Ionic exchanges of turtle shell in vitro and their relevance to shell function in the anoxic turtle

To understand more fully the role of the in vivo turtle shell in buffering lactic acid produced during prolonged anoxia, powdered turtle shell was incubated in vitro at constant pH (6.0, 6.5, 7.0, 7. 5 or 8.0) in electrolyte solutions simulating extracellular fluid. Exchanges of ions and CO2 between the shell and solution were evaluated by measuring pre- and post-incubation solution concentrations of calcium, magnesium, sodium, potassium, chloride, phosphate and lactate. The production of CO2 from the shell and lactate within the shell were also measured. We observed that calcium and magnesium, but not phosphate, were released from the shell in association with CO2 and that the magnitude of release of each increased with solution acidity. The amount of acid titration required to maintain constant pH also increased as solution pH fell. The CO2 loss, in mmol, was approximately half the acid titration (in mmol), indicating that the evolved CO2 derives from carbonate. When the incubating solution contained lactate (50 mmol l-1), lactate entered the shell and again the amount entering the shell increased with solution acidity. Shell samples containing high initial lactate levels lost lactate to the solution and at high pH (7.5) acidified the solution and required NaOH titration for pH-stat control. These results are consistent with observations on anoxic turtles in vivo and confirm the important role of the shell as a source of buffer and as a storage site for lactate.