Signal transduction in smooth muscle - Selected contribution: Time course and heterogeneity of contractile responses in cultured human airway smooth muscle cells

We measured the time course and heterogeneity of responses to contractile a nd relaxing agonists in individual human airway smooth muscle (HASM) cells in culture. To this end, we developed a microrheometer based on magnetic tw isting cytometry adapted with a novel optical detection system. Ferromagnet ic beads (4.5 mum) coated with Arg-Gly-Asp peptide were bound to integrins on the cell surface. The beads were twisted in a sinusoidally varying magne tic field at 0.75 Hz. Oscillatory bead displacements were recorded using a phase-synchronized video camera. The storage modulus (cell stiffness; G'), loss modulus (friction; G"), and hysteresivity (eta; ratio of G" to G') cou ld be determined with a time resolution of 1.3 s. Within 5 s after addition of histamine (100 muM), G' increased by 2.2-fold, G" increased by 3.0-fold , and h increased transiently from 0.27 to 0.34. By 20 s, eta decreased to 0.25, whereas G' and G" remained above baseline. Comparable results were ob tained with bradykinin (1 muM). These changes in G', G", and eta measured i n cells were similar to but smaller than those reported for intact muscle s trips. When we ablated baseline tone by adding the relaxing agonist dibutyr yl cAMP (1 mM), G' decreased within 5 min by 3.3-fold. With relaxing and co ntracting agonists, G' could be manipulated through a contractile range of 7.3-fold. Cell populations exhibited a log-normal distribution of baseline stiffness (geometric SD = 2.8) and a heterogeneous response to both contrac tile and relaxing agonists, partly attributable to variability of baseline tone between cells. The total contractile range of the cells (from maximall y relaxed to maximally stimulated), however, was independent of baseline st iff-ness. We conclude that HASM cells in culture exhibit a clear, although heterogeneous, response to contractile and relaxing agonists and express th e essential mechanical features characteristic of the contractile response observed at the tissue level.