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Receptor agonists induce myosin phosphorylation-dependent and phosphorylation-independent contractions in vascular
1Department of Veterinary Pharmacology, Faculty of Agriculture, University of Tokyo, Japan.
Abstract:
In isolated rat aorta, 72.7 mM KCI, 10 microM prostaglandin F2 alpha, 30 nM endothelin-1 and 1 microM norepinephrine increased muscle tension, cytosolic Ca++ concentration ([Ca++]i) and 20 kDa myosin light chain (MLC) phosphorylation. The levels of contractile tension and MLC phosphorylation at a given [Ca++]i were greatest in the presence of endothelin-1 followed by prostaglandin F2 alpha greater than norepinephrine greater than high K+. Verapamil inhibited the high K(+)-induced increments to their respective resting levels. Verapamil also almost completely inhibited the receptor agonist-induced increments in [Ca++]i and MLC phosphorylation, although a part of the contraction was not inhibited. Ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid further decreased [Ca++]i and muscle tension, suggesting that a part of the contraction is regulated by [Ca++]i below a resting level. Receptor agonists induced sustained contraction in the absence of external Ca++ which was not followed by the increase in [Ca++]i or MLC phosphorylation. This contraction was followed by the increments in shortening velocity and stiffness. In the rabbit mesenteric artery permeabilized with Staphylococcus aureus, alpha-toxin, norepinephrine and endothelin-1 shifted the Ca(++)-tension curve to the left in the presence of GTP. From these results, it is suggested that high K(+)-induced sustained contraction of vascular smooth muscle is attributable to an increase in [Ca++]i followed by an increase in MLC phosphorylation. In addition to this fundamental mechanism, receptor agonists increase Ca+ sensitivity of MLC phosphorylation when [Ca++]i is higher than resting level resulting in a greater contraction than that induced by high K+ for a given increase in [Ca++]i.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
Vascular smooth muscle contraction involves increased cytosolic calcium (Ca++) and myosin light chain (MLC) phosphorylation. Receptor agonists enhance contraction beyond high K+ by increasing Ca++ sensitivity of MLC phosphorylation.
Area of Science:
- Pharmacology
- Physiology
- Biochemistry
Background:
- Vascular smooth muscle contraction is crucial for regulating blood pressure.
- Calcium ions (Ca++) and myosin light chain (MLC) phosphorylation are key regulators of smooth muscle contraction.
Purpose of the Study:
- To investigate the mechanisms of vascular smooth muscle contraction induced by various agonists.
- To compare the effects of high K+, prostaglandin F2 alpha, endothelin-1, and norepinephrine on muscle tension, [Ca++]i, and MLC phosphorylation.
Main Methods:
- Isolated rat aorta and permeabilized rabbit mesenteric artery preparations were used.
- Measurements included muscle tension, cytosolic Ca++ concentration ([Ca++]i), and MLC phosphorylation.
- Pharmacological agents like verapamil and ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid were employed.
Main Results:
- High K+, prostaglandin F2 alpha, endothelin-1, and norepinephrine all increased muscle tension, [Ca++]i, and MLC phosphorylation.
- Endothelin-1 induced the greatest tension and MLC phosphorylation at a given [Ca++]i, followed by prostaglandin F2 alpha, norepinephrine, and high K+.
- Receptor agonists induced sustained contraction even without external Ca++, associated with increased shortening velocity and stiffness.
Conclusions:
- High K+-induced contraction relies on increased [Ca++]i and subsequent MLC phosphorylation.
- Receptor agonists augment contraction by increasing Ca++ sensitivity of MLC phosphorylation, leading to greater tension than high K+ for equivalent [Ca++]i increases.