UA ECOL 437 - Lactic Acid Buffering by Bone and Shell in Anoxic Softshell and Painted Turtles

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290Lactic Acid Buffering by Bone and Shell in Anoxic Softshell andPainted TurtlesD. C. Jackson1,*A. L. Ramsey1J. M. Paulson1C. E. Crocker1,2G. R. Ultsch21Department of Molecular Pharmacology, Physiology, andBiotechnology, Brown University, Providence, Rhode Island02912;2Department of Biological Sciences, University ofAlabama, Tuscaloosa, Alabama 35487Accepted 3/1/00ABSTRACTWe tested two hypotheses: first, that the inferior anoxia tol-erance of the softshell turtle, Apalone spinifera, compared tothe western painted turtle, Chrysemys picta bellii, is related toits less mineralized shell, and second, that turtle bone, like itsshell, stores lactate during prolonged anoxia. Lactate concen-trations of blood, hindlimb bone, and shell were measured onnormoxic Apalone and Chrysemys and after anoxic submer-gence at 107C for 2 and 9 d, respectively. Blood and shellconcentrations of Ca21,Mg21,Na1,K1, and inorganic phos-phate (Pi; for shell only) were also measured. Because a pre-liminary study indicated lactate distribution in Chrysemysthroughout its skeleton during anoxia at 207C, we used hind-limb bones as representative skeletal samples. Apalone shell,though a similar percentage of body mass as Chrysemys shell,had higher water content (76.9% vs. 27.9%) and only 20%–25%as much Ca21,Mg21,CO2, and Pi. When incubated at constantpH of 6.0 or 6.5, Apalone shell powder released only 25% asmuch buffer per gram wet weight as Chrysemys shell. In ad-dition, plasma [Ca21] and [Mg21] increased less in Apaloneduring anoxia at an equivalent plasma lactate concentration.Lactate concentrations increased in the shell and skeletal bonein both species. Despite less mineralization, Apalone shell tookup lactate comparably to Chrysemys. In conclusion, a weakercompensatory response to lactic acidosis in Apalone correlates*To whom correspondence should be addressed; e-mail: [email protected] and Biochemical Zoology 73(3):290–297. 2000. q 2000 by TheUniversity of Chicago. All rights reserved. 1522-2152/2000/7303-9984$03.00with lower shell mineralization and buffer release and maypartially account for the poorer anoxia tolerance of this species.IntroductionAlthough freshwater turtles in general possess considerable tol-erance to anoxia (Belkin 1963), significant differences in thiscapacity exist among species (Ultsch et al. 1984). These differ-ences may relate to their individual environments and habitsand may be explained by varying expression of traits contrib-uting to survival during anoxia.A particularly striking contrast exists between the paintedturtle, Chrysemys picta, and the softshell turtle, Apalone spi-nifera. Chrysemys is the most anoxia-tolerant turtle yet studiedand can recover from submergences in O2-depleted water last-ing3moat37C and 10 d at 107C (Herbert and Jackson 1985a)and can remain alive under these conditions for as long as 5mo and 17 d, respectively (Ultsch et al. 1984). The softshellturtle, in contrast, survived only 2.6 d of anoxia at 107C (Ultschet al. 1984), and submergence durations from which it couldrecover are undoubtedly considerably less. Compared to Chrys-emys, Apalone has a more gas-permeable integument, and byselecting microhabitats with adequate aquatic oxygen levels, thisturtle can support aerobic metabolism by oxygen uptake fromthe water. In aerated water at 107C, for example, most sub-merged Apalone survived more than 100 d, and their limit wasnot established (Ultsch et al. 1984). Chrysemys, on the otherhand, has an integument less suited for aquatic gas exchange,and the advantages it gains from aquatic oxygen are less dra-matic. During hibernation it may select refuges in anoxic mud(Ernst 1972), although this is not always the case (St. Clair andGregory 1990; Crocker et al. 2000).A key physiological difference between Chrysemys and Apa-lone during anoxic submergence at 107C is the rate at whichplasma lactate concentration increases, an index of anaerobicmetabolic rate. In Chrysemys, lactate increased over the 10 dof submergence by 0.28 mmol L h21, which was less than 30%the rate of increase in Apalone over 1 d of anoxia (Jackson etal. 1984). As a consequence, blood pH fell more rapidly inApalone, and this may have been a major factor limiting survivalin this animal. The low rate of lactate accumulation in Chrys-emys and the associated depressed rate of anaerobiosis (Herbertand Jackson 1985b) are fundamental traits possessed by thisturtle that permit long anoxic submergences.Bone and Shell Buffering in Anoxic Turtles 291Compensatory buffering capability may be another physio-logical difference between these turtles that helps explain theirrespective tolerances to anoxia. Chrysemys has higher extra-cellular [ ] than Apalone (Ultsch et al. 1984), and of even2HCO3greater importance, Chrysemys has a large calcified shell thataccounts for the bulk of lactic acid buffering by this animalduring long-term anoxic submergence at low temperature(Jackson 1997). The shell functions in two ways: first, calciumand magnesium carbonates are released from the shell to bufferlactic acid in the extracellular fluid, and second, lactic acidenters the shell where it is buffered and sequestered during theanoxic period (Jackson 1997; Jackson et al. 1996). The softshellturtle, Apalone, as its name indicates, has a less mineralizedshell and, therefore, may lack the capacity for shell bufferingpossessed by Chrysemys. This lack may be a second factor ac-counting for the poorer performance of Apalone during anoxia.A primary object in this study, therefore, was to quantitativelycompare shell composition and function during anoxia in thesetwo chelonian species in order to consider the hypothesis thatdifferences contribute to the disparity in anoxia tolerance. Asecond related objective was to determine whether the turtle’sskeleton (that portion not incorporated into the shell) alsofunctions as a storage site for lactate during anoxia and tocompare the behavior of the two species in this regard.Material and MethodsAnimalsWestern painted turtles (Chrysemys picta bellii) and softshellturtles (Apalone spinifera aspera) were obtained from com-mercial sources in Wisconsin and Alabama, respectively. Theywere housed before the study under a 10L : 14D photoperiodin large tanks with shallow water and were fed 3–4 times perweek with earthworms. The studies were performed in lateDecember and early January.Experimental Protocols—In VivoEach of the projects involved analysis of blood,


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UA ECOL 437 - Lactic Acid Buffering by Bone and Shell in Anoxic Softshell and Painted Turtles

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