Taurine and osmoregulation. IV. Cerebral taurine transport is increased in rats with hypernatremic dehydration

Abstract

Taurine is an organic osmolyte in brain cells. We studied whether cerebral taurine transport is enhanced as part of the cell volume regulatory adaptation to hypernatremia. Hypernatremic dehydration was induced for 48 h. Synaptosomes, metabolically active nerve terminal vesicles, were isolated by homogenization of brain and purification on a discontinuous Ficoll gradient. Taurine transport was evaluated in vitro using a rapid filtration assay. After 48 h of hypernatremia, there was a 22.4% increment in Na(+)-specific taurine transport from 2.99 +/- 0.16 to 3.66 +/- 0.13 mumol/mg protein/30 min (p less than 0.001). Dehydration for 48 h without hypertonic saline loading had no effect on taurine uptake. Glycine transport was unaltered by hypernatremia. The adaptation in taurine uptake resulted from an enhanced Vmax of the high affinity-low capacity transport system [265 +/- 17, control versus 337 +/- 19 nmol/min/mg protein, experimental (p less than 0.03)] without a change in the Km (approximately 60 microM). Under both control and hypernatremic conditions, Na+ and Cl- were required for maximal total Na(+)-mediated taurine uptake. Oubain (1 mM) decreased taurine uptake by 25%, whereas addition of beta-alanine or hypotaurine (500 microM) to the external media reduced taurine transport by 45-65% in both control and experimental conditions (p less than 0.01). Synaptosomal taurine uptake in hypernatremic rats was inhibited by 15-20% (p less than 0.01) after addition of 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (0.1 mM) or 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (0.1 mM) or 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (0.1 mM) to the external medium.(ABSTRACT TRUNCATED AT 250 WORDS)