Tissue Glycogen and Extracellular Buffering Limit the Survival of Red-Eared Slider Turtles during Anoxic Submergence at 3°C

The goal of this study was to identify the factors that limit the survival of the red-eared slider turtle Trachemys scripta during long-term anoxic submergence at 3 C. We measured blood acid-base status and tissue lactate and glycogen contents after 13, 29, and 44 d of submergence from ventricle, liver, carapace (lactate only), and four skeletal muscles. We also measured plasma Ca , Mg , Na , K , Cl , inorganic phosphate (Pi), lactate, and glucose. After 44 d, one of the six remaining turtles died, while the other turtles were in poor condition and suffered from a severe acidemia (blood from 7.77) caused pH p 7.09 by lactic acidosis (plasma lactate 91.5 mmol L ). An initial respiratory acidosis attenuated after 28 d. Lactate rose to similar concentrations in ventricle and skeletal muscle (39.3–46.1 mmol g ). Liver accumulated the least lactate (21.8 mmol g ), and carapace accumulated the most lactate (68.9 mmol g ). Plasma Ca and Mg increased significantly throughout submergence to levels comparable to painted turtles at a similar estimated lactate load. Glycogen depletion was extensive in all tissues tested: by 83% in liver, by 90% in ventricle, and by 62%–88% in muscle. We estimate that the shell buffered 69.1% of the total lactate load, which is comparable to painted turtles. Compared with painted turtles, predive tissue glycogen contents and plasma concentrations were low. We believe these difHCO3 ferences contribute to the poorer tolerance to long-term anoxic submergence in red-eared slider turtles compared with painted turtles. * Corresponding author. Present address: Department of Anesthesia, University of California, San Francisco, California 94143; e-mail: [email protected]. Present address: Department of Biological and Physical Sciences, Kennesaw State University, Kennesaw, Georgia 30144. Physiological and Biochemical Zoology 79(4):736–744. 2006. 2006 by The University of Chicago. All rights reserved. 1522-2152/2006/7904-5113$15.00 Introduction For most animals, tolerance to hypoxia or anoxia is determined by their ability to downregulate metabolism, maintain and utilize stored glycogen, and manage the end products of anaerobic metabolism, principally lactate and H (Hochachka 1986; Jackson 2002). Freshwater turtles are extreme examples of anoxiatolerant animals, and the western painted turtle Chrysemys picta, in particular, can survive from 3 to 6 mo at 3 C without oxygen (Jackson 2000b). Painted turtles dramatically reduce their metabolic rates, utilize extremely high tissue glycogen contents, and exploit their large skeleton to buffer lactic acid during anoxic submergence. These traits allow the painted turtle to survive winters trapped in ice-covered ponds that may become severely hypoxic or anoxic. A closely related species, the red-eared slider Trachemys scripta, is found in habitats that are similar in most respects to that of the painted turtle except that they are found in regions of the country that do not routinely freeze during winter months (Conant and Collins 1998). One might expect, therefore, that these turtles cannot tolerate periods of anoxic submergence to the same extent as painted turtles. Indeed, it has been shown that adult red-eared slider turtles can survive anoxic submergence at 3 C for only 25–41 d (Ultsch 1985). We hypothesized that the poorer tolerance of the red-eared slider could be due to the deficiencies in one or more of the traits contributing to anoxia tolerance in the painted turtle. Our objective in this study, therefore, was to perform a comprehensive analysis of the slider turtle’s response to anoxic submergence at 3 C, including blood ionic and acid-base status, tissue lactate and glycogen contents, and the contributions of the shell to the buffering of lactic acid. Our results indicate that glycogen depletion and low predive are major facHCO3 tors limiting anoxic survival in this species. Material and Methods