Dynamic cytosolic Ca and force responses to muscarinic stimulation in airway smooth muscle.

During agonist stimulation of airway smooth muscle (ASM), agonists such as ACh induce a transient increase in cytosolic Ca concentration ([Ca]), which leads to a contractile response [excitation-contraction (E-C) coupling]. Previously, the sensitivity of the contractile response of ASM to elevated [Ca] (Ca sensitivity) was assessed as the ratio of maximum force to maximum [Ca]. However, this static assessment of Ca sensitivity overlooks the dynamic nature of E-C coupling in ASM. In this study, we simultaneously measured [Ca] and isometric force responses to three concentrations of ACh (1, 2.6, and 10 μM). Both maximum [Ca] and maximum force responses were ACh concentration dependent, but force increased disproportionately, thereby increasing static Ca sensitivity. The dynamic properties of E-C coupling were assessed in several ways. The temporal delay between the onset of ACh-induced [Ca] and onset force responses was not affected by ACh concentration. The rates of rise of the ACh-induced [Ca] and force responses increased with increasing ACh concentration. The integral of the phase-loop plot of [Ca] and force from onset to steady state also increased with increasing ACh concentration, whereas the rate of relaxation remained unchanged. Although these results suggest an ACh concentration-dependent increase in the rate of cross-bridge recruitment and in the rate of rise of [Ca], the extent of regulatory myosin light-chain (rMLC) phosphorylation was not dependent on ACh concentration. We conclude that the dynamic properties of [Ca] and force responses in ASM are dependent on ACh concentration but reflect more than changes in the extent of rMLC phosphorylation.