ABSENCE OF CALPONIN PHOSPHORYLATION IN CONTRACTING OR RESTING ARTERIAL SMOOTH-MUSCLE

We have tested the hypothesis of Winder and Walsh [(1990) J. Boil. Chem. 265, 10148] that the contractile state of smooth muscle is regulated by calponin phosphorylation. Porcine carotid arterial muscles were highly labeled with P-32, then contracted with four different agents for various times. No radioactivity was detected in calponin isolated by 2D or 1D gel electrophoresis from the muscles. Similarly, resting muscles showed no [P-32]phosphate in calponin. Apparently the sites of calponin available for phosphorylation in vitro are rendered unavailable in the intact muscle.