Control of myocardial contraction: the sensitivity of cardiac actomyosin to calcium ion.

The control by calcium ion of the adenosine triphosphatase activity of cardiac actomyosin is similar to that of white skeletal actomyosin. This finding indicates that the slower contraction and relaxation of heart muscle do not reflect different levels to which free calcium ion concentration around the myofibrils must be adjusted during contraction and relaxation and suggests a mechanism whereby myocardial contractility may be regulated.