Summary
The relationship between calcium concentration ([Ca2+]) and force in smooth muscle can be studied by permeabilizing the sarcolemma and bathing the preparation in a mock intracellular solution. Normally [Ca2+] is set in these solutions using the Ca2+ chelator EGTA in the concentration range of 4–10 mm. This study shows that lowering total EGTA concentration ([EGTA]t) below 10 mm depresses Ca2+-activated force generated in 0.1 μm Ca2+. The observed threshold for the effect of EGTAt is 0.2 mm, and the effect is maximal at approximately 10 mm. BAPTA, another Ca2+ chelator, also produces this effect. Tension production in smooth muscle is controlled by acto-myosin interaction. This in turn is mediated by the relative activities of myosin light chain kinase (MLCK) and phosphatase (MLCP). Inhibiting MLCP with Microcystin LR (10 μm), an increase [EGTAt] from 0.2 mm to 10 mm still enhanced force. This suggests that EGTA promotes phosphorylation of myosin by the activation of MLCK and not by inhibition of MLCP.
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Smith, G.L., Crichton, C.A. Ca-EGTA affects the relationship between [Ca2+] and tension in α-toxin permeabilized rat anococcygeus smooth muscle. J Muscle Res Cell Motil 14, 76–84 (1993). https://doi.org/10.1007/BF00132182
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DOI: https://doi.org/10.1007/BF00132182