The mechanochemistry of cardiac muscle. I. The isometric contraction.

The utilization of creatine phosphate (CP) and adenosine triphosphate (ATP) was studied in the iodoacetate (IAA) and nitrogen (N(2))-treated cat papillary muscle. Under these conditions the net production of ATP does not occur, and the net utilization of ATP is reflected in a fall in CP concentration. The rate of energy utilization of the IAA-N(2)-treated cat papillary muscle resting without tension was 0.68 micromole CP/g/min. This rate was increased to 1.07 micromole/g/min when muscles were passively stretched with 2 g of tension. In a series of isometrically contracting muscles CP utilization was found to be proportional to the number of activations and the summated contractile element work. These rates of CP utilization were 0.083 micromole/g/activation and 0.0059 micromole/g-cm of work. The calculated mechanochemical coupling efficiency was 33%.