Comparison between uptake2 and rOCT1: effects of catecholamines, metanephrines and corticosterone

Abstract

Active and specialized transmembrane transport systems are responsible for the functional inactivation of catecholamines. Uptake2, the classical extraneuronal uptake system, and rOCT1, a recently cloned organic cation transporter, share a number of properties. The present study was undertaken to investigate putative differences between these two transporters that might clarify their relative physiological roles. Uptake of [3H]MPP+ ([3H]1-methyl-4-phenylpyridinium) by Caki-1 cells (to study uptake2) and by primary cultured rat hepatocytes (to study rOCT1) was kinetically and pharmacologically characterized. In both cell types, [3H]MPP+ was avidly taken up and accumulated. All compounds tested (catecholamines, metanephrines and corticosterone) inhibited [3H]MPP+ uptake, albeit with different potencies. In Caki-1 cells, the ranking order of inhibitory potency was: (-)isoprenaline> (-)adrenaline >> (-)noradrenaline > dopamine. Metanephrine and normetanephrine were equipotent. Corticosterone had an IC50 of 102 nM. In cultured hepatocytes, the ranking order of inhibitory potency was: (-)isoprenaline > dopamine > (-)adrenaline >> (-)noradrenaline. Metanephrine was about seven times more potent than normetanephrine. Corticosterone had an IC50 of 72 microM, being about 700-fold less potent in inhibiting rOCT1 than uptake2. The results showed that uptake2 and rOCT1 can be clearly distinguished on a functional basis. On the one hand, uptake2 prefers adrenaline among the endogenous catecholamines, whereas rOCT1 has similar affinity for adrenaline and dopamine. On the other hand, corticosterone and normetanephrine are significantly more potent in inhibiting uptake2 than rOCT1. The results are compatible with a possible physiological role of corticosteroids in the modulation of adrenaline effects in tissues equipped with uptake2, without significant interference with the hepatic and renal excretion of catecholamines.

Caki-1 Cells as a Model System for the Interaction of Renally Secreted Drugs with OCT3

“…All kinetic data correlated well with what has been previously reported for OCT3 in Caki-1 cells and other model systems for OCT3 [15, 20]. The uptake was further characterized to be carrier-mediated by the significant decrease in the uptake observed at 4°C.…”

“…Decynium-22, a known selective inhibitor of OCT3, was chosen to confirm the [ 3 H]-MPP + uptakeobserved was mediated by OCT3. An IC 50 of approximately 0.05 µ M was determined, correlating well with what has been previously published in the literature [15]. …”

“…Important to note here is that the Caki-1 cells have previously been used for the characterization of EMT/OCT3 interactions with monoamines and steroid hormones [15]. For these purposes, MPP + and decynium-22 were selected as prototypical substrate and specific inhibitor, respectively; in the current work, kinetic parameters (K m and IC 50 ) were performed as initial experiments to prove the functionality of OCT3 in the Caki-1 cells.…”

Corticosterone Potentiation of Cocaine-Induced Reinstatement of Conditioned Place Preference in Mice is Mediated by Blockade of the Organic Cation Transporter 3

“…This is important, because, while normetanephrine is a potent inhibitor of OCT3-mediated transport (Martel et al, 1999), it has been reported to have other pharmacological effects, including weak agonist actions at alpha-1 adrenergic receptors (Langer and Rubio 1973) and lowaffinity antagonism of alpha-2 adrenergic receptors (Lenz et al, 1991). The present findings indicate that, to the extent that these secondary pharmacological effects occur, the regulation of cocaine-induced reinstatement by normetanephrine is likely the result of OCT3 blockade, thereby validating the use of this compound as a pharmacological tool for studying OCT3-mediated processes.…”

TRANSPORT OF THE DOPAMINE D2 AGONIST PRAMIPEXOLE BY RAT ORGANIC CATION TRANSPORTERS OCT1 AND OCT2 IN KIDNEY

“…OCT2 is predominantly expressed in the kidney in rodents and humans. In rat kidney, rOCTs are expressed in the basolateral membrane of the S1 and S2 segments (rOCT1) and S2 and S3 segments (rOCT2) of the proximal tubule, and they have a significant role in the first step of tubular secretion (Martel et al, 1999;Chen et al, 2002;Slitt et al, 2002). A study using OCT gene-disrupted mice demonstrated that the kidney-to-plasma concentration ratios of prototypic organic cation tetraethylammonium (TEA) in OCT1(Ϫ/Ϫ), OCT2(Ϫ/Ϫ), and OCT1/2(Ϫ/Ϫ) mice at steady state were 2-, 2-, and 6-fold lower, respectively, than that in wild-type mice, suggesting that both OCT1 and OCT2 are involved in the uptake of TEA by the kidney (Jonker et al, 2003).…”