Polyamine Uptake in Cultured Astrocytes

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Abstract
The properties and regulation of the polyamine transport system in brain are still poorly understood. The present study shows, for the first time, the existence of a polyamine transport system in cerebellar astrocytes and suggests that polyamine uptake is mediated by a single and saturable high‐affinity transport system for putrescine, spermine, and spermidine (Km = 3.2, 1.2, and 1.8 μM, respectively). Although substitution of NaCl by choline chloride produced a decrease in the putrescine, spermine, and spermidine uptake, it seems that polyamine transport in cerebellar astrocytes is not mediated by an Na+ cotransport as in the presence of Na+ and cholinium, polyamine uptake was much lower than when measured in a sucrose‐based medium. On the other hand, ouabain, gramicidin (a Na+ ionophore), and ionomycin (a Ca2+ ionophore) produced a strong inhibition of polyamine uptake, suggesting that membrane potential could have an important role in the functioning of the astroglial polyamine uptake system. Moreover, protein kinase C inhibition produced an enhancement of polyamine uptake, whereas stimulation of protein kinase C with phorbol esters inhibited polyamine uptake. Alternatively, the tyrosine kinase inhibitor genistein caused a marked reduction in the uptake. No effects on polyamine uptake were observed with inhibitors and activators of cyclic AMP‐dependent protein kinase or when Ca2+/calmodulin‐dependent protein kinase II was inhibited with KN‐62. These results suggest that the polyamine uptake system in cerebellar astrocytes could be modulated by protein kinase C and tyrosine kinase activities.