Jeffrey M. Duerr

Ion Transport Processes of Crustacean Epithelial Cells

Epithelial cells of the gut, antennal glands, integument, and gills of crustaceans regulate the movements of ions into and across these structures and thereby influence the concentrations of ions in the hemolymph. Specific transport proteins serving cations and anions are found on apical and basolateral cell membranes of epithelia in these tissues. In recent years, a considerable research effort has been directed at elucidating their physiological and molecular properties and relating these characteristics to the overall biology of the organisms. Efforts to describe ion transport in crustaceans have focused on the membrane transfer properties of Na+/H+ exchange, calcium uptake as it relates to the molt cycle, heavy metal sequestration and detoxification, and anion movements into and across epithelial cells. In addition to defining the properties and mechanisms of cation movements across specific cell borders, work over the past 5 yr has also centered on defining the molecular nature of certain transport proteins such as the Na+/H+ exchanger in gill and gut tissues. Monovalent anion transport proteins of the gills and gut have received attention as they relate to osmotic and ionic balance in euryhaline species. Divalent anion secretion events of the gut have been defined relative to potential roles they may have in hyporegulation of the blood and in hepatopancreatic detoxification events involving complexation with cationic metals.

Mitochondrial physiology of diapausing and developing embryos of the annual killifish Austrofundulus limnaeus: implications for extreme anoxia tolerance

Diapausing embryos of the annual killifish Austrofundulus limnaeus have the highest reported anoxia tolerance of any vertebrate and previous studies indicate modified mitochondrial physiology likely supports anoxic metabolism. Functional mitochondria isolated from diapausing and developing embryos of the annual killifish exhibited VO2, respiratory control ratios (RCR), and P:O ratios consistent with those obtained from other ectothermic vertebrate species. Reduced oxygen consumption associated with dormancy in whole animal respiration rates are correlated with maximal respiration rates of mitochondria isolated from diapausing versus developing embryos. P:O ratios for developing embryos were similar to those obtained from adult liver, but were diminished in mitochondria from diapausing embryos suggesting decreased oxidative efficiency. Proton leak in adult liver corresponded with that of developing embryos but was elevated in mitochondria isolated from diapausing embryos. In metabolically suppressed diapause II embryos, over 95% of the mitochondrial oxygen consumption is accounted for by proton leak across the inner mitochondrial membrane. Decreased activity of mitochondrial respiratory chain complexes correlates with diminished oxidative capacity of isolated mitochondria, especially during diapause. Respiratory complexes exhibited suppressed activity in mitochondria with the ATP synthase exhibiting the greatest inhibition during diapause II. Mitochondria isolated from diapause II embryos are not poised to produce ATP, but rather to shuttle carbon and electrons through the Kreb’s cycle while minimizing the generation of a proton motive force. This particular mitochondrial physiology is likely a mechanism to avoid production of reactive oxygen species during large-scale changes in flux through oxidative phosphorylation pathways associated with metabolic transitions into and out of dormancy and anoxia.

Anoxia Tolerance During Vertebrate Development - Insights from Studies on the Annual Killifish Austrofundulus limnaeus

With a rare few exceptions, vertebrates are extremely sensitive to a lack of oxygen and can survive for only brief episodes of oxygen deprivation (Nilsson and Lutz, 2004). In fact, when differences in body temperature are taken into account, endotherm and ectotherm vertebrates share a very similar survival time in anoxia, and similar symptoms of cellular and tissue damage and death (Nilsson and Lutz, 2004). Thus, there appears to be a common limit to survival of anoxia in most vertebrates that may be supported by common limits to metabolic and physiologic systems. The few exceptions to this rule (all aquatic vertebrates) have developed novel mechanisms to support tolerance of long-term anoxia (Nilsson and Lutz, 2004; Podrabsky et al., 2007).

Osmotic Effects on Mitochondria from Two Species of Amphibian, Bufo marinus and Rana catesbeiana

The effects of ionic and/or osmotic change on skeletal muscle mitochondrial performance were investigated. Two substrates, pyruvate and glutamate, and variation in osmotic pressure from 205 to 360 mosm in KCl or mannitol/sucrose media had no effect on maximal respiratory rate (state 3) or coupling (respiratory control ratio) in either species. Over an equivalent range of osmolalities associated with dehydration, organismic maximal O₂-consumption rates are severely diminished with increasing osmolalities. The data do not support a mitochondrial limit to organismic O₂ consumption under dehydration. There were interspecies differences in state 3 respiration and coupling that were similar to diferences noted in mitochondria isolated from fish red and white muscle, with toad mitochondria behaving more like red muscle and frog mitochondria behaving more like white muscle.