Excitation-contraction in rat myocardium: Alterations with adult aging

Simultaneous transmembrane action potential (TAP) and isometric contraction measurements were made in thin right ventricular papillary muscles isolated from senescent (S, 24-26 mo) and young adult (YA, 6-8 mo) rat hearts. At the peak of the length-tension curve in the steady state at 24 min^-1 at 29°C in perfusate [Ca²⁺] ([Ca²⁺](e)) of 0.375 or 2.5 mM, contractile tension developed in response to excitation (DT) and the maximum rate of tension development (dT/dt) were not age-related; contraction duration (CD) was 17 and 27% greater in S than YA in the low and high [Ca²⁺](e), respectively (P < 0.001); resting membrane potential was not age related. TAP time above zero mV, integrated area above zero mV, and times to 75% (T₇₅) and 90% (T₉₀) repolarization were approximately twofold greater in S than in YA in both [Ca²⁺](e) (P < 0.001). Changes in steady-state T₇₅ and T₉₀ due to an increase in [Ca²⁺](e) from 0.375 to 2.5 mM were significantly correlated with those in CD, DT, and dT/dt in S but not in YA. We conclude that in the senescent myocardium a prolonged and greater extent of depolarization is related to the prolonged CD and may also be a determinant of the peak force developed in response to excitation under some conditions.