The impact of age and frailty on ventricular structure and function in C57BL/6J mice

Abstract

Key points: Heart size increases with age (called hypertrophy), and its ability to contract declines. However, these reflect average changes that may not be present, or present to the same extent, in all older individuals. That aging happens at different rates is well accepted clinically. People who are aging rapidly are frail and frailty is measured with a 'frailty index'. We quantified frailty with a validated mouse frailty index tool and evaluated the impacts of age and frailty on cardiac hypertrophy and contractile dysfunction. Hypertrophy increased with age, while contractions, calcium currents and calcium transients declined; these changes were graded by frailty scores. Overall health status, quantified as frailty, may promote maladaptive changes associated with cardiac aging and facilitate the development of diseases such as heart failure. To understand age-related changes in heart structure and function, it is essential to know both chronological age and the health status of the animal.

Abstract: On average, cardiac hypertrophy and contractile dysfunction increase with age. Still, individuals age at different rates and their health status varies from fit to frail. We investigated the influence of frailty on age-dependent ventricular remodelling. Frailty was quantified as deficit accumulation in adult (≈7 months) and aged (≈27 months) C57BL/6J mice by adapting a validated frailty index (FI) tool. Hypertrophy and contractile function were evaluated in Langendorff-perfused hearts; cellular correlates/mechanisms were investigated in ventricular myocytes. FI scores increased with age. Mean cardiac hypertrophy increased with age, but values in the adult and aged groups overlapped. When plotted as a function of frailty, hypertrophy was graded by FI score (r = 0.67-0.55, P < 0.0003). Myocyte area also correlated positively with FI (r = 0.34, P = 0.03). Left ventricular developed pressure (LVDP) plus rates of pressure development (+dP/dt) and decay (-dP/dt) declined with age and this was graded by frailty (r = -0.51, P = 0.0007; r = -0.48, P = 0.002; r = -0.56, P = 0.0002 for LVDP, +dP/dt and -dP/dt). Smaller, slower contractions graded by FI score were also seen in ventricular myocytes. Contractile dysfunction in cardiomyocytes isolated from frail mice was attributable to parallel changes in underlying Ca²⁺ transients. These changes were not due to reduced sarcoplasmic reticulum stores, but were graded by smaller Ca²⁺ currents (r = -0.40, P = 0.008), lower gain (r = -0.37, P = 0.02) and reduced expression of Cav1.2 protein (r = -0.68, P = 0.003). These results show that cardiac hypertrophy and contractile dysfunction in naturally aging mice are graded by overall health and suggest that frailty, in addition to chronological age, can help explain heterogeneity in cardiac aging.

Keywords: ageing; excitation-contraction coupling; frailty; ventricular myocyte.

Figure 1. Frailty increases with age

The distribution of frailty index (FI) scores in adult (203 ± 21 days; n = 24; filled symbols) and aged (811 ± 11 days; n = 54; open symbols) C57BL/6J mice are shown as a scatterplot (left) and as box plots. Mean FI scores, indicated by a horizontal line in the scatterplot, were higher in aged mice compared to adult mice (P < 0.001) although there was overlap in FI scores in the two groups. The box and whisker plot illustrates, from top to bottom: the 90th percentile, the 75th percentile, the median, the 25th percentile and the 10th percentile. Differences between age groups were assessed with a Mann–Whitney Rank Sum test.

Figure 2. Cardiac hypertrophy increases with age and is directly proportional to FI scores

A–C, mean HW, HW:BW and HW:TL ratios were higher in aged mice compared to adult mice (P < 0.003 for all). D–F, scatterplots show that FI scores are positively correlated with HW, HW:BW and HW:TL ratios (n = 10 adult and 30 aged mice). Differences between age groups were assessed with a t test or Mann–Whitney Rank Sum test; correlations were evaluated with linear regression analysis. HW = heart weight; HW:BW = heart weight to body weight ratio; HW:TL = heart weight to tibia length ratio. Filled symbols indicate adult mice and open symbols indicate aged mice.

Figure 3. Myocyte hypertrophy, characterized by larger cell widths and areas, increases as FI scores increase

A–C, scatterplots and box and whisker plots indicate that cell length, width and area were not affected by age. D, cell length was not correlated with FI scores. E–F, scatterplots show that cell width and area were positively correlated with frailty (n = 11 adult and 32 aged myocytes). Differences between age groups were assessed with a t test; the relationship between FI and indices of hypertrophy were evaluated with linear regression. Filled symbols indicate adult mice and open symbols indicate aged mice.

