Uptake, release, and binding of taurine in degenerated rat retinas

High-affinity uptake, potassium-stimulated release and receptor binding of gamma-aminobutyric acid (GABA) and taurine were studied in retinas of rats treated with monosodium glutamate (MSG) or iodoacetate-malate (IAM). Such treatments produce a selective damage of the inner retinal layers (MSG) or of the photoreceptor layer (IAM). MSG treatment decreased GABA uptake by more than 60%. Also, the potassium-stimulated release of this amino acid was markedly diminished (90%) in retinas lesioned by MSG. MSG treatment decreased 3H-GABA binding by 45%. Uptake, release or binding of GABA were unaffected by IAM-treatment. Taurine uptake was reduced by a similar degree by both MSG and IAM treatment, 57% and 38% respectively. Potassium-stimulated release of taurine was unchanged by MSG and 50% reduced by IAM. Both treatments reduced the spontaneous release of endogenous taurine. Binding of taurine to receptors in retinal membranes was practically unaffected by any of the treatments utilized. These results are consistent with a neurotransmitter action of GABA at the inner retinal layers, while they do not support a major role for taurine as a synaptic transmitter in retina.

Nucleotide content of isolated bovine rod outer segments

Endogenous nucleotide levels of isolated intact ROS were analyzed by high pressure liquid chromatography. Intact bovine ROS showed a total nucleotide concentration of 1.0 mM, ATP and GTP being the major components (0.1-0.2 mol per mol of rhodopsin). When intact ROS resuspended in sucrose-ficoll medium were diluted in a Ringer-Krebs bicarbonate, the nucleotide ratios were markedly changed, but the total concentration remained unchanged. The concentration of cyclic nucleotides (cAMP and cGMP) was markedly enhanced when ROS were placed in Ringer buffer, whereas ATP and GTP concentration was reduced. Total nucleotide concentration is 100% higher in intact than in leaky plasma membrane ROS. Upon illumination, no change was observed in the nucleotide levels of ROS in sucrose-ficoll medium. However ATP, GTP and cAMP levels were reduced when ROS in Ringer medium were exposed to light while cGMP concentration showed no change. Relevance of relative nucleotide content and ionic concentration to the transduction phenomenon in photoreceptor is discussed.

High-affinity taurine uptake in developing retina

A specific system for taurine transport is present at the early stages of development in both chick and rat retinas. The results obtained with taurine analogs indicate a high degree of specificity of taurine uptake. Two transport systems were detected for the adult rat retina: a high-affinity (Km 21 microM) and a low-affinity transport system (Km 312 microM). On the other hand, in the adult chick retina, only a low-affinity transport system (Km 580 microM) could be detected. Nevertheless, embryo chick retina accumulated [3H]taurine by two different kinetic mechanisms with Kms of 242 microM and 21 microM for the low- and high-affinity processes, respectively. Taurine uptake systems were absolutely Na+ dependent. The sodium-dependence curve for taurine uptake was sigmoid. These mechanisms appear not to be mediated by a Na+ cotransport system. In spite of the differences observed in taurine uptake in both species, in each of them it closely parallels the changes brought about by the morphological and functional maturation of the retina.

The effect of colchicine and cytochalasin B on the release of taurine from the chick retina

The effect of colchicine (0.5 mM) and of cytochalasin B (10(-4) M) on the release of [35S]taurine from the isolated chick retina, upon stimulation by 68.5 mM-KCl, 10(-5) M-veratridine, and 10 mM-glutamate, was studied. Cytochalasin and colchicine effects on taurine release were compared with those on K+-stimulated release of [3H]dopamine and [3H]GABA. Colchicine caused a marked decrease of the [35S]taurine release evoked by the three stimulatory agents; it also decreased [3H]dopamine release without affecting that of [3H]GABA. Cytochalasin B significantly decreased the efflux of [35S]taurine stimulated by glutamate or veratridine without altering that evoked by 68.5 mM-KCl. Cytochalasin practically suppressed the [3H]dopamine-stimulated release and slightly decreased that of [3H]GABA. This drug produced a high increase in the spontaneous release of labeled GABA and taurine. These results suggest that the release of taurine and GABA from the chick retina probably occurs though different mechanisms. It is suggested that taurine release may be related to a process involving contractile proteins.

Glutamate decarboxylase activity in chick brain and retina. Inhibition of the immature enzyme by Triton-X-100

We have studied the effect of Triton-X-100 on glutamate decarboxylase (GAD) activity in brain and retina from chick embryos of 12 and 16 days' incubation and from chicks 4--6 weeks old. GAD activity was measured in five different homogenization media. Triton-X-100 inhibited the enzyme by about 60% in both brain and retina of 12-day embryos and by about 50% in 16-day embryos, independently of the homogenization medium. In chicks only about 20% inhibition by the detergent was observed in brain whereas no effect was found in retina. These results indicate that the evaluation of the experimental conditions of enzyme assays at different ages is essential for developmental studies of GAD activity in nervous tissue.

Potassium-stimulated release of GABA, glycine, and taurine from the chick retina

The effect of depolarizing potassium concentration on the release of [14C]glycine, [3H]GABA, and [3S]taurine was investigated in the whole chick retina and in a synaptosomal fraction prepared from the chick retina. In the whole retina, increasing potassium concentration above 40 mM resulted in an increased release of the three amino acids. The release of glycine was the most stimulated and that of taurine, the least. The potassium-evoked release of glycine and GABA was calcium dependent. In the synaptosomal fraction, 68.5 mM potassium significantly stimulated the efflux of GABA and glycine by a calcium-dependent mechanism. The release of taurine from this fraction was unaffected by high potassium.

Cysteine sulphinate decarboxylase in chick and rat retina during development

The activity, properties and developmental pattern of cysteine sulphinate decarboxylase (CSD) were studied in chick and rat retina. Retinal CSD shows properties similar to those of the enzyme in brain with respect to optimum pH, saturating substrate concentrations and stimulation by pyridoxal phosphate, CSD activity increased 3-fold from the 10th day of embryogenesis to hatching in chicks and in postnatal development in rats. The developmental pattern of CSD activity in both species is coincident with the functional maturation of visual function.

Regulation of arginase activity by intermediates of the arginine biosynthetic pathway in Neurospora crassa

It has been found that, in Neurospora crassa, arginine synthesized from exogenous citrulline was not as effectively hydrolyzed as exogenous arginine. This was explained by the observed inhibition of arginase in vitro and in vivo by citrulline. The high arginine pool formed from exogenous citrulline feedback inhibits the arginine pathway. These two factors allow exogenous citrulline to be used adventitiously and efficiently as an arginine source. Finally, it was found that ornithine was a strong inhibitor of arginase. This suggests that the characteristically high ornithine pool of minimal cultures of Neurospora may act to control a potentially wasteful catabolism of endogenous arginine by arginase.