Comparison between (RS)-nipecotic acid and GABA transport in cultured astrocytes: coupling with two sodium ions.

The sodium ion dependency of the uptake of (RS)-nipecotic acid into astrocytes in primary cultures has been studied by performing kinetic analysis at different sodium ion concentrations (16--151 mM). Vmax of the saturable component of the astroglial (RS)-nipecotic acid uptake is clearly affected by the sodium ion concentration whereas Km surprisingly remains unaffected. At high (RS)-nipecotic acid concentrations (Greater Than or Equal To 50 mu M), uptake rates as a function of the sodium ion concentration were clearly sigmoid. This sigmoid shape was not obvious at lower concentrations of (RS)-nipecotic acid. The calculated Hill coefficients corresponding to all (RS)-nipecotic acid concentrations studied were approximately two. From these results it is concluded that (RS)-nipecotic acid uptake into astrocytes in primary cultures, like astroglial GABA uptake, requires the binding of at least two sodium ions per (RS)-nipecotic acid molecule transported.