ALBERT

Description: Chemical exchange saturation transfer (CEST) from glutamate to water (GluCEST) is a powerful tool for mapping glutamate concentration and intracellular pH. GluCEST could also be helpful to understand the physiology of lower aquatic vertebrates and invertebrates. Therefore, this study aimed to investigate the GluCEST effect and the exchange rate ksw from amine protons of glutamate to water in a broad range of temperatures (1–37°C) and pH (5.5–8.0). Z-spectra were measured from glutamate solutions at different pH values and temperatures and analysed by numerically solving the Bloch–McConnell equation. As expected, a strong dependence of the GluCEST effect and the determined ksw values on pH and temperature was observed. In addition, a strong dependence of the GluCEST effect on phosphate buffer concentration was confirmed. The in vitro data show that GluCEST is detectable in the whole temperature range, even at 1°C. An interpolation function for the exchange rate ksw was determined for the considered range of temperatures and pH values, showing a bijective relation between the exchange rate and pH at a given temperature. To investigate the specificity of GluCEST imaging at low temperatures, the CEST effect was investigated for several metabolites relevant for CEST imaging of the brain. As an example, the contribution of GluCEST to the total CEST effect at 3ppm was estimated for zebrafish (Danio rerio). It is shown that also at lower temperatures glutamate is the major contributor to the total CEST effect, particularly if the experimental parameters are optimized.

Description: 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 (pHi) 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 contribu- tion 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 OAm = 3300 μatm; high hypercapnia OAh = 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 pHi or metabolite concentration. Consecutive recordings of 1H 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 pHi in the brain of polar cod during exposure to CO2.

Description: Objectives: Temperature dependent chemical shifts of important brain metabolites measured by localised 1H MRS were investigated to test how the use of incorrect prior knowledge on chemical shifts impairs the quantication of metabolite concentrations. Materials and methods: Phantom measurements on solutions containing 11 metabolites were performed on a 7 T scanner between 1 and 43 °C. The temperature dependence of the chemical shift differences was fitted by a linear model. Spectra were simulated for di erent temperatures and analysed by the AQSES program (jMRUI 5.2) using model functions with chemical shift values for 37 °C. Results: Large differences in the temperature dependence of the chemical shift differences were determined with a maximum slope of about ±7.5 × 10−4 ppm/K. For 32–40 °C, only minor quantification errors resulted from using incorrect chemical shifts, with the exception of Cr and PCr. For 1–10 °C considerable quantification errors occurred if the temperature dependence of the chemical shifts was neglected. Conclusion: If 1H MRS measurements are not performed at 37 °C, for which the published chemical shift values have been determined, the temperature dependence of chemical shifts should be considered to avoid systematic quantification errors, particularly for measurements on animal models at lower temperatures.

Description: Chemical exchange saturation transfer from taurine to water (TauCEST) is primarily detectable in the low temperature range. Since, TauCEST asymmetry is bijective in the physiological pH-range (6.8-7.5), TauCEST is a potential candidate for in vivo studies on brain of polar fish. The specificity of TauCEST MRI on the brain of polar cod at 1.5°C shows a taurine contribution of 65%. TauCEST in brain of polar cod significantly increased under elevated CO2 concentrations by about 1.34%-3.17% in comparison to control, reflecting pHi changes since localized 1H NMR spectra show no significant changes in metabolite concentration for the different treatments.

Description: Chemical exchange saturation transfer (CEST) at about 2.8 ppm downfield from water is characterized besides other compounds by exchanging amine protons of relatively high concentration amino acids and is determined by several physiological (pH, T) and experimental (B0, B1, tsat) parameters. Although the weighting of the CEST effect observed in vivo can be attributed mainly to one compound depending on the organism and organ, there are still several other amino acids, proteins and molecules that also contribute. These contributions in turn exhibit dependences and thus can lead to possible misinterpretation of the measured changes in the CEST effect. With this in mind, this work aimed to determine the exchange rates of six important amino acids as a function of pH and temperature, and thus to create multi-pool models that allow the accurate analysis of the CEST effect concerning different physiological and experimental parameters for a wide variety of organisms. The results show that small changes in the above parameters have a significant impact on the CEST effect at about 2.8 ppm for the chosen organisms, i.e. the human brain (37 °C) and the brain of polar cod (1.5 °C), furthermore, the specificity of the CEST effect observed in vivo can be significantly affected. Based on the exchange rates ksw(pH, T) determined for six metabolites in this study, it is possible to optimize the intensity and the specificity for the CEST effect of amino acids at about 2.8 ppm for different organisms with their specific physiological characteristics. By adjusting experimental parameters accordingly, this optimization will help to avoid possible misinterpretations of CEST measurements. Furthermore, the multi-pool models can be utilized to further optimize the saturation.

Description: Chemical exchange saturation transfer (CEST) ist ein Bildkontrast, der die indirekte Detektion von Änderungen im pH ermöglicht. CEST bietet daher die Möglichkeit, die Säure-Basen-Regulation im Fischgehirn unter CO2-Konzentrationen, wie sie durch den Klimawandel bewirkt werden, zu verfolgen. Ziel dieser Studie war es, einen geeigneten Metaboliten zu finden, um Änderungen im intrazellulären pH-Wert mit hoher zeitlicher und räumlicher Auflösung im Fischgehirn bei 1.5°C zu detektieren. Der TauCEST-Effekt erwies sich als geeignet und wurde zum ersten Mal in vivo angewendet.