Fig. 4 Summary figure for rainbow trout plasma-accessible carbonic anhydrase atlas study. Tissues coloured in light grey in top row (all four gill arches and stomach) showed no evidence for plasma-accessible carbonic anhydrase. All other tissues, coloured in dark grey in bottom row (pyloric caeca, anterior, mid, and posterior intestine, heart, brain, liver, kidney, and white muscle) showed evidence for plasma-accessible carbonic anhydrase. Red outlines indicate the CA-IV isoform present is likely involved in primarily gas exchange roles and blue outlines indicate the CA-IV isoform present is likely involved in primarily acid–base/ion exchange roles.
Charlotte Nelson, Angelina M. Dichiera, Ellen H Jung, Colin J. Brauner. 2023. An atlas of plasma‑accessible carbonic anhydrase availability in the model teleost, the rainbow trout. Journal of Comparative Physiology B
The unique teleost oxygenation system that permits enhanced oxygen unloading during stress comprises three main characteristics: pH-sensitive haemoglobin, red blood cell (RBC) intracellular pH (pHi) protection, and a heterogeneous distribution of plasma-accessible carbonic anhydrase (paCA). A heterogeneous distribution of paCA is essential; its presence permits enhanced oxygen unloading during stress, while its absence at the gills maintains conditions for oxygen uptake by pHsensitive haemoglobins. We hypothesised that paCA would be absent in all four gill arches, as has been previously indicated for arch two, and that paCA would be present in all other tissues. Through a suite of biochemical and molecular methods, we confirmed the absence of paCA from all four arches. We also found evidence for paCA in nine other tissues and a lack of paCA availability in the stomach. Expression was highly variable between tissues and suggests these differences may be associated with their respective metabolic activities. Additionally, we analysed the specific CA-IV isoform expressed within each tissue and showed almost complete separation of expression between tissues; CA-IVa was detected in the heart, brain, anterior intestine, and liver, whereas CA-IVb was detected in all intestine sections, pyloric caeca, kidney, and white muscle. This adds to a growing collection of work suggesting CA-IVa and b play divergent roles in gas exchange and ion/acid–base balance, respectively. The current study represents the first comprehensive atlas of paCA availability within the circulatory system of the model teleost, rainbow trout, and fills important gaps in our knowledge of this unique oxygenation system.