Polyamine uptake in cultured astrocytes: Characterization and modulation by protein kinases

The properties and regulation of the polyamine transport system in brain ar e still poorly understood. The present study shows, for the first time, the existence of a polyamine transport system in cerebellar astrocytes and sug gests that polyamine uptake is mediated by a single and saturable high-affi nity transport system for putrescine, spermine, and spermidine (K-m = 3.2, 1.2, and 1.8 mu M, respectively). Although substitution of NaCl by choline chloride produced a decrease in the putrescine, spermine, and spermidine up take, it seems that polyamine transport in cerebellar astrocytes is not med iated by an Naf cotransport as in the presence of Na+ and cholinium, polyam ine 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 t hat membrane potential could have an important role in the functioning of t he astroglial polyamine uptake system. Moreover, protein kinase C inhibitio n produced an enhancement of polyamine uptake, whereas stimulation of prote in kinase C with phorbol esters inhibited polyamine uptake. Alternatively, the tyrosine kinase inhibitor genistein caused a marked reduction in the up take. No effects on polyamine uptake were observed with inhibitors and acti vators of cyclic AMP-dependent protein kinase or when Ca2+/calmodulin-depen dent 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.