The effect of nifedipine and verapamil on KC1-induced rhythmic contractions of guinea pig ureter in vitro.

Addition of KC1 (40 mM) produced rhythmic contractions of guinea-pig ureters in vitro which were unaffected by phentolamine, atropine or tetrodotoxin. KC1 failed to elicit rhythmic contractions of ureters incubated in a Krebs solution with no added Ca++; in these conditions the addition of CaC12 in concentrations of 1.5 mM, or higher, produced rhythmic contractions whose frequency, but not amplitude, was proportional to CaC12 concentration in the bathing medium. EDTA reduced the frequency of KC1-induced rhythmic contractions without affecting their amplitude. Nifedipine and verapamil reduced both the frequency and the amplitude of KC1-induced rhythmic contraction; verapamil was more effective than nifedipine in reducing their amplitude. Urethane reduced the amplitude without significantly affecting the frequency of KC1-induced rhythmic contractions. An increase in the extracellular Ca++ concentration reverted the suppressive effect of all drugs under study. These results suggest that an influx of Ca++ from the extracellular space is responsible for the initiation of KC1-induced rhythmic contractions and is involved in the mechanism(s) which regulates their frequency, but that a separate mechanism regulates their amplitude.