Impaired Cell Shortening and Relengthening with Increased Pacing Frequency are Intrinsic to the Senescent Mouse Cardiomyocyte

Increased heart rate enhances cardiac contractility and accelerates relaxation. Both the force– and relaxation–frequency relationships are critical to myocardial function, especially during stress, and have been shown to be impaired in senescent myocardium. While senescent myocardium is characterized by decreased sarcoplasmic reticulum calcium ATPase activity, it is unclear if altered calcium regulation is directly responsible for the attenuated contractility and relaxation observed with increasing pacing frequency in aged myocardium. We examined this issue using freshly dissociated left ventricular myocytes, isolated from young adult and senescent mouse hearts. Myocytes were paced from 2 to 9 Hz at 37°C, and cell shortening and [Ca2+]iwere simultaneously measured using video edge-detection and fura-2 fluorescence, respectively. In adult myocytes, increasing the pacing rate resulted in a progressive increase in percent cell shortening (CS) (P<0.01). This positive CS–frequency relationship was paralleled by an increase in [Ca2+]itransient amplitude (P<0.05). In contrast, the CS–frequency relationship was blunted in senescent myocytes with no increase in percent CS or [Ca2+]itransient amplitude with increasing pacing rate. With increased pacing, the decreases in time constants (τ) of cell relengthening and Ca2+transient decay were much steeper in adult compared to senescent myocytes (P<0.05). This study demonstrates that adult mouse myocytes exhibit augmented intracellular Ca2+transient amplitude and enhanced intracellular Ca2+removal with increasing pacing frequency, resulting in increased cell shortening and enhanced relengthening with frequency. In contrast, senescent mouse myocytes exhibit impaired calcium handling with increasing pacing frequency, which correlated with impairment of both cell shortening and relengthening.