Fig. 7.

The old myocardium presents blunted contractile force frequency relationship. A: isometric contraction of ventricular trabeculae obtained from adult and old dogs stimulated at 1-Hz pacing frequency. Twitches are superimposed in the inset. B: data are shown as mean ± SE (adult: n = 9 muscles, 6 dogs, 5.8 ± 0.6 yr; old: n = 10 muscles, 7 dogs, 13.2 ± 0.1 yr). *P < 0.05. C: isometric contraction (top traces) and first derivative of developed tension (bottom traces) recorded at progressively higher stimulation frequencies in adult and old trabeculae. D and E: quantitative data for developed tension (D) and time to 90% relaxation (E) normalized with respect to CL 2,000 ms are shown as mean ± SE (adult: n = 7 muscles, 5 dogs, 5.1 ± 0.6 yr; old: n = 6 muscles, 4 dogs, 12.2 ± 0.4 yr). *P < 0.05 vs. adult; †P < 0.001 for various frequencies within the same group.