Phosphatase-Mediated Modulation of Actin-Myosin Interaction in Bovine Aortic Actomyosin and Skinned Porcine Carotid Artery

Publisher Website
Google Scholar
Abstract
Since the Ca2+-regulatory mechanism for actin-myosin interaction in smooth muscle involves phosphoryaltion of the 20,000 Da [dalton] myosin light chains, it was hypothesized that such interaction should be influenced by myosin phosphatase. Accordingly, the effects of an aortic myosin light-chain phosphatase on Ca1+-dependent actin-myosin interaction was studied in detergent-skinned porcine carotid artery and bovine aortic native actomyosin. In skinned preparations, the aortic phosphatase (16 U/ml) markedly inhibited the rate of isometric contraction in low Ca2+ (6.8 .times. 10-7 M) and responsiveness to Ca2+ (foce attained with 6.8 .times. 10-7 Ca2+/force attained with 1.6 .times. 10-6 M Ca2+); relaxation was accelerated, Ca2+-dependent actomyosin ATPase activity and phosphorylation of the light chains were significantly and progressively depressed in the presence of increasing concentrations of phosphatase (0.1-0.9 U/ml). The concentration of Ca2+ (1.1 .times. 10-6 M) required for half-maximal activation of either ATPase activity or light-chain phosphorylation increased by 70% in the presence of 0.1 U phosphatase/ml. Neither the maximal rate of Ca2+-sensitive ATP hydrolysis (39 .+-. 0.8 nmol/min/mg actomyosin) nor the extent of phosphorylation (0.68 .+-. 0.05 mole PO4/mole light chain) was altered at > 5 10-6 M Ca2+, ATPase activity was correlated to light chain phosphorylation under diverse conditions including the presence or absence of 1 .mu.M calmodulin, different concentrations of phosphatase (0-0.9 U/ml), and different concentrations of Ca2+ (10-8 to 1.25 .times. 10-5 M). Significant phosphorylation was present (20-25% of maximum) in the absence of Ca2+-dependent ATPase activity and only 15% of the maximal rate of ATP hydrolysis was expressed until phosphorylation attained 50% of its maximal value. These findings are consistent with the ordered model of myosin phosphorylation suggested by A. Persechini and D.J. Hartshorne. Myosin phosphatase may participate in modulating actin-myosin interactions in vascular smooth muscle.