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After the Devonian tetrapod land invasion, groups of terrestrial air-breathing and endothermic mammals repeatedly went back to live in the sea, relying on air intake at the surface for extended breath-hold dives to forage underwater, often at great depths and even in the coldest oceans. Studies on the physiological mechanisms behind prolonged breath-hold diving have a long history, including August Krogh's estimates of the maximal dive duration of the blue whale. Yet the molecular underpinnings of such extreme physiological adaptations are only beginning to be understood. The present review focuses on the molecular properties of the respiratory protein myoglobin that has repeatedly evolved an elevated net positive surface charge in several distantly related groups of diving mammals. This has enabled substantial increases of maximal myoglobin concentration in muscle cells, and hence muscle oxygen storage capacity and maximal dive duration. Using myoglobin net surface charge as a marker has allowed unprecedented insights into the evolution of mammal diving capacity and into the general mechanisms of adaptive protein evolution. From these findings it is argued, in an extension of the August Krogh principle, that for a large number of problems in molecular and evolutionary physiology there will be some protein of choice, or a few such proteins, on which it can be most conveniently studied., (Crown Copyright © 2020. Published by Elsevier Inc. All rights reserved.)

Red blood cells (RBCs) from most vertebrates restore volume upon hypertonic shrinkage and the mechanisms underlying this regulatory volume increase (RVI) have been studied extensively in these cells. Despite the phylogenetically interesting position of reptiles, very little is known about their red cell function. The present study demonstrates that oxygenated RBCs in all major groups of reptiles exhibit no or a very reduced RVI upon approximately 25% calculated hyperosmotic shrinkage. Thus, RBCs from the snakes Crotalus durissus and Python regius, the turtle Trachemys scripta and the alligator Alligator mississippiensis showed no statistically significant RVI within 120 min after shrinkage, while the lizard Tupinambis merianae showed 22% volume recovery after 120 min. Amiloride (10(-4) M) and bumetanide (10(-5) M) had no effect on the RVI in T. merianae, indicating no involvement of the Na(+)/H(+) exchanger (NHE) or the Na(+)/K(+)/2Cl(-) co-transporter (NKCC) or insentive transporters. Deoxygenation of RBCs from A. mississippiensis and T. merianae did not significantly affect RVI upon shrinkage. Deoxygenation per se of red blood cells from T. merianae elicited a slow volume increase, but the mechanism was not characterized. It seems, therefore, that the RVI response based on NHE activation was lost among the early sauropsids that gave rise to modern reptiles and birds, while it was retained in mammals. An RVI response has then reappeared in birds, but based on activation of the NKCC. Alternatively, the absence of the RVI response may represent the most ancient condition, and could have evolved several times within vertebrates.

This review highlights the use of analytical tools, recently developed in the comparative method of evolutionary biology, for the study of haemoglobin (Hb) adaptation. It focuses on the functional consequences of a previously largely ignored structural feature of Hb, namely the degree and positional specificity of histidine (His) substitution in Hb chains. The importance of His side chains for hydrogen ion buffering, blood CO(2) transport capacity and the molecular mechanism of the Bohr effect in vertebrate Hbs is discussed. Using phylogenetically independent contrasts, a significant correlation between the specific buffer value of Hb and the number of predicted physiological buffer groups from Hb sequence data is shown. In a new result, the evolution of the number of physiological buffer groups in 77 vertebrate species is reconstructed on a phylogenetic tree. The analysis predicts that teleost fishes, passeriform birds and some snakes have independently evolved a much-reduced specific buffer value of Hb, possibly for enhancing the efficiency of an acid load to change oxygen affinity via the Bohr effect. This analysis demonstrates how in comparative physiology analysis of genetic databases in an evolutionary framework can identify candidate species for further experimental in vitro and whole animal studies.

The structure and function of the pseudobranch has long interested scientists, but its overall role has remained a mystery. Previous studies have attributed respiratory, endocrine, osmoregulatory and sensory roles to the pseudobranch, and the present review concentrates on new findings. Perfusion experiments on the pseudobranch of the rainbow trout (Oncorhynchus mykiss) using both erythrocyte suspensions and Ringer solution have shown that this organ is able to generate values for the respiratory quotient (RQ) greater than 1.0. The release of carbon dioxide into the perfusate was found to be largely independent of flow between perfusion rates of 120-190 microl/min and could be inhibited by acetazolamide (10(-5) M), indicating a role for carbonic anhydrase. Noradrenaline (10(-5) M) had no effect on oxygen consumption or carbon dioxide release of the pseudobranch. The rate of carbon dioxide release was also dependent on the pH of the pre-pseudobranch perfusate, carbon dioxide release being reduced at lower perfusate pH values. Based on the glucose balance of the isolated saline-perfused rainbow trout pseudobranch and on the enzyme profiles for the rainbow trout, cod, swordfish and deep-water grenadier pseudobranch, it is suggested that the pentose phosphate shunt might be a source of carbon dioxide, yielding the high RQ values found for this organ. Most evidence now available indicates that the pseudobranch is integrally linked with the choroid rete and the supply of oxygen to the retina of the fish eye.