Effect of calcium on oxytocin-induced contraction of mammary gland myoepithelium as visualized by NBD-phallacidin.

The effect of calcium on oxytocin-induced contraction of myoepithelial cells was visualized with NBD-phallacidin, a fluorescent stain for filamentous actin. In the absence of oxytocin, the cells appeared relaxed; long, branching processes radiated from the cell bodies. In the presence of 50 nM-oxytocin, myoepithelial cells contracted into smaller spoke-shaped bodies in which the arms were shorter and thicker. Electron microscopy confirmed the morphological differences between oxytocin-treated and untreated myoepithelium. To determine a role for extracellular calcium, tissue was incubated in EGTA, then exposed to oxytocin, with or without added calcium. Contraction occurred in the presence of oxytocin plus additional calcium but not in the absence of calcium. When the tissue was incubated with the calmodulin antagonist trifluoperazine (TFP) in calcium-containing medium, oxytocin did not induce myoepithelial cell contraction. These data support previous results obtained with a myosin light-chain phosphorylation assay implicating calcium and calmodulin in oxytocin-induced contraction. Furthermore, NBD-phallacidin visualization of myoepithelial cells demonstrates that the effect of calcium on contraction is physiologically significant.