Dynamic Assessment of Ca2+ Sensitivity of Isometric Force in Intact Airway Smooth Muscle Using Phase Loop Plots

Abstract

A transient elevation of cytosolic Ca2+ concentration ([Ca2+]cyt) triggers a force response in excitation‐contraction coupling. The Ca2+ sensitivity of muscle reflects the relationship between [Ca2+]cyt and force, and in skeletal and cardiac muscle is often defined as the [Ca2+]cyt at which 50% force is generated (ED50). In smooth muscle, Ca2+ sensitivity has been assessed as the ratio of peak force to peak cytosolic [Ca2+]cyt. However, this conventional measurement overlooks the dynamic nature of Ca2+ regulation of force generation. In the present study, in order to provide a dynamic index of Ca2+ sensitivity in airway smooth muscle (ASM), we simultaneously measured [Ca2+]cyt and force and determined their temporal dependence using a phase loop plot. We examined the phase loop plot assessment of Ca2+ sensitivity in ASM under the condition in which the conditional assessment indicated an increase (Calyculin A treatment) in Ca2+ sensitivity. Intact ASM strips were isolated from porcine tracheas obtained from local abattoir. ASM strips were loaded with Fura‐2 AM (Ca2+ fluorescent indicator) and stimulated using 1 mM ACh. Isometric force and [Ca2+]cyt responses were simultaneously measured using a Guth Muscle Research System. ASM strips were exposed to Calyculin A (phosphatase inhibitor; 100 nM for 10 min), which was previously reported to alter static Ca2+ sensitivity. Phase loop plots were calculated from simultaneous [Ca2+]cyt and force responses. The extent of rMLC phosphorylation was determined at different time periods after ACh stimulation using Phos‐tag SDS‐PAGE (SuperSep Phos‐tag; Wako Pure Chemical Industries, Ltd.) and Western blotting. Calyculin A increased ACh‐induced isometric force in ASM strips, but it had no effect on the amplitude of evoked [Ca2+]cyt transients. These results suggested that Calyculin A increased static Ca2+ sensitivity. However, the phase loop plots showed no significant shift in dynamic Ca2+ sensitivity with Calyculin A. These results indicate that the phase loop plot provides a valuable tool to assess dynamic Ca2+ sensitivity of isometric force generation in ASM, which better reflects the complex process of excitation‐contraction coupling in smooth muscle. These results further indicate that dynamic Ca2+ sensitivity of force generation in ASM is independent of rMLC phosphorylation.Support or Funding InformationSupported by NIH grant HL126451 (GCS)This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.