The effects of scyliorhinins I (SCY I) and II (SCY II) on longitudinal (LM) and circular muscle (CM) strips isolated from the carp intestinal bulb were investigated in vitro and compared with that of substance P (SP). 2. SP (0.3 nM-1 microM), SCY I (0.3-300 nM) and SCY II (0.3 nM-1 microM) caused transient concentration-dependent contractions of LM strips. The EC50 values for SP, SCY I and SCY II were 16 nM, 15 nM and 39 nM, respectively. Tetrodotoxin and atropine partly decreased the contractile responses to SP, neurokinin A and neurokinin B, but did not change those to SCY I and SCY II. Spantide, methysergide, pyrilamine and naloxone did not decrease the contractile responses to SP, SCY I and SCY II. SP-induced desensitization selectively decreased the responsiveness of LM strips to SCY I and SCY II, and in addition, SCY I- or SCY II-induced desensitization decreased that to SP, SCY I and SCY II. 3. SP, SCY I and SCY II (1 nM-1 microM) caused concentration-dependent contraction of CM strips. The time course of the contractile response of CM strips was different from that of LM strips. Neither tetrodotoxin, atropine, methysergide nor spantide decreased the contractile responses to these tachykinins. 4. These results indicate that SCY I and SCY II act directly on tachykinin receptors located on smooth muscle cells and thus cause the excitatory response in the carp intestinal bulb.
