Lactic acid buffering by bone and shell in anoxic softshell and painted turtles

We tested two hypotheses: first, that the inferior anoxia tolerance of the softshell turtle, Apalone spinifera, compared to the western painted turtle , Chrysemys picta bellii, is related to its less mineralized shell, and sec ond, that turtle bone, like its shell, stores lactate during prolonged anox ia. Lactate concentrations of blood, hindlimb bone, and shell were measured on normoxic Apalone and Chrysemys and after anoxic submergence at 10 degre es C for 2 and 9 d, respectively. Blood and shell concentrations of Ca2+, M g2+, Na+, K+, and inorganic phosphate (P-p; for shell only) were also measu red. Because a preliminary study indicated lactate distribution in Chrysemy s throughout its skeleton during anoxia at 20 degrees C, we used hindlimb b ones as representative skeletal samples. Apalone shell, though a similar pe rcentage of body mass as Chrysemys shell, had higher water content (76.9% v s. 27.9%) and only 20%-25% as much Ca2+, Mg2+, CO2, and P-i. When incubated at constant pH of 6.0 or 6.5, Apalone shell powder released only 25% as mu ch buffer per gram wet weight as Chrysemys shell. In addition, plasma [Ca2] and [Mg2+] increased less in Apalone during anoxia at an equivalent plasm a lactate concentration. Lactate concentrations increased in the shell and skeletal bone in both species. Despite less mineralization, Apalone shell t ook up lactate comparably to Chrysemys. In conclusion, a weaker compensator y response to lactic acidosis in Apalone correlates with lower shell minera lization and buffer release and may partially account for the poorer anoxia tolerance of this species.