Anomalous stabilizing action of Ca on sphincter smooth muscle of the rat iris

This paper concerns the first measurement of spontaneous electrical activity in the smooth muscle cells of the mammalian iris sphincter. Although the membrane potential was quite stable, at - 59 mV, under physiological conditions in vitro, bursts of rhythmic electrical activity occurred when barium was substituted for all of the calcium in the medium (Ba-Krebs' solution). These bursts consisted initially of spike potentials, but these were followed by a long-lasting depolarization. Although the amplitude of the spike potentials did not decrease even when the external Na concentration was lowered to 25 mM, addition of a small amount of Ca (greater than 0.3 mM) to the Ba-Krebs' depressed the rhythmic contractions which followed the cessation of the spike potentials. Strontium also depressed the rhythmicity but to a lesser extent. These results show that the smooth muscle of the rat iris sphincter has an ability to generate spike potentials, and that the properties of the latter differ from those observed in other smooth muscles; i.e. the activity is suppressed by Ca ions at far lower concentrations than those in the physiological environment. This anomalous effect is presumably due to the stabilizing action of Ca.