Measurement of [Ca²+]i in smooth muscle strips using front-surface fluorimetry

In regulating the contraction of smooth muscle cells, changes in the cytosolic concentrations of Ca(2+) ([Ca(2+)](i)) play a primary role as the initiation of contraction is associated with Ca(2+) binding to calmodulin with the subsequent activation of myosin light chain kinase. During the contraction induced by receptor-mediated stimulation, however, there are temporal changes in the relationship between [Ca(2+)](i) and the developed tension. Furthermore, the receptor-mediated stimulation also produces a proportionately greater tension for a given change in [Ca(2+)](i) than does K(+) depolarization. Therefore, it is important to measure [Ca(2+)](i) and tension simultaneously in order to determine the molecular and cellular mechanisms in both the regulation of contraction and relaxation of smooth muscle. For this purpose, front-surface fluorimetry of fura-2, a [Ca(2+)](i) indicator dye, has been performed on small smooth muscle strips.