title: Phosphatase-mediated modulation of actin-myosin interaction in bovine aortic actomyosin and skinned porcine carotid artery.
abstract: Since the Ca2+-regulatory mechanism for actin-myosin interaction in smooth muscle involves phosphorylation of the 20,000-Da myosin light chains, it was hypothesized that such interaction should be influenced by myosin phosphatase. Accordingly, we studied the effects of an aortic myosin light-chain phosphatase on Ca1+-dependent actin-myosin interaction 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 X 10(-7) M) and responsiveness to Ca2+ (force attained with 6.8 X 10(-7) Ca2+/force attained with 1.6 X 10(-6) M Ca2+), whereas 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 X 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 nmole/min/mg actomyosin) nor the extent of phosphorylation (0.68 +/- 0.05 mole PO4/mole light chain) was altered at greater than 5 X 10(-6) M Ca2+. ATPase activity was correlated to light-chain phosphorylation under diverse conditions including the presence or absence of 1 microM calmodulin, different concentrations of phosphatase (0-0.9 U/ml), and different concentrations of Ca2+ (10(-8) to 1.25 X 10(-5) M). However, 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 [Science (Washington, DC), 213:1383-285, 1961] (36). They also suggest that myosin phosphatase may participate in modulating actin-myosin interactions in vascular smooth muscle.
authors.forename: C
authors.initials: C
authors.lastname: Bialojan
authors.forename: J C
authors.initials: JC
authors.lastname: Rüegg
authors.forename: J
authors.initials: J
authors.lastname: Di Salvo
mesh_terms: D000199:Actins / Q000378:metabolism*; D000205:Actomyosin / Q000378:metabolism*; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D001011:Aorta / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology; D002339:Carotid Arteries / Q000378:metabolism*; D002417:Cattle; D007700:Kinetics; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D043263:Myosin-Light-Chain Phosphatase; D009218:Myosins / Q000378:metabolism*; D010749:Phosphoprotein Phosphatases / Q000378:metabolism*; D010766:Phosphorylation; D013552:Swine
chemical_list: D000199:Actins; D000205:Actomyosin; D010749:Phosphoprotein Phosphatases; D043263:Myosin-Light-Chain Phosphatase; D000251:Adenosine Triphosphatases; D009218:Myosins; D002118:Calcium