PREJUNCTIONAL AND POSTJUNCTIONAL EFFECTS OF VIP-LIKE PEPTIDES IN GUINEA-PIG TRACHEAL SMOOTH-MUSCLE

To determine the role of VIP-like peptides on neurotransmission of vag us nerve, we evaluated the effects of helodermin, helospectin, and vas oactive intestinal peptide (VIP) on the contraction of guinea pig trac heal smooth muscle evoked by electrical field stimulation (EFS) or the exogenous application of acetylcholine (ACh). Isometric tension of tr acheal strips was measured in the presence of indomethacin (10(-6) M) and of guanethidine (10(-6) M. VIP (10(-9) M to 10(-7) M) significantl y suppressed the contraction evoked by EFS. VIP, at concentrations of 10(-9) M and 10(-8) M, did not affect the ACh-evoked contraction, but a concentration of 10(-7) M suppressed ACh-evoked contraction. Helospe ctin and helodermin (10(-8)M and 10(-7) M significantly suppressed the EFS-evoked contraction, but 10(-9) M showed no effect. Helospectin an d helodermin had no effect on the ACh sensitivity of smooth muscle up to 10(-8) M, but a concentration of 10(-7) M suppressed the ACh-evoked contraction. These results indicate that helodermin, helospectin, and VIP exert both pre- and post-junctional inhibitory effects on the air way smooth muscle of guinea pigs. These peptides, thus, inhibited trac heal smooth muscle contraction prejunctionally at low concentrations, and acted postjunctionally at higher concentrations.