Osmoregulatory role of amino acids in brain of the elasmobranch, Raja erinacea

CO2 induced pHi changes in the brain of polar fish: a TauCEST application

Chemical exchange saturation transfer (CEST) from taurine to water (TauCEST) can be used for in vivo mapping of taurine concentrations as well as for measurements of relative changes in intracellular pH (pH_i ) at temperatures below 37°C. Therefore, TauCEST offers the opportunity to investigate acid-base regulation and neurological disturbances of ectothermic animals living at low temperatures, and in particular to study the impact of ocean acidification (OA) on neurophysiological changes of fish. Here, we report the first in vivo application of TauCEST imaging. Thus, the study aimed to investigate the TauCEST effect in a broad range of temperatures (1-37°C) and pH (5.5-8.0), motivated by the high taurine concentration measured in the brains of polar fish. The in vitro data show that the TauCEST effect is especially detectable in the low temperature range and strictly monotonic for the relevant pH range (6.8-7.5). To investigate the specificity of TauCEST imaging for the brain of polar cod (Boreogadus saida) at 1.5°C simulations were carried out, indicating a taurine contribution of about 65% to the in vivo expected CEST effect, if experimental parameters are optimized. B. saida was acutely exposed to three different CO₂ concentrations in the sea water (control normocapnia; comparatively moderate hypercapnia OA_m  = 3300 μatm; high hypercapnia OA_h  = 4900 μatm). TauCEST imaging of the brain showed a significant increase in the TauCEST effect under the different CO₂ concentrations of about 1.5-3% in comparison with control measurements, indicative of changes in pH_i or metabolite concentration. Consecutive recordings of ¹ H MR spectra gave no support for a concentration induced change of the in vivo observed TauCEST effect. Thus, the in vivo application of TauCEST offers the possibility of mapping relative changes in pH_i in the brain of polar cod during exposure to CO₂ .

The little skate Raja erinacea exhibits an extrahepatic ornithine urea cycle in the muscle and modulates nitrogen metabolism during low-salinity challenge

Urea synthesis via the hepatic ornithine urea cycle (OUC) has been well described in elasmobranchs, but it is unknown whether OUC enzymes are also present in extrahepatic tissues. Muscle and liver urea, trimethylamine oxide (TMAO), and other organic osmolytes, as well as selected OUC enzymes (carbamoyl phosphate synthetase III, ornithine transcarbamoylase, arginase, and the accessory enzyme glutamine synthetase), were measured in adult little skates (Raja erinacea) exposed to 100% or 75% seawater for 5 d. Activities of all four OUC enzymes were detected in the muscle. There were no changes in muscle OUC activities in skates exposed to 75% seawater; however, arginase activity was significantly lower in the liver, compared to controls. Urea, TMAO, and several other osmolytes were significantly lower in the muscle of little skates exposed to 75% seawater, whereas only glycerophosphorylcholine was significantly lower in the liver. Urea excretion rates were twofold higher in skates exposed to 75% seawater. Taken together, these data suggest that a functional OUC may be present in the skeletal muscle tissues of R. erinacea. As well, enhanced urea excretion rates and the downregulation of the anchor OUC enzyme, arginase, in the liver may be critical in regulating tissue urea content under dilute-seawater stress.

Taurine blocks spontaneous cone contraction but not horizontal cell dark suppression in isolated goldfish retina

The objective of this study was to investigate the effects of taurine on cone retinomotor movements and the responses of cone-driven horizontal cells in dark-adapted teleost retina. In isolated goldfish retina preparations maintained in the dark, cones spontaneously contracted, and the responses of horizontal cells were suppressed. Addition of 5 mM taurine to the physiological solution blocked the spontaneous contraction of cones in the dark but did not block the dark-suppression of horizontal cell responses. These results indicate that the mechanism that leads to horizontal cell dark suppression is not sensitive to taurine. Although both cone retinomotor position and horizontal cell responsiveness are known to be modulated by dopamine, the present results do not support the hypothesis that taurine inhibits dopamine release in the dark because only spontaneous cone contraction was affected by taurine. These results also indicate that spontaneous cone contraction in the dark is not the cause of horizontal cell dark suppression because, in the presence of taurine, cones were elongated yet horizontal cell responses were still suppressed. Consequently, these results make it clear that horizontal cell dark suppression is not an artifact produced by incubating isolated teleost retina preparations in taurine-free physiological solution.

Beta-alanine transport in the isolated hepatocytes of the elasmobranch Raja erinacea

Cells were isolated from the liver of the skate and the uptake of beta-alanine followed using [14C]-beta-alanine. The isolated hepatocytes showed good viability, were found to accumulate beta-alanine from the incubation medium, and did so in a manner indicating a transport system involving a saturable carrier. The data for the rate of beta-alanine uptake suggest that this may be a rate-limiting step in the oxidation of the amino acid by the liver. Experiments indicated that the transport system could distinguish beta-alanine from certain structurally similar molecules (L-alanine and taurine, but not gamma-amino butyrate). Cells isolated from fish adapted to a diluted environment (50% seawater) showed no significant change in the uptake rate. However, evidence indicates that, over the range of beta-alanine concentrations occurring in the fish, the uptake rate would be acutely sensitive to small changes in the concentration in the blood, thus forming a self-regulating system for the metabolism of beta-alanine.

Cell volume regulation in fish heart ventricles with special reference to taurine

A review of the cell volume regulation mechanism in heart ventricles of teleosts reveal that the mechanism is not only restricted to euryhaline species in a changing salinity regime but also is manifested in fresh-water fish. Taurine is the dominating amino acid and the main cellular osmo-effector in teleost hearts (accounting for 40-50% of the osmolality change). During hypo-osmotic regulation, cellular taurine is reduced by an efflux from the cells, whereas intracellular synthesis of taurine most probably accompanies hyper-osmotic regulation. Vertebrate hearts seem to have a high concentration of taurine and it may in general in vertebrate hearts also play a pivotal role in cellular osmoregulatory function.

Postprandial level of free amino acids in fish brain

1. Free amino acids were examined in common carp (Cyprinus carpio L.) brain 3, 6, 24 and 72 hr after a meal. 2. In common carp brain glutamic acid (409.6 +/- 37.6 mg/100 g dry matter), threonine (232.8 +/- 24.6) and histidine (191.3 +/- 70.1) were the most abundant free amino acids. 3. The ratios between the brain and the whole blood concentrations (B/W) expressed as the amounts of moles per tissue water were examined. 4. Glutamic acid, histidine, threonine, alanine, methionine, serine and glycine were found in consistently higher concentration in fish brain than in blood.