CEREBRAL TAURINE TRANSPORT IS INCREASED DURING STREPTOZOCIN-INDUCED DIABETES IN RATS

Taurine is a cerebral osmolyte whose intracellular content changes in parallel with plasma osmolality. We conducted experiments to assess whether cerebral taurine transport is modified during chronic hyperglycemia. Rats with STZ-induced diabetes were studied after 1 wk of sustained hyperglycemia. Cerebral taurine uptake in synaptosomes (metabolically active nerve terminal vesicles) was measured using a rapid filtration technique. The synaptosomes were isolated by homogenization of the brain and purification on discontinuous Ficoll gradients (n = 8 synaptosome preparations). Diabetic rats (n = 13) displayed a 15-25% increase in synaptosomal taurine uptake compared with normoglycemic control animals (n = 12) at all time points assayed between 5 and 120 min. Thus, after a 30-min incubation, cerebral taurine uptake increased from a control level of 3.53 +/- 0.23 to 4.10 +/- 0.24-mu-mol/mg protein (n = 10) in hyperglycemic rats, P < 0.03. The magnitude of the plasma-to-brain cell taurine gradient was unchanged in diabetic animals. The intrasynaptosomal taurine concentration (approximately 2-mu-M) and taurine efflux from the synaptosomes were no different in hyperglycemic versus control rats; efflux amounted to <2.5% of the uptake value at corresponding time points. Maximal brain taurine uptake under both control and experimental hyperglycemic conditions required the presence of external Na+ and Cl-. Synaptosomal taurine transport was reduced by competing beta-amino acids such as beta-alanine, beta-aminoisobutyric acid, and hypotaurine (P < 0.01). Addition of oubain and the anionic binding site inhibitors, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid and 4,4'diisothio-cyanatostilbene-2,2'-disulfonic acid, also decreased cerebral taurine uptake under normoglycemic and hyperglycemic conditions (P < 0.01). The increased synaptosomal taurine uptake by diabetic rats was not a result of generalized membrane dysfunction because glycine transport was not elevated in hyperglycemic rats. The enhanced transport rate was attributable to a 35 and 81% increase in the V(max) of the high- and low-affinity taurine transporters, respectively (P < 0.01), without significant change in the K(m) of the carrier systems. Treatment of hyperglycemic rats (n = 5) with ultra-long-acting insulin to normalize the serum glucose concentration restored synaptosomal taurine uptake to the level observed in normoglycemic controls. The effect of insulin was attributable to correction of hyperglycemia, because addition of insulin (500 mU/ml) to the in vitro assay system did not alter synaptosomal taurine uptake. We conclude that taurine uptake into the brain is enhanced during chronic hyperglycemia in order to expand the cytosolic pool of this osmoprotective molecule and limit brain cell shrinkage during this hyperosmolal condition. This adaptation in taurine transport, mediated by increased activity of the beta-amino acid carrier system, is reversible because insulin therapy that achieves euglycemia prevents the change.