Biomedical subjects

S Quester

Publications and source records attributed to S Quester.

4 recordsLinked to original sources

PubMed1997

Cloning and characterization of two human polyspecific organic cation transporters.

Previously we cloned a polyspecific transporter from rat (rOCT1) that is expressed in renal proximal tubules and hepatocytes and mediates electrogenic uptake of organic cations with different molecular structures. Recently a homologous transporter from rat kidney (rOCT2) was cloned but not characterized in detail. We report cloning and characterization of two homologous transporters from man (hOCT1 and hOCT2) displaying approximately 80% amino acid identity to rOCT1 and rOCT2, respectively. Northern blots showed that hOCT1 is mainly transcribed in liver, while hOCT2 is found in kidney. Using in situ hybridization and immunohistochemistry, expression of hOCT2 was mainly detected in the distal tubule where the transporter is localized at the luminal membrane. After expression in Xenopus laevis oocytes, hOCT1 and hOCT2 mediate tracer influx of N-1-methylnicotinamide (NMN), tetraethylammonium (TEA), and 1-methyl-4-phenylpyridinium (MPP). For cation transport by hOCT2 apparent K(m) and K(i) values were determined in tracer flux measurements. In addition, electrical measurements were performed with voltage-clamped oocytes. Similar to rOCT1, cation transport by hOCT2 was pH independent, electrogenic, and polyspecific; however, the cation specificity was different. In voltage-clamped hOCT2-expressing oocytes, inward currents were induced by superfusion with MPP, TEA, choline, quinine, d-tubocurarine, pancuronium, and cyanine863. Cation transport in distal tubules is indicated for the first time. Here hOCT2 mediates the first step in cation reabsorption. hOCT1 may participate in hepatic excretion of organic cations.

Amino Acid Sequence↗

PubMed1996

Electrogenic properties and substrate specificity of the polyspecific rat cation transporter rOCT1.

The previously cloned rat cation transporter rOCT1 detected in renal proximal tubules and hepatocytes (Gründemann, D., Gorboulev, V., Gambaryan, S., Veyhl, M., and Koepsell, H. (1994) Nature 372, 549-552) was expressed in Xenopus oocytes, and transport properties were analyzed using tracer uptake studies and electrophysiological measurements. rOCT1 induced highly active transport of a variety of cations, including the classical substrates for cation transport, such as N-1-methylnicotinamide, 1-methyl-4-phenylpyridinium (MPP), and tetraethylammonium (TEA), but also the physiologically important choline. In oocytes rOCT1 also mediated efflux of MPP, which could be trans-stimulated by MPP and TEA. Cation transport via rOCT1 was electrogenic. In voltage-clamped oocytes, transport of TEA and choline via rOCT1 produced inwardly directed currents, which were independent of extracellular ion composition or pH. The choline- and TEA-induced currents were voltage-dependent at nonsaturating concentrations, and the apparent affinity of these cations was decreased at depolarized voltages. Other substrates transported by rOCT1 were the polyamines spermine and spermidine. Interestingly, the previously described potent inhibitors of rOCT1, cyanine 863, quinine, and D-tubocurarine were substrates themselves. The data indicate that rOCT1 is an effective transport system that is responsible for electrogenic uptake of a wide variety of organic cations into epithelial cells of renal proximal tubules and hepatocytes.

Animals↗

PubMed1996

Monoamine neurotransmitter transport mediated by the polyspecific cation transporter rOCT1.

The polyspecific cation transporter rOCT,1 which is localized in the basolateral membrane of rat renal proximal tubules and in sinusoidal membranes of hepatocytes, was analyzed for transport of monoamine neurotransmitters. In voltage-clamp experiments with rOCT1-expressing Xenopus oocytes, superfusion with dopamine, serotonin, noradrenaline, histamine and the permanent cation acetylcholine induced saturable inwardly directed currents with apparent Km values ranging from 20 to 100 microM. Transport of dopamine was also demonstrated by uptake measurements in oocytes and in the mammalian cell line (HEK 293) which was permanently transfected with rOCT1. The high uptake rates measured in rOCT1-expressing oocytes and in transfected HEK 293 cells suggest that rOCT1 is a high capacity transporter which mediates the first step in the excretion of monoamine neurotransmitters.

Acetylcholine↗

PubMed1986

Allotypic differences in murine mu genes.

We report the complete DNA sequence of a c-DNA clone of the heavy chain mu b allele of the C57BL/6 mouse. Comparisons have been made with the nucleotide sequences of the germ line BALB/c mu a and the plasmacytoma TEPC-183 mu a alleles reported elsewhere over the entire length of the coding and the 3' untranslated region. In contrast to the extensive differences between the gamma 2a a and b alleles we have reported earlier we see a very high degree of homology between the mu alleles. Only one of the nucleotide differences between C57BL/6 mu b and BALB/c mu a leads to an amino acid substitution. This single amino acid exchange must form the allotypic determinant of the mu b allele. A comparison of four different DNA sequences indicates that they are all distinct IgM alleles.

Amino Acid Sequence↗