Cation dependence of hypotaurine uptake in mouse brain slices

Release of preloaded taurine and hypotaurine from astrocytes in primary culture: stimulation by calcium-free media

The spontaneous and stimulated release of taurine and hypotaurine from astrocytes in primary cultures were investigated. Spontaneous efflux was slow, less than one half of preloaded labeled taurine and hypotaurine still remaining in the cells after a 60-min efflux period. The release processes of both amino acids were in principle similar. No homo- or heteroexchange with extracellularly added taurine, hypotaurine or GABA could be detected, and depolarizing potassium concentrations failed to stimulate taurine or hypotaurine release. On the other hand, omission of calcium ions from medium increased efflux of taurine and hypotaurine about three- and twofold, respectively, in both high-K+ and normal-K+ media.

Taurine and hypotaurine transport by a single system in cultured neuroblastoma cells

The kinetics of mutual inhibition of taurine and hypotaurine uptake were studied using neuroblastoma C1300 cells as neuronal model. Hypotaurine and GABA inhibited taurine uptake competitively, increasing the apparent Km. High-affinity uptake of hypotaurine was completely abolished and the low-affinity component competitively inhibited by taurine. GABA affected noncompetitively low-affinity hypotaurine uptake, whereas the effect on high-affinity uptake was competitive, with an increase in the apparent Km. All structural analogues tested inhibited taurine and hypotaurine uptakes similarly. The most potent inhibitors were beta-alanine and 2-guanidinoethanesulphonic acid. The mutual inhibition and similar specificity profiles of taurine and hypotaurine uptakes showed that these amino acids employ a single transport system in neuroblastoma cells. Competitive inhibition by GABA of the high-affinity uptake of taurine and hypotaurine further suggests that also GABA uses the same carrier system.

Hypotaurine transport in mouse brain synaptosomal preparations

The transport of hypotaurine, the precursor of the probable neuromodulator taurine, was investigated using mouse brain synaptosomal preparations. Hypotaurine uptake was concentrative, energy- and sodium-dependent and strongly inhibited by GABA, L-DABA (L-2,4-diaminobutyric acid) and beta-alanine, suggesting interactions with GABA uptake systems. The uptake consisted of only one saturable transport component both in adult and in 6-day-old brain, being more efficient in the latter.

beta-Alanine uptake by mouse brain slices

beta-[3H]Alanine uptake by mouse brain slices was studied in Krebs-Ringer-HEPES-glucose medium (pH 7.4) under O2. The uptake was temperature-sensitive and consisted of two saturable transport components, high- and low-affinity, with kinetic parameters comparable to those of amino acid neurotransmitter candidates. beta-Alanine uptake was strictly sodium-dependent and also inhibited by the omission of potassium and presence of ouabain, suggesting that the transport is mainly fuelled by cation gradients. Sodium ions showed positive cooperative effects in beta-alanine uptake, indicating the association of at least two sodium ions in the transfer of one molecule of beta-alanine. The uptake was strongly inhibited by gamma-aminobutyrate and hypotaurine, the high-affinity uptake component completely disappearing in the presence of hypotaurine. Taurine had no measurable effect. The results suggest that the high-affinity transports of beta-alanine and hypotaurine may be mediated by the same system.

Taurine, hypotaurine, and GABA uptake by cultured neuroblastoma cells

Uptake of [3H]taurine, [35S]hypotaurine, and [3H] gamma-aminobutyric acid (GABA) was studied in neuroblastoma C1300 cells in Krebs-Ringer-Hepes-glucose medium (pH 7.4). The uptakes consisted of nonsaturable penetration (taurine and hypotaurine) and two saturable transport components: high affinity for taurine, hypotaurine, and GABA and low affinity for hypotaurine and GABA. The affinity of the high-affinity uptake was highest for hypotaurine but the transport capacity was greatest for taurine. GABA uptake was almost abolished by taurine and hypotaurine. Hypotaurine also strongly inhibited taurine uptake, whereas GABA had only a moderate inhibitory effect on taurine and hypotaurine uptakes. The mutual inhibition suggests that these amino acids use the same transport sites when entering the cells.

Effects of cations on taurine, hypotaurine, and GABA uptake in mouse brain slices

The effects of cations on taurine, hypotaurine and GABA uptake were studied in mouse brain slices under identical experimental conditions. The uptakes were all strictly sodium-dependent. The omission or excess of K+ inhibited similarly taurine, hypotaurine and GABA uptake. The effects of omission of Ca2+ or Mg2+ were less pronounced. In both normal-sodium and low-sodium media all uptakes were saturable, consisting of both low- and high-affinity transport components. The Km constants for both low- and high-affinity transport components of hypotaurine and GABA increased in low-sodium medium, suggesting that sodium ions are necessary for their attachment to possible carrier sites in plasma membranes. In the case of taurine, however, the translation rate rather than the affinity of carrier sites was affected in Na+-free media. More than two sodium ions may be involved in the transport of one hypotaurine and one GABA molecule, whereas the coupling ratio between sodium and taurine was at least three. In its cation dependence hypotaurine uptake thus resembled more GABA uptake than taurine uptake.

Hypotaurine transport in brain slices: comparison with taurine and GABA

Hypotaurine uptake was compared to taurine and GABA uptakes in brain slices under identical experimental conditions. The slices effectively concentrated both hypotaurine and GABA from the medium, whereas taurine was taken up more slowly. The uptakes of these three structurally related amino acids were all saturable, consisting of one low- and one high-affinity transport component. The kinetic parameters of hypotaurine uptake were of the same order of magnitude as those of GABA uptake. All uptake systems were sensitive to temperature, metabolic poisons, and sodium omission. Hypotaurine uptake was inhibited by GABA, L-2,4-diaminobutyric acid (L-DABA), cysteic acid, and beta-alanine, but not by taurine. Taurine uptake was strongly reduced by hypotaurine, beta-alanine, and L-DABA, as well as by GABA, whereas GABA uptake was affected only by cystamine, L-DABA, and nipecotic acid. The uptake processes of hypotaurine, taurine, and GABA were thus fairly similar and showed properties characteristic for neurotransmitter uptake. Hypotaurine uptake resembled more GABA than taurine uptake. The present inhibition studies suggest that there may exist only one common two-component transport system for these three amino acids.