Comparison of the caldesmon content of cardiac and smooth muscle

The high abundance of caldesmon in smooth muscle and its ability to inhibit actomyosin ATPase activity have led to the hypothesis that caldesmon modulates contractile activity. It has also been proposed, however, that caldesmon acts as a structural protein in muscle and non-muscle cells. We have determined the caldesmon content of mammalian cardiac muscle and have found that caldesmon is 200-fold less abundant in cardiac muscle than it is in gizzard smooth muscle. This finding argues against a role for caldesmon in the modulation of cardiac contractility.