Interaction of rate and preload on developed tension in isometric papillary muscle

Initial fiber length and heart rate are important intrinsic determinants of cardiac muscle performance. To date, the effect of their interaction has not been considered. To study this interaction, peak developed tension (F), peak first derivative of tension development (dF/dt), time to peak tension (TPF), and time to decline to 1/2 of peak developed tension (T1/2P) were plotted as functions of a series of increments of frequency at several different preloads for 15 kitten papillary muscles (mean cross sectional area 0.4 mm²). At high preloads F and dF/dt reached peaks at significantly lower frequencies than at corresponding low preloads; similarly, dF/dt rose to its peak and TPF and T1/2P declined at significantly greater rates. Postcontrol results returned toward those of the precontrol experiments. Results from 7 additional muscles studied at 2 levels of oxygenation suggest that differences in slopes were not due to relative hypoxia at high preloads. These data indicate that preload determines the extent and direction of inotropic change produced by a given increment in frequency and define the interaction of these 2 intrinsic variables on performance.