γ-Aminobutyric Acid Transporter 2 Mediates the Hepatic Uptake of Guanidinoacetate, the Creatine Biosynthetic Precursor, in Rats

Abstract

Guanidinoacetic acid (GAA) is the biosynthetic precursor of creatine which is involved in storage and transmission of phosphate-bound energy. Hepatocytes readily convert GAA to creatine, raising the possibility that the active uptake of GAA by hepatocytes is a regulatory factor. The purpose of this study is to investigate and identify the transporter responsible for GAA uptake by hepatocytes. The characteristics of [14C]GAA uptake by hepatocytes were elucidated using the in vivo liver uptake method, freshly isolated rat hepatocytes, an expression system of Xenopus laevis oocytes, gene knockdown, and an immunohistochemical technique. In vivo injection of [14C]GAA into the rat femoral vein and portal vein results in the rapid uptake of [14C]GAA by the liver. The uptake was markedly inhibited by γ-aminobutyric acid (GABA) and nipecotinic acid, an inhibitor of GABA transporters (GATs). The characteristics of Na+- and Cl−-dependent [14C]GAA uptake by freshly isolated rat hepatocytes were consistent with those of GAT2. The Km value of the GAA uptake (134 µM) was close to that of GAT2-mediated GAA transport (78.9 µM). GABA caused a marked inhibition with an IC50 value of 8.81 µM. The [14C]GAA uptake exhibited a significant reduction corresponding to the reduction in GAT2 protein expression. GAT2 was localized on the sinusoidal membrane of the hepatocytes predominantly in the periportal region. This distribution pattern was consistent with that of the creatine biosynthetic enzyme, S-adenosylmethionine∶guanidinoacetate N-methyltransferase. GAT2 makes a major contribution to the sinusoidal GAA uptake by periportal hepatocytes, thus regulating creatine biosynthesis in the liver.

Alcohol Consumption Decreases Rat Hepatic Creatine Biosynthesis Via Altered Guanidinoacetate Methyltransferase Activity

“…EtOH intake had no effect on the kidney GAA synthesis. Recent studies have identified γ‐aminobutyric acid transporter 2 as the transporter responsible for GAA uptake by the hepatocytes (Tachikawa et al., ). There are no reports to date whether EtOH consumption affects this transporter level or activity in the liver and will be the focus of our future studies.…”

Impact of SLC6A Transporters in Physiological Taurine Transport at the Blood–Retinal Barrier and in the Liver

“…Previously, rat GAT2 was reported to be involved in the uptake of GAA, γ-butyrobetaine, β-Ala and GABA by hepatocytes, with K m values of 134, 9.3, 35.3, and 22.5 µM, respectively. [62][63][64] Because of the blood concentrations of GAA (3.73 µM), γ-butyrobetaine (0.84-6 µM), β-Ala (<0.5 µM), GABA (0.13 µM) and taurine (130 µM), 22,[65][66][67][68][69] the K m values indicate that GAT2-mediated transport in hepatocytes is not completely saturated under physiological conditions, and the uptake of taurine by rat GAT2 seems to depend on the blood concentration of taurine, revealing the possible endorsement that taurine and its conjugated bile acids are increased by the systemic administration of taurine. 70,71) Immunohistochemistry showed that rat GAT2 is preferentially localized on the sinusoidal membrane of the periportal region (Fig.…”

Partucularities of g-aminobutyric acid metabolism in the liver of rats during different types of alcohol withdrawal

“…При обсуждении сдвигов в содержании пула свободных аминокислот в печени следует учитывать то, что ГАМК и другие изученные аминокислоты могут выполнять в печени функции нутриентов [3,4,8]. Принимая во внимание множественность функций неспецифических ГАМК-Т и ЯПА-ДГ в ткани печени [11,13,14,24], можно предположить, что наблюдаемые метаболические сдвиги являются следствием неспецифической адаптации клеток печени к интенсивной алкогольной нагрузке и периодам её отмены. Следовательно, наблюдаемые изменения при отмене алкоголя и прерывистой алкоголизации могут свидетельствовать о нарушениях утилизации не только ГАМК, но и других её метаболитов, относящихся к нутриентам.…”