title: Increased GABA transport activity in rat calvarial osteoblasts cultured under hyperglycemic conditions.
abstract: Several independent lines of evidence indicate the direct impairment by extracellular glucose at high concentrations of different osteoblastic functions with a marked decrease in bone mass toward osteoporosis, while the underlying mechanisms are not well clarified to date. We have previously demonstrated the functional expression of the neural amino acid gamma-aminobutyric acid (GABA) signaling system including betaine/GABA transporter-1 (BGT-1) with a temperature-, sodium- and chloride-dependent activity of [(3)H]GABA accumulation in cultured rat calvarial osteoblasts. In this study, therefore, we attempted to demonstrate the possible involvement of BGT-1 isoform in bone dysfunctions due to impaired mineralization in rat calvarial osteoblasts cultured under hyperglycemic conditions. No significant change was seen in [(3)H]GABA accumulation in osteoblasts cultured for 7 d in vitro (DIV) under hyperglycemic conditions (glucose=25.5-50.5 mM) compared to those cultured in normoglycemic (glucose=5.5 mM) and hyperosmotic (mannitol=25.5-50.5 mM) conditions. In osteoblasts cultured for 14 DIV under hyperglycemic conditions, however, [(3)H]GABA accumulation was significantly increased compared to those cultured under normoglycemic and hyperosmotic conditions. Kinetic analysis revealed that hyperglycemic cultivation resulted in a significant increase in V(max) values from 2.85 nmol/min/mg protein for normoglycemic conditions to 4.17 nmol/min/mg protein for hyperglycemic conditions without affecting K(m) values. However, experimental hyperglycemia did not significantly affect the expression of mRNA for BGT-1 isoform by osteoblasts. These results suggest that GABA transport system may at least in part play a role in pathological malfunctions and abnormalities through a mechanism not directly related to gene expression in osteoblasts under hyperglycemia.
authors.affiliation: Laboratory of Molecular Pharmacology, Kanazawa University Graduate School of Natural Science and Technology, Kanazawa, Ishikawa 920-1192, Japan.
authors.forename: Sayumi
authors.initials: S
authors.lastname: Fujimori
authors.forename: Masato
authors.initials: M
authors.lastname: Osawa
authors.forename: Mika
authors.initials: M
authors.lastname: Iemata
authors.forename: Eiichi
authors.initials: E
authors.lastname: Hinoi
authors.forename: Yukio
authors.initials: Y
authors.lastname: Yoneda
mesh_terms: D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism; D002478:Cells, Cultured; D003470:Culture Media; D050485:GABA Plasma Membrane Transport Proteins; D005947:Glucose / Q000378:metabolism*; D010006:Osteoblasts / Q000187:drug effects* / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D012886:Skull / Q000166:cytology / Q000378:metabolism*; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; D003470:Culture Media; D050485:GABA Plasma Membrane Transport Proteins; C072207:betaine plasma membrane transport proteins; D005680:gamma-Aminobutyric Acid; D005947:Glucose; D002118:Calcium