Ca2+ can control vascular smooth-muscle thin filaments without caldesmon phosphorylation

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Abstract
The Ca2+-dependent regulation of the activation of myosin MgATPase by vascular-smooth-muscle thin filaments involves caldesmon. This efect may be due to the direct interaction of caldesmon with a Ca2+-binding protein such as calmodulin or phosphorylation of caldesmon by a Ca2+-dependent kinase. I have found that Ca2+ switches on aorta thin filaments in less than 10 s, whereas the caldesmon in the thin filaments is phosphorylated only slowly (half-time > 10 min) and the maximum phosphorylation is very low (1 molecule per 7 molecules of caldesmon). I conclude that the phosphorylation of caldesmon hypothesis is untenable.