An analysis of carbon dioxide transport in arterial and venous blood of the rainbow trout, Oncorhynchus mykiss, following exhaustive exercise

Hypoxia delays hematopoiesis: retention of embryonic hemoglobin and erythrocytes in larval rainbow trout, Oncorhynchus mykiss, during chronic hypoxia exposure

In rainbow trout development, a switch occurs from high-affinity embryonic hemoglobin (Hb) and round, embryonic erythrocytes to lower-affinity adult Hb and oval, adult erythrocytes. Our study investigated the early ontogeny of rainbow trout blood properties and the hypoxia response. We hypothesized that hypoxia exposure would delay the ontogenetic turnover of Hb and erythrocytes because retention of high-affinity embryonic Hb would facilitate oxygen loading. To test this hypothesis we developed a method of efficiently extracting blood from individual embryos and larvae and optimized several techniques for measuring hematological parameters on microliter (0.5 - 2.0 μl) blood samples. In chronic hypoxia (30% of oxygen saturation), stage-matched embryos and larvae possessed half the Hb concentration, erythrocyte counts, and hematocrit observed in normoxia. Hypoxia-reared larvae also had 3 to 6 fold higher mRNA expression of the embryonic Hb α-1, β-1, and β-2 subunits relative to stage-matched normoxia-reared larvae. Furthermore, in hypoxia the round embryonic erythrocytic shape persisted into later developmental stages. Despite these differences, Hb-oxygen affinity (P50), cooperativity, and the Root effect were unaltered in hypoxia-reared O. mykiss. The data support our hypothesis that chronic hypoxia delays the ontogenetic turnover of Hb and erythrocytes but without the predicted functional consequences (i.e. higher than expected P50). These results also suggest that the Hb-oxygen affinity is protected during development in chronic hypoxia to favour oxygen unloading at the tissues. We conclude that in early trout development, the blood-oxygen transport system responds very differently to chronic hypoxia relative to adults, possibly because respiration depends relatively more on oxygen diffusion than convection.

CO2 transport and excretion in rainbow trout (Oncorhynchus mykiss) during graded sustained exercise

A quantitative analysis of CO2 transport and excretion was conducted in seawater acclimated rainbow trout (Oncorhynchus mykiss) swimming at different sustained swimming velocities. CO2 excretion increased linearly with cardiac output during exercise but arterial P(CO2) (Pa(CO2)) and total CO2 levels also increased indicating a diffusion limitation to CO2 excretion. The elevated Pa(CO2) was not accompanied by a decrease in pH, indicating that the acid-base compensation was rapid. Mixed-venous P(CO2) increased to a greater extent than Pa(CO2) resulting in a large increase in the venous arterial difference in P(CO2) (Pv(CO2) - Pa(CO2)). The Pv(CO2) - Pa(CO2) difference was used to calculate the proportion of total CO2 excreted comprised of dissolved CO2 which accounted for less than 1% of total CO2 excreted in fish swimming at 11 cm sec(-1) but increased to about 9% at the greatest swimming velocity indicating that the pattern of CO2 excretion changes during exercise. There was no effect of exercise on the proportion of CO2 excreted which was dependent upon HCO3-/Cl- exchange (54%) or that which was dependent upon the dehydration of HCO3- that resided within the red cell prior to gill blood entry (42%). The large proportion of total CO2 excreted that was dependent upon HCO3-/Cl- exchange is significant because this is thought to be the rate limiting step in CO2 excretion.

Different red blood cell characteristics in a primitive agnathan (M. glutinosa) and a more recent teleost (O. mykiss) influence their strategies for blood CO2 transport

This study examines how the different red blood cell (rbc) characteristics in two lower vertebrates, the phylogenetically primitive hagfish and a more recent teleost, the rainbow trout, influence their strategies for blood CO2 transport. Deoxygenation of the blood resulted in a significant increase in rbc CO2 content in hagfish, but there were no significant changes in the CO2 content of plasma or whole blood under these conditions. In contrast, deoxygenation increased the CO2 content of the rbc, plasma and whole blood in the trout. These results demonstrate that the Haldane effect is much less important for CO2 transport in the hagfish as compared to the trout. The relative importance of the rbc and plasma in blood CO2 transport were roughly similar in hagfish and trout and were very different from that previously documented in another primitive vertebrate, the lamprey. In trout, however, the role of the rbc in CO2 carriage was increased upon the addition of the beta-adrenergic agonist isoproterenol (10(-5) M) to the blood. Taken together, these results and those recently collected for lampreys demonstrate that changes in rbc characteristics during vertebrate evolution have probably resulted in several important transitions in the strategy for blood CO2 transport.