naptic cleft, suggesting that DAT may play a role in clearing excess dopamine in the vicinity of neurons.
Physiologically, DAT is involved in the various functions that are attributed to the dopaminergic system, including mood, behavior, reward, and cognition. Drugs that interact with DAT include cocaine and its analogs, amphetamines, and the neurotoxin MPTP.
SLC6A4 (SERT) SERT plays a role in the reuptake and clearance of serotonin in the brain.
Like the other SLC6A family members, SERT transports its substrates in a Na+-dependent fashion and is dependent on Cl–and possibly on the countertransport of K+. Substrates of SERT include serotonin (5-HT), various tryptamine derivatives, and neurotoxins such as 3,4-methylene-dioxymethamphetamine (MDMA; ecstasy) and fenfluramine. SERT is the specific target of the selective serotonin reuptake inhibitors (e.g., fluoxetine and paroxetine) and one of several targets of tricyclic antidepressants (e.g., amitriptyline). Genetic variants of SERT have been associated with an array of behavioral and neurological disorders. The precise mechanism by which a reduced activity of SERT, caused by either a genetic variant or an antidepressant, ultimately affects mood and behavior is not known.
BLOOD–BRAIN AND BLOOD–CSF BARRIERS
Drugs acting in the CNS must either cross the BBB or the blood–CSF barrier, which are formed by brain capillary endothelial cells or epithelial cells of the choroid plexus, respectively. Efflux trans-porters play a role in these dynamic barriers. P-glycoprotein extrudes its substrate drugs on the luminal membrane of the brain capillary endothelial cells into the blood, complicating CNS ther-apy for some drugs (see Chapter 1). Other transporters in the BBB and the blood–CSF barrier include members of organic anion transporting polypeptide (OATP1A4 and OATP1A5) and organic anion transporter (OAT3) families, which facilitate the uptake of organic compounds such as b-lactam antibiotics, statins, PAH, H2-receptor antagonists, and bile acids on the plasma mem-brane facing the brain–CSF. Further understanding of influx and efflux transporters in these barri-ers should translate into more effective delivery of drugs to the CNS while avoiding undesirable CNS side effects and may help to define the mechanisms of drug–drug interactions and interindi-vidual differences in the therapeutic CNS effects.
For a complete Bibliographical listing see Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 11th ed., or Goodman & Gilman Online at www.accessmedicine.com.
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