Effects of CPA on spontaneous contractions in circular smooth muscles of the rabbit urethra. (Aa) A large circular smooth muscle strip generating spontaneous phasic contractions. CPA (30 μM) initially increased both the frequency and amplitude of phasic contractions, but subsequently reduced their frequency. (Ab) In another large preparation that developed spontaneous contractions, CPA (10 μM) initially increased both the frequency and amplitude of contractions and then abolished their generation after about 15 min. (Ba) In a smaller circular smooth muscle strip, spontaneous phasic contractions were developed. CPA increased the amplitude and frequency of the contractions. (Bb) In the same preparations that had been exposed to CPA for 30 min, spontaneous contractions with increased amplitude and reduced frequency were still generated. (C) In another smaller circular smooth muscle strip that exhibited spontaneous contractions, CPA initially increased the amplitude and frequency of the contractions, but then abolished them.