The effect of Ca2+ antagonists on spontaneous motility from sheep duodenum.

Longitudinal smooth muscle of the sheep duodenum showed a rhythmic spontaneous activity with an average frequency of 5.6 +/- 0.55 phasic movements min-1 and a mean value of the amplitude of phasic contractions of 0.956 +/- 0.1 g. When the strips were incubated in Ca(2+)-free medium, the spontaneous motility amplitude (SMA) was reduced to 37 +/- 8.2% of control values. In Ca(2+)-free medium plus EDTA (1 or 2 mM), the SMA was strongly reduced to 21.9 +/- 8.3 and 1.8 +/- 1.8%, respectively. Verapamil, nifedipine and diltiazem diminished the SMA. The EC50 value for verapamil was 10(-9) M, whereas that for diltiazem was 2 x 10(-9) M and for nifedipine was 3 x 10(-14) M. Trifluoperazine and TMB-8 reduced the SMA with EC50 values of 7 x 10(-6) and 3 x 10(-5) M, respectively. The spontaneous activity in the sheep duodenum seemed to be mediated by influx extracellular Ca2+, which enters through potential-dependent channels and intracellular Ca2+ release.