Possible expression of a particular gamma-aminobutyric acid transporter isoform responsive to upregulation by hyperosmolarity in rat calvarial osteoblasts.
Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter in the brain, but widely distributed in different peripheral organs. We have previously shown the functional expression of GABA(B) receptors required for GABAergic signal input by cultured rat calvarial osteoblasts. This study focused on the possible functional expression of the machinery required for GABAergic signal termination such as GABA transporters. In rat calvarial osteoblasts cultured for 7 days, [(3)H]GABA accumulation was observed in a temperature-, sodium- and chloride-dependent manner, consisting of a single component with a K(m) value of 789.6+/-9.0 microM and a V(max) value of 4.4+/-0.1 nmol/min/mg protein, respectively. Both nipecotic and L-2,4-diaminobutyric acids significantly inhibited [(3)H]GABA accumulation in a concentration-dependent manner. Constitutive expression was seen with mRNA for the betaine/GABA transporter-1 (BGT-1) and taurine transporter (TauT), while hyperosmotic cultivation led to significant increases in both [(3)H]GABA accumulation and BGT-1 mRNA expression without affecting TauT mRNA expression. Highly immunoreactive cells were detected for the BGT-1 isoform at the surface of trabecular bone of neonatal rat tibias. Sustained exposure to GABA significantly inhibited alkaline phosphatase (ALP) activity, but not cellular viability, at concentrations above 0.1 mM in osteoblasts cultured for 3 to 28 days. Nipecotic acid not only decreased ALP activity alone, but also further decreased ALP activity in osteoblasts cultured in the presence of GABA. These results suggest that the BGT-1 isoform may be functionally expressed by rat calvarial osteoblasts to play a hitherto unidentified role in mechanisms underlying hyperosmotic regulation of osteoblastogenesis.
Fujimori
Sayumi
S
Laboratory of Molecular Pharmacology, Division of Pharmaceutical Sciences, Kanazawa University Graduate School of Natural Science and Technology, Kakuma-machi, Kanazawa, Ishikawa 920-1192, Japan.
Hinoi
Eiichi
E
Takarada
Takeshi
T
Iemata
Mika
M
Takahata
Yoshifumi
Y
Yoneda
Yukio
Y
D000469:Alkaline Phosphatase / Q000378:metabolism; D000818:Animals; D001846:Bone Development / Q000187:drug effects; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000096:biosynthesis; D002470:Cell Survival / Q000187:drug effects; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000378:metabolism; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology; D005260:Female; D050485:GABA Plasma Membrane Transport Proteins / Q000096:biosynthesis* / Q000235:genetics; D007150:Immunohistochemistry; D007536:Isomerism; D009557:Nipecotic Acids / Q000494:pharmacology; D009994:Osmolar Concentration; D010006:Osteoblasts / Q000378:metabolism*; D011247:Pregnancy; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D051381:Rats; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000187:drug effects; D012886:Skull / Q000166:cytology / Q000254:growth & development / Q000378:metabolism*; D015854:Up-Regulation / Q000187:drug effects; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D002352:Carrier Proteins; D018690:Excitatory Amino Acid Agonists; D050485:GABA Plasma Membrane Transport Proteins; D009557:Nipecotic Acids; D012333:RNA, Messenger; C072207:betaine plasma membrane transport proteins; C030278:nipecotic acid; D005680:gamma-Aminobutyric Acid; D000242:Cyclic AMP; D000469:Alkaline Phosphatase; D002118:Calcium