31P-NMR determinations of cytosolic phosphodiesters in turtle hearts

As part of our ongoing research on cardiac hypoxia tolerance we have conducted ³¹P nuclear magnetic resonance (NMR) studies of isolated, perfused, working hearts from freshwater turtles, animals that are well known for their ability to tolerate prolonged periods of anoxia. A striking feature of turtle heart spectra is an extremely high concentration of NMR visible phosphodiesters (PDEs). Cardiac spectra from mammals, on the other hand, typically exhibit only a small resonance in the PDE region. Our aim in this study was to compare myocardial PDE profiles between the highly hypoxia tolerant western painted turtle (Chrysemys picta bellii) and the relatively hypoxia sensitive softshelled turtle (Trionyx spinifer) in order to begin to test the hypothesis that high constitutive levels of cytosolic PDEs may play a role in conferring hypoxia and ischemia tolerance on the myocardium. We also collected ³¹P-NMR spectra of PCA extracts of tissue from these species and from Kemp's ridley sea turtles (Lepidochelys kempi), as well as spectra from isolated hearts and PCA extracts of redeared sliders (Trachemys [formerly Pseudemys] scripta]). Total NMR visible phosphodiesters make up 24 ± 8.6% of the total NMR visible phosphorus in chrysemys hearts, 20.7 ± 5.9% in Trachemys hearts, but only 12.2 ± 5.1% in Trionyx hearts (P < 0.05). We have identified three distinct PDEs in turtle hearts: glycerophosphorylcholine (GPC); glycerophosphorylethanolamine (GPE); and serine ethanolamine phosphodiester (SEP). SEP is the dominant compound in Chrysemys and Trachemys (79.3 ± 10.2% and 84.7 ± 3.7% of total PDE, respectively), while GPC is most abundant in Trionyx (74.0 ± 4.3% of total PDE) and Lepidochelys (not quantitated). The function of this class of compounds is unclear but it has been suggested that cytosolic PDEs may function as lysophospholipase inhibitors, a role that would decrease the rate of membrane phospholipid turnover. Our comparative data suggest that cytosolic PDEs could play a role in phospholipid sparing during anoxic or ischemic stress in turtles but a direct test of this hypothesis awaits future experimentation.