Temperature-dependent pH regulation in stenothermal Antarctic and eurythermal temperate eelpout (Zoarcidae): an in-vivo NMR study

Temperature-dependent adjustments of intracellular pH are thought to play a major role in the maintenance of protein function. Comparative studies wer e carried out in two species from the same fish family (Zoarcidae), the ste nothermal Antarctic eelpout (Pachycara brachycephalum) and the eurythermal eelpout (Zoarces vivparus), to find out whether pH regulation is modified b y temperature in the closely related species and to what extent the respect ive pattern differs between eurytherms and stenotherms. Previous invasive s tudies had compared individual animals sampled at various temperatures and suggested that a decrease in intracellular pH (pHi) values occurs at rising temperatures, as predicted by the alpha-stat hypothesis of acid-base regul ation. The present study used non-invasive in vivo P-31-NMR spectroscopy in non-anaesthetized, unrestrained fish for long-term online recordings in in dividual specimens. Control spectra obtained at T=0 degreesC for P. brachyc ephalum and at 12 degreesC for Z. viviparus indicated low stress conditions , as well as a high stability of energy and acid-base status over time peri ods longer than I week. Temperature changes had no influence on the concent ration of high-energy phosphates like phosphocreatine or ATP. Temperature-i nduced pH changes were monitored continuously in a range between 0 and 6 de greesC for polar, and 12 and 18 degreesC for temperate eelpout. A pHi chang e of around -0.015 pH units/degreesC was observed within both species, in a ccordance with the alpha-stat hypothesis; however, extrapolation to the sam e temperature revealed different set points of pH regulation in the two spe cies. These findings confirm that an alpha-stat pattern of pH regulation ca n be found in stenothermal Antarctic animals, at set points deviating from an alpha-stat pattern, however, in a between-species comparison.