Myocardial energetics during isometric twitch contractions of cat papillary muscle

During myocardial tetanus, when activation is maximum and constant, there is a linear relationship throughout contraction between oxygen consumption (MVo2) and the cumulative product of active tension and time (integral of AT). The goal of this study was to determine the relation of MVo2 to integral of AT during isometric myocardial twitch contractions. Ten right ventricular cat papillary muscles were studied in a flow respirometer. MVo2 was determined during contractions unloaded from Lmax to a slack length at successive 100-ms intervals after stimulation. In contrast to the linear relationship observed during tetanus, MVo2/integral of AT varied during twitch contractions: when the muscles were made slack 100 ms after stimulation MVo2/integral of AT was 389 +/- 51 (SE) (nl of O2/mg of dry muscle)/(N of active tension/mm2.s of active tension). This value was 94 +/- 7 at peak active tension and was constant thereafter. There was a continuous increase in cumulative MVo2 as integral of AT increased; before integral of AT began, MVo2 was 0.41 +/- 0.04 (nl of O2/mg)/contraction at Lmax and 0.22 +/- 0.04 at a slack length; at peak isometric tension MVo2 was 1.84 +/- 0.19; for a complete contraction MVo2 was 2.89 +/- 0.25. These data support two concepts 1) activation energy is small and dependent on initial length and tension; and 2) integral of AT is variably energy dependent throughout the entire isometric twitch contraction.