Multiple lines of evidence now indicate that
Multiple lines of evidence now indicate that mitochondrial decay is a fundamental factor that leads to alterations in energy metabolism in the aged heart (Hagen et al., 2002, Judge and Leeuwenburgh, 2007, Lesnefsky et al., 2001c). Nevertheless, the exact biochemical events that cause such an alteration in energetics are not completely understood. Furthermore, the existence of two cardiac mitochondrial subpopulations which display different functional features (Palmer et al., 1977, Riva et al., 2005) increases the complexity of a thorough characterization of the molecular events underlying the age-related deterioration of mitochondrial function. Histologically, subsarcolemmal mitochondria (SSM) are associated with the sarcolemma, and appear to provide energy for the regulation of myocellular ion exchange and homeostasis (Kaasik et al., 2001, Sasaki et al., 2001). On the other hand, interfibrillar mitochondria (IFM) are intercalated along the myofibrils, and are believed to supply ATP for the myocardial contraction–relaxation angiopoietin (Hoppel et al., 1982, Kaasik et al., 2001). A number of studies show that IFM functionally deteriorate with age. This subpopulation exclusively displays decreased ADP-stimulated respiration, altered electron transport chain (ETC) components, higher rates of oxidant appearance, and increased susceptibility to both permeability transition and damage during ischemia and reperfusion (Fannin et al., 1999, Hofer et al., 2009, Lesnefsky et al., 2001a, Lesnefsky et al., 2001b, Suh et al., 2003).
To our knowledge, no direct characterization as to how myocardial aging affects CPT1 activity and/or substrate utilization in the two mitochondrial subpopulations has been undertaken. Thus, it is not known whether the aging lesion for fatty oxidation lies in a particular mitochondrial subpopulation or more generally results from cardiac decrements in l-carnitine levels which decline with age in humans (Costell et al., 1989, Opalka et al., 2001) and also in rats (Maccari et al., 1990, Tanaka et al., 2004). Thus, the age-related loss of myocardial carnitine levels may diminish overall CPT1 activity, and/or exacerbate enzyme catalytic dysfunction in a particular mitochondrial subpopulation. If carnitine levels indeed contribute to lower CPT1 activity, then it is equally possible that general CPT1-mediated fatty acid oxidation can be remediated by increasing myocardial l-carnitine content. In this regard, cardiac mitochondrial bioenergetics in aged rats has been improved following dietary supplementation with the l-carnitine analogue, acetyl-l-carnitine (ALCAR) (Hagen et al., 2002, Paradies et al., 1994, Paradies et al., 1995, Paradies et al., 1999).
In the present work, the effects of aging and ALCAR supplementation on the activity of CPT1 were investigated in rat heart SSM and IFM. Our data show that aging selectively decreases CPT1 activity in IFM by reducing enzyme catalytic efficiency for palmitoyl-CoA utilization without inducing significant alterations in the kinetic parameters for l-carnitine. These findings suggest that the decline in IFM CPT1 activity could be a key factor in the mechanism by which fatty acid utilization decreases, and as a consequence, induces lipid toxicity in the aging heart (Slawik and Vidal-Puig, 2006, Wende and Abel, 2010).
Materials and methods
Discussion The present work provides a new perspective on the age-associated decline in fatty acid-supported cardiac bioenergetics. To the best of our knowledge, this is the first side-by-side characterization of CPT1 activity in two sublocalized mitochondrial populations of the aging heart. Our data show that age induces a ∼30% decline in CPT1 activity, but only in IFM (Table 1 and Fig. 5). While this activity loss is consistent with previous observations for diminished fatty acyl-CoA supported bioenergetics in hearts from old rats and mice (Abu-Erreish et al., 1977, McMillin et al., 1993, Odiet et al., 1995), our results now indicate that the aging lesion for CPT1 is specific to mitochondria intercalated along the myofibrils. As the IFM supply ATP to the actomyosin complex during the contraction–relaxation cycle (Hoppel et al., 1982, Kaasik et al., 2001), cardiac pump function may therefore be even more adversely affected with age than the subtle loss in overall CPT1 activity would generally indicate. These results thus have important implications for myocardial energy reserve capacity.