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

Abstract

The appearance of automatic activity in mammalian iris sphincter muscle cells upon treatment with Ba 2+ 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 Ca 2+-free solution. Application of Ba 2+ 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. Ca 2+ and Mg 2+ had dual effects on such a Ba 2+-induced automaticity; that is, excitatory action in very low doses (cattle < 0·2 m m; rabbit < 0·3 m m) and inhibitory action in high doses (cattle, rabbit > 0·5 m m). 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 Ba 2+. These results may suggest that Ca 2+ and/or Mg 2+ present in high concentrations in the aqueous fluid probably leads to stabilization of the iris sphincter muscle membrane and masking of the spontaneous activity.