Turnover of muscle lipids and response to exercise differ between neutral and polar fractions in a model songbird, the zebra finch

The turnover rates of tissues and their constituent molecules give us insights into animals' physiological demands and their functional flexibility over time. Thus far, most studies of this kind have focused on protein turnover, and few have considered lipid turnover despite an increasing appreciation of the functional diversity of this class of molecules. We measured the turnover rates of neutral and polar lipids from the pectoralis muscles of a model songbird, the zebra finch (Taeniopygia guttata, N= 65), in a 256 day C-3/C-4 diet shift experiment, with tissue samples taken at 10 time points. We also manipulated the physiological state of a subset of these birds with a 10 week flight training regimen to test the effect of exercise on lipid turnover. We measured lipid delta C-13 values via isotope ratio mass spectrometry (IRMS) and estimated turnover in different fractions and treatment groups with non-linear mixed-effect regression. We found a significant difference between the mean retention times (tau) of neutral and polar lipids (t(119) = -2.22, P = 0.028), with polar lipids (tau = 11.80 +/- 1.28 days) having shorter retention times than neutral lipids (tau = 19.47 +/- 3.22 days). When all birds were considered, we also found a significant decrease in the mean retention time of polar lipids in exercised birds relative to control birds (difference = -2.2 +/- 1.83 days, t(56) = -2.37, P = 0.021), but not neutral lipids (difference = 4.2 +/- 7.41 days, t(56) = 0.57, P = 0.57). A larger, more variable neutral lipid pool and the exposure of polar lipids in mitochondrial membranes to oxidative damage and increased turnover provide mechanisms consistent with our results.