Figure 4. Left ventricular contractile function declines with age and is graded by frailty

A, representative examples of pressure recorded from perfused hearts isolated from mice with different FI scores. B, pressure recordings illustrated in A shown at an expanded time scale to illustrate differences in +dP/dt and –dP/dt. C–E, scatterplots plus box and whisker plots demonstrate that LVDP, +dP/dt and –dP/dt decreased with age. F–H, regression lines show that LVDP, +dP/dt and –dP/dt were graded by FI score and declined as frailty increased (n = 11 adult and 30 aged mice). Differences between age groups were assessed with either a t test or a Mann–Whitney Rank Sum test; correlations were performed with linear regression analysis. Filled symbols indicate adult mice and open symbols indicate aged mice.

Figure 5. The age‐dependent decline and slowing of contraction is graded by frailty in voltage‐clamped ventricular myocytes

A, representative recordings of myocyte contractions recorded from mice with different FI scores. B, peak contractions normalized to cell size were smaller in aged animals compared to adults. C and D, velocities of shortening and lengthening were slower in the aged group compared to the adult group. E–G, scatterplots show that peak contractions, as well as the velocities of shortening and lengthening, were inversely proportional to FI score. Differences between age groups were assessed with a t test and correlations were assessed by linear regression (n = 10 adult and 24 aged myocytes). Filled symbols indicate adult mice and open symbols indicate aged mice.

Figure 6. The peaks and rates of rise of the Ca²⁺ transients are attenuated by age and graded by FI score in voltage‐clamped ventricular myocytes

A, representative recordings of Ca²⁺ transients from myocytes isolated from mice with different FI scores. B, scatterplots plus box and whisker plots show that peak Ca²⁺ transients declined with age. C and D, the Ca²⁺ transient rates of rise declined with age, but decay rates were not affected. E–G, Ca²⁺ transient amplitudes and rates of rise were graded by FI, but decay rates were not. Differences between age groups were assessed using a Mann–Whitney Rank Sum test and correlations were evaluated with linear regression (n = 9 adult and 32 aged myocytes). Filled symbols indicate adult mice and open symbols indicate aged mice.

Figure 7. Frailty and age have no effect on either fractional SR Ca²⁺ release or SR Ca²⁺ content

A, representative examples of caffeine‐induced Ca²⁺ transients recorded from myocytes isolated from mice with different FI scores. B and D, SR Ca²⁺ content was similar in myocytes from the adult and aged groups and showed no obvious relationship with FI score. C and E, fractional SR Ca²⁺ release was not affected by either age or frailty. Differences between age groups were assessed using a t test and Mann–Whitney Rank Sum test and linear regression analysis was used to evaluate the impact of FI score (n = 6 adult and 11 aged myocytes). Filled symbols indicate adult mice and open symbols indicate aged mice.

Figure 8. Frailty grades the age‐dependent reduction in peak Ca²⁺ current, Ca²⁺ flux and the gain of SR Ca²⁺ release in voltage‐clamped myocytes

A, representative Ca²⁺ currents recorded from cells from mice with different FI scores. B and C, peak Ca²⁺ current and the integral of the Ca²⁺ current (Ca²⁺ flux) declined with age. D, the gain of SR Ca²⁺ release decreased with age. E and F, the age‐related declines in peak Ca²⁺ current and Ca²⁺ flux were graded by FI score. G, the gain of SR Ca²⁺ release was negatively correlated with frailty score. Differences between age groups were assessed using a Mann–Whitney Rank Sum test and correlations were evaluated with linear regression analysis (n = 11 adult and 32 aged myocytes). Filled symbols indicate adult mice and open symbols indicate aged mice.

Figure 9. Frailty scores grade the age‐related decline in peak contractions, Ca²⁺ transients, Ca²⁺ currents and gain when the pacing rate is increased to 4 Hz in voltage‐clamped myocytes

A–D, peak contractions, Ca²⁺ transients, Ca²⁺ currents and the gain of SR Ca²⁺ release were graded by FI score when the pacing rate was increased from 2 to 4 Hz. Contractions (P = 0.005) and Ca²⁺ currents (P = 0.03) also declined with age at 4 Hz, whereas Ca²⁺ transients and gain did not (data not shown). Correlations were evaluated with linear regression analysis (n = 14–15 adult and 19–23 aged myocytes). Filled symbols indicate adult mice and open symbols indicate aged mice.

Figure 10. Frailty grades the age‐dependent decline in expression of Cav1.2 protein in the mouse heart

A, representative Western blots for Cav1.2 protein expression in mice with varying FIs. Ponceau S staining was used as a loading control in all experiments (lower panels). B, mean expression of Cav1.2 decreased with age in the mouse heart. C, cardiac Cav1.2 expression was closely graded by frailty score in the mouse. Differences between age groups were assessed using a Mann–Whitney Rank Sum test and correlations were evaluated with linear regression analysis (n = 8 adult and 8 aged hearts). Filled symbols indicate adult mice and open symbols indicate aged mice.