On the inhibitory mechanism of bassianolide, a cyclodepsipeptide, in acetylcholine-induced contraction in guinea-pig taenia coli.

The effect of bassianolide (BASS), a cyclodepsipeptide, was investigated on mechanical response, membrane potential, intracellular Na and K contents and 45Ca uptake in response to acetylcholine (ACh) in guinea-pig taenia coli. BASS (10(-5) M) as well as verapamil (5 X 10(-7) M) and papaverine (3 X 10(-5) M) selectively inhibited the tonic component of the contraction induced by ACh (10(-5) M), but scarcely affected the phasic one. In contrast, atropine (3 X 10(-8) M) inhibited both components of contraction. BASS did not modify the change in membrane potential by ACh. BASS, ACh and the combination of both did not influence the intracellular Na and K contents and the 45Ca uptake. These data show that BASS seems unlikely to have the property of an ionophore. BASS slightly inhibited both the tonic and phasic components of contraction induced by 60 mM K in a non-selective manner, though verapamil and papaverine inhibited the tonic component more potently than the phasic one. Verapamil decreased the increased 45Ca uptake in the muscle soaked in 60 mM K medium, but BASS did not. Since BASS selectively inhibits the tonic component of the ACh-induced contraction, the inhibitory mechanism of BASS seems to be different from that of verapamil, papaverine or atropine; and the mechanism may be beyond the interactions with a binding activity of ACh to the muscarinic receptor, membrane potential and the contractile machinery of the intestinal smooth muscle.