Effects of divalent cations on the spontaneous synchronization in mammalian iris sphincter muscle cells

The appearance of automatic activity in mammalian iris sphincter muscle cells upon treatment with Ba2+ was investigated in vitro, using tension measurement and the sucrose-gap method. In isolated cattle and rabbit sphincter muscles (0.7-1 mm wide, 5-mm long), contraction induced by electrical nerve stimulation (0.3 msec) was inhibited in Ca2+-free solution. Application of Ba2+ produced a rapid contraction in a dose-dependent manner followed by spontaneous activities, even in the presence of 10(-6) M atropine and/or tetrodotoxin, thereby indicating the generation of myogenic automaticity. Ca2+ and Mg2+ had dual effects on such a Ba2+-induced automaticity; that is, excitatory action in very low doses (cattle less than 0.2 mM; rabbit less than 0.3 mM) and inhibitory action in high doses (cattle, rabbit greater than 0.5 mM). In case of the iris sphincter from albino rabbits, we recorded action potentials and the following plateau, using sucrose-gap method. Although the iris sphincter is classed as a multi-unit smooth muscle and is inactive without neuronal influences, in the present experiments this muscle did exhibit spontaneity without neuronal influences, in the presence of Ba2+. These results may suggest that Ca2+ and/or Mg2+ present in high concentrations in the aqueous fluid probably leads to stabilization of the iris sphincter muscle membrane and masking of the spontaneous activity.