Figure 4.

Maximal actomyosin Mg-ATPase activity and Hill coefficients varied with age in a sex-specific fashion, but EC₅₀ values were unaffected. (A) Mean data show that maximal actomyosin ATPase activity increased with age in females but not males. There was also a sex difference at younger ages where activity was higher in middle-aged males than in middle-aged females. (B) EC₅₀ values were similar in all four groups. (C) Average values for the Hill coefficients declined markedly with age in males and were significantly lower in older males compared to older females. Values denote the mean ± SEM in each case. Data were analyzed with a two-way ANOVA with age and sex as main factors (post-hoc test was Holm-Sidak). The * denotes p < 0.05. Samples were hearts from 5 middle-aged male mice, 6 older males, 5 middle-aged females and 5 older females, respectively.