Effect of aging on power output properties in rat skinned cardiac myocytes

Abstract

Aging is generally associated with a decline in several indices of cardiac function. The cellular mechanisms for this decline are not completely understood. The ability of the myocardium to perform external work (power output) is a critical aspect of ventricular function. The purpose of this study was to determine the effect of aging on loaded shortening and power output properties. We measured force-velocity properties in permeabilized (skinned) myocytes from the hearts of 9-, 24-, and 33-month-old male Fisher 344 × Brown Norway F1 hybrid rats (F344BN) during loaded contractions using a force-clamp technique. Power output was calculated by multiplying force and shortening velocity values. We found that peak power output normalized to maximal force was significantly decreased by 18% and 31% in myocytes from 24- and 33-month-old group, respectively, compared with 9-month group (p < .05). These results suggest that aging is associated with a significant decrease in the ability of the myocardium to do work.

Figure 1.

Composite force–velocity curves for myocytes from different aged rats. Data were compiled from 60, 49, and 55 myocytes for 9-, 24-, and 33-month-old rats, respectively. Isotonic shortening velocity values at each load were averaged from all myocytes in each group. Data points are presented as mean ± SD. • and dashed line = 33 months old; ♦ and dotted line = 24 months old; ○ and solid line = 9 months old. Lines are the best-fit regression line using the Hill equation as described in Methods.

Figure 2.

Force–power curve constructed from force–velocity data. In each myocyte at each load, force values (expressed as P/P_o) were multiplied times mean velocity values (expressed ML/s) to result in a value of power output for that load. Data points are means ± SD for all cells at that age. • and dashed line = 33 months old; ♦ and dotted line = 24 months old; ○ and solid line = 9 months old. Lines are the best-fit regression line using the Hill equation as in Methods. Peak power output was taken from the highest point in the best-fit line.

Figure 3.

(a) Representative 6% sodium dodecyl sulfate–polyacrylamide gel electrophoresis showing the distribution of myosin heavy chain (MHC) isoforms in ventricular homogenates from 9-, 24-, and 33-month-old rats. We found significant differences in MHC isoform content in the 24- and 33-month-old rats compared with the 9-month-old rats. (b) Bar graph representation of the MHC isoform distribution in heart homogenates from 9-, 24-, and 33-month-old rats. Values are expressed as the % of total MHC that is αMHC. *Significantly different (p < .05) from 9 month.