Linking metabolic and contractile dysfunction in aged cardiac myocytes

Abstract

Aging is associated with declining cardiac contractile function as well as changes in metabolism and mitochondrial function. The relationship between age-related changes in cardiac metabolism and declining cardiac contractile function has not been determined. In order to define the role energetics play in changes in contractile function, we measured mitochondrial NADH, [NADH]_m, during continuous contractions of isolated left ventricular myocytes from young (Y) and old (O) FBN rats. Second, we explored the role of metabolic disruption with rotenone and increased workload with isoproterenol (ISO) had on age-related changes in myocytes shortening. Single, intact myocytes were stimulated for 10 min of continuous contraction at either 2 Hz or 4 Hz while being perfused with Ringer's solution. Properties of shortening (peak shortening and rate of shortening) were measured at the onset (T0) and after 10 min (T10) of continuous contraction, and the decline in shortening over time (T10/T0) was determined. Although young and old myocytes had similar contractile function under resting conditions, old myocytes demonstrated decrements in [NADH]_m during continuous stimulation, while young myocytes maintained constant [NADH]m over this time. In addition, old myocytes exhibited impaired contractile function to a workload (ISO) and metabolic (rotenone) stress compared to young myocytes. Taken together, these results demonstrated that old myocytes are susceptible to stress-induced contractile dysfunction which may be related to altered cellular energetics.

Keywords: Aging; cardiac myocytes; contractile function; mitochondrial function.

Figure 1

Representative trace of an old cardiac myocyte during simultaneous contractile and NADH measurements. T0 measurements occur after 30 sec of contraction in order to ensure steady‐state NADH tracings and T10 measurements occurred after 10 min of continuous stimulation. The “Max” and “Min” represent the maximum and minimum NADH fluorescence obtained using cyanide and FCCP, respectively, as described in Methods. The Max and Min measures were performed after the 10 min of continuous stimulation while the myocyte was quiescent. In this case, T0 and T10 NADH levels were 63% and 23% of max, respectively.

Figure 2

Effects of recovery period on peak shortening. After 10 min of continuous stimulation, peak shortening decreased by ~13% at T10 compared to T0. Following 10 min of quiescence, we restimulated the myocytes and measured peak shortening, and found that peak shortening recovered to ~99% of the value at T0. n = 18 myocytes. Values represent means ± SE. *P < 0.05 main effect of continuous stimulation.

Figure 3

Single‐cell NADH concentrations at T0 and after 10 min (T10) of continuous stimulation. NADH values are expressed as a percent of maximum NADH values as described in Methods. n = 34 and 30 cells in old myocytes versus young myocytes, respectively. *P < 0.05 old versus young at same time point. †P < 0.05 T10 versus T0 within group. Values represent means ± SE

Figure 4

Rotenone was used to determine the effect of metabolic disruption on contractile performance. Contractile properties are expressed as absolute values before and after rotenone administration. Statistical analyses were performed on the change in contractile properties before and after rotenone administration (rotenone/without rotenone) between young and old myocytes. n = 28 cells and 26 cells in old myocytes versus young myocytes, respectively. *P < 0.05 main effect of rotenone treatment; ‡P < 0.05 between young and old myocytes. Values are means ± SE

Figure 5

Isoproterenol was administered to increase cellular stress on young (open bars) and old myocytes (slashed bars). Contractile properties are expressed as a ratio of the value after 5 min of isoproterenol exposure and the initial value (T5/T0). n = 40 cells in old myocytes and young myocytes, respectively *P < 0.05 old versus young. Values indicate means ± SE