ABSTRACT
Th e extant lungfi sh – that comprise the predominantly water-breathing Neoceratodus forsteri from Australia and the obligate air - breathing Protopterus and Lepidosiren from Africa and South America, respectively – face extraordinary variations in exogenous factors (O2 and water availability and temperature) and endogenous constraints (bimodal breathing and estivation in moist or dried mud). Th ese circumstances predictably result in inordinate variations in factors (blood levels of pH, O2 and CO2 tensions, urea and lactate levels and osmolality) that mandatorily aff ect O2 and CO2 binding by the circulating hemoglobin (Hb). Th is treatise focuses on the distinctive, compensatory adaptations in the gas-transporting functions of lungfi sh blood and Hb and the underlying molecular mechanisms that support aerobic metabolism in lungfi sh under harsh conditions. Keywords: lungfi sh, hemoglobin, oxygen affi nity, carbon dioxide, estivation
In transporting respiratory gases, hemoglobin (Hb) in blood bridges wide and independent variations in tensions of these gases at the respiratory surfaces
and in the tissues. In meeting these requirements the gas binding properties of Hbs from ectotherm vertebrates display remarkable adaptability (Johansen and Lenfant 1971; Weber 1992). Th is applies particularly to lungfi sh, a sister group to tetrapod vertebrates, that have lungs and gills and are capable of bimodal breathing. Some lungfi sh species moreover survive drought by estivating in dried mud for extended periods [up to 7 years (Lomholt 1993)] risking subjection to strongly increased levels of excretory products (CO2, protons, lactate, ammonium, urea) and body fl uid osmolality (decreased water activity) – factors that are known to perturb the gas binding properties of Hb. Lungfi sh thus are ideal for studying the mechanistic basis underlying the ability of animals to survive combinations of harsh exogenous factors (oxygen and water availability, temperature, etc.) and endogenous constraints (activity, suspended animation, breathing mode, etc.). Th e extant lungfi shes that comprise Neoceratodus forsteri (family Ceratodontidae) found in Australia, and four species of Protopterus (P. dolloi, P. aethiopicus, P. annectans and P. amphibius) from Africa and Lepidosiren forsteri from South America (members of the family Epidosirenidae) diff er in their breathing mode. Compared to the Neoceratodus that is a facultative air-breather, Protopterus and Lepidosiren are obligatory air-breathers with reduced gills. Th e roles the lungs compared to that of the gills and skin in gas exchange thus vary greatly in fi sh living in water with access to air. Whereas O2 uptake and CO2 elimination occur via gills and skin in Neoceratodus, O2 uptake is predominantly pulmonary in Protopterus and almost entirely pulmonary in Lepidosiren – where the gills remain a major route for CO2 elimination (Lenfant et al. 1970). Th us, in Lepidosiren, pulmonary gas exchange accounts for more than 95% of total O2 uptake at elevated temperatures (Johansen and Lenfant 1967; Bassi et al. 2005) and 99% of the total morphological diff usion capacity lies in the lungs, compared to <1% in the skin and 0.001% in the gills (de Moraes et al. 2005). Th is chapter focuses on respiratory properties of lungfi sh blood and Hbs, specifi cally the adaptations to exogenous and endogenous factors (environmental conditions, air/water breathing, estivation, etc.) and the implicated cellular and molecular mechanisms.
