CO2 excretion and postcapillary pH equilibration in blood-perfused turtle lungs

Blood and Gill Carbonic Anhydrase in the Context of a Chondrichthyan Model of CO2 Excretion

Pacific spiny dogfish (Squalus suckleyi) have been widely used as a representative species for chondrichthyan CO₂ excretion. Pacific spiny dogfish have a slower red blood cell (RBC) carbonic anhydrase (CA) isoform than teleost fishes, extracellular CA activity, no endogenous plasma CA inhibitor, and plasma-accessible CA IV at the gills. Thus, both the RBC and plasma compartments contribute to bicarbonate ion (HCO3-) dehydration at the gills for CO₂ excretion in contrast to teleost fishes, in which HCO3- dehydration is restricted to RBCs. We compared CA activity levels, subcellular localization, and presence of plasma CA inhibitors in the blood and gills of 13 chondrichthyans to examine the hypothesis that the dogfish model of CO₂ excretion applies broadly to chondrichthyans. In general, blood samples from the 12 other chondrichthyans examined had lower RBC CA activity than teleosts, some extracellular CA activity, and no endogenous plasma CA inhibitor. While type IV-like membrane-associated CA was found in the gills in all four of the chondrichthyans examined, S. suckleyi had three times more CA activity (183±13.2 μmol CO₂ min^-1 mg protein^-1) in the microsomal (membrane) fraction of gills than the other three. In addition, unexpected variation in CA characteristics was observed between chondrichthyan species. Thus, in general, it appears that the pattern of CA distribution in fishes can be generally categorized as either chondrichthyan or teleost models. However, further studies should examine the functional significance of the within-chondrichthyan differences we observed and investigate whether CO₂ excretion patterns exist along a continuum or in discrete groups.

The structure and function of carbonic anhydrase isozymes in the respiratory system of vertebrates

Carbonic anhydrase is a ubiquitous metalloenzyme that catalyzes the reversible hydration/dehydration of carbon dioxide. To date, 16 different CA isozymes have been identified in mammals, and several novel isozymes have also been identified in non-mammalian vertebrates. These isozymes are involved in many physiological processes; however, one of the most important roles is facilitating the transport and subsequent excretion of carbon dioxide. As such, CA isozymes are found at virtually every step of the process, including the metabolic site of CO(2) production (muscle), the circulating red blood cells, and the primary respiratory surface (gills/lungs). This review will examine the structural characteristics that are integral to CAs participation in respiration, as well as highlight the specific roles and tissues that the different CA isozymes are involved in.

Pulmonary carbonic anhydrase in vertebrate gas exchange organs

Carbonic anhydrase (CA) catalyzes the interconversion of CO(2) and HCO(3)(-). Intracellular (extravascular) and intravascular (extracellular) CA has been identified and localized in the lungs of reptiles and mammals. Less information is known, however, on the presence of intravascular CA in the lungs of amphibians and avians. In the present study, perfusion studies were used to compare the catalytic activity of pulmonary intravascular CA in reptiles and mammals. In addition, SDS-resistant CA activity was examined in microsomal fractions prepared from gill/lung tissue from representative animals in each vertebrate class. Finally, the CNO(-) sensitivity of the microsomal CA activity was compared. No SDS-resistant CA activity was found in gill microsomal fractions of several fish species. In contrast, the data suggest that SDS-resistant, intravascular pulmonary CA activity is present in air-breathing vertebrates with vastly differing lung morphologies and that the kinetics of inhibition is remarkably comparable between the vertebrate classes.

Pulmonary carbonic anhydrase in the garter snake, Thamnophis sirtalis

This study examined whether the snake lung possesses intravascular carbonic anhydrase (CA). Lungs were perfused with control salines and with salines containing CA inhibitors. Perfusion with control salines resulted in a stable CO(2) excretion, whereas CA inhibitors significantly reduced pulmonary CO(2) excretion. Membrane-permeable and membrane-impermeable CA inhibitors produced comparable decreases in CO(2) excretion, suggesting that extracellular, intravascular CA participated in the pulmonary CO(2)-HCO3(-)-H(+) reactions. Treatment of lungs with phosphatidylinositol specific-phospholipase C (PI-PLC) significantly decreased CO(2) excretion, indicating that CA was connected to the luminal endothelial cell membrane by a phosphatidylinositol glycan linkage. Taken together, these results are the first to demonstrate the presence of membrane-bound, intravascular CA (CA IV) in the snake lung.