Involvement of the carrier-mediated process in the retina-to-blood transport of spermine at the inner blood-retinal barrier

The elimination of spermine, the end product of cellular polyamine, from the retina to the blood across the blood-retinal barrier (BRB) was investigated. The invivo microdialysis study revealed that the elimination of [³H]spermine from vitreous humor after vitreous bolus injection was in a biexponential manner. The rate constant for the elimination of [³H]spermine during the terminal phase was estimated to be 1.67-fold greater than that of [¹⁴C]d-mannitol, a bulk flow marker, and the difference in the terminal elimination rate constant between [³H]spermine and [¹⁴C]d-mannitol was reduced in the presence of 50mM spermine, suggesting a retina-to-blood transport system for [³H]spermine across the BRB. The retina-to-blood transport of [³H]spermine was also supported by a study of the retinal uptake index (RUI). The invitro transport study with TR-iBRB2 cells, a model cell line of the inner BRB, revealed time-, concentration- and temperature-dependent transport of [³H]spermine, suggesting the involvement of carrier-mediated processes in spermine transport across the inner BRB. The invitro study also suggested that the transport of spermine at the inner BRB is pH-, membrane potential- and Cl^--sensitive and Na⁺-insensitive, and these functional properties of spermine transport suggest only a minor contribution of spermine transporters, such as CCC9 (SLC12A8), the expression of which was suggested at the inner BRB. In the inhibition study, [³H]spermine transport was markedly inhibited by putrescine, spermidine, spermine and agmatine while substrates of a well-characterized organic cation transporter (OCTs/SLC22A) and a cationic amino acid transporter (CATs/SLC7A) had no effect, suggesting the involvement of unknown transporters in spermine elimination from the retina.