Characterization of guanine and guanosine transport in primary cultured rat cortical astrocytes and neurons

In this study, we examined the transport mechanisms for guanine and guanosine in rat neurons and astrocytes, and compared their characteristics. In the both types of cell, the uptake of [H-3]guanine and [H-3]guanosine was time-, temperature-, and concentration-dependent, and Na+-independent. Their uptake decreased on the addition of purine and pyrimidine nucleobases or nucleosides, and the inhibitory effect of the purine analogues was greater than that of the pyrimidine ones. In both cell types, equilibrative nucleoside transporter (ENT) 1 and ENT2 expression was confirmed at the mRNA level, and nitrobenzylmercaptopurine riboside, a representative inhibitor for ENT, decreased their uptake at concentrations of over 10 mu M. Comparing uptake characteristics between the substrates, [H-3]guanine uptake exhibited higher affinity and clearance than [H-3]guanosine uptake in each type of cell. Although between neurons and astrocytes, there was no difference in the apparent uptake clearance for [H-3]guanine and [H-3]guanosine, which was calculated based upon the cellular protein content, the cellular uptake clearance was significantly greater in astrocytes than in neurons. These findings indicate that guanine and guanosine, of which the former is a preferable substrate, are taken up into both neurons and astrocytes via ENT2, and that the extracellular concentrations of guanine and guanosine are mainly regulated by astrocytes to maintain brain physiology. (C) 2007 Wiley-Liss, Inc.