Increased GABA transport activity in rat calvarial osteoblasts cultured under hyperglycemic conditions

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.