The relationship between asynchronous Ca waves and force development in intact smooth muscle bundles of the porcine trachea

Fluctuations in intracellular calcium ion concentration ([Ca]i) constitute the main link in excitation-contraction coupling in the airway smooth muscle cells (ASMC). It has been recently reported that acetylcholine induces asynchronous recurring Ca waves in the intact ASMC of murine bronchioles. Using a novel technique that allows us to simultaneously measure sub-cellular [Ca]i and force generation in ASMC located within an intact tracheal muscle bundle, we examined a similar pattern of Ca signaling in the trachea. We found that application of acetylcholine (ACh) resulted in the generation of recurring intracellular Ca waves progressing along the longitudinal axis of the ribbon-shaped intact ASMC. These Ca waves were not synchronized between neighboring cells and induction of wave-like [Ca]i oscillations was temporally associated with development of force by the tracheal muscle bundle. By comparing the concentration-dependence of force generation and the parameters characterizing the [Ca]i oscillations, it was found that the concentration-dependent increase in AChinduced force development by the tracheal smooth muscle bundle is Achieved by differential recruitment of intact ASMC to initiate Ca waves and by enhancement in the frequency of the [Ca]i oscillations and elevation of inter-spike [Ca]i once the cells are recruited. Our findings demonstrate that asynchronous recurring Ca waves underlie E-C coupling in ACh-induced contraction of the intact tracheal smooth muscle bundle. Furthermore, in contrast to what was reported in enzymatically-dissociated ASMC, Ca influx through the L-type voltage-gated Ca channel was not an obligatory requirement for the generation of [Ca]i oscillations and development of force in ACh-stimulated intact ASMC.