Substrates for polarographic assays of OXPHOS through complex I (glutamate), complex II (succinate), complex III (DHQ), and complex IV (TMPD) are shown (A). ADP-stimulated (2 mM) rates of state 3 respiration were determined in freshly isolated mitochondria from 6 month (light bars, n=8) and 24 month (dark bars, n=8). Uncoupled rates determined after the addition of DNP are shown for substrates: glutamate, succinate, and DHQ. ADP-stimulated (2 mM) rates of state 3 respiration are shown for the substrate combinations: pyruvate and malate (P+M), α-ketoglutarate (αKG) and malonate, acetyl-carnitine and malate (AC+M), palmitoyl-carnitine and malate (PC+M), and palmitoyl-CoA with malate (PCoA+M) and carnitine (B). Bars represent the mean value of each experimental group and the error bars indicate the standard deviation from the mean. P-values <0.05 are denoted by an asterisk (*) and p-values <0.01 are denoted by a cross (†) above the indicated pair and were calculated from non-paired two-tailed student t-test.