The effects of vasoactive intestinal polypeptide (VIP) antagonists [AC-Tyr1, D-Phe2]-GRF(1-29)-NH2 and [4-Cl-D-Phe6, Leu17]-VIP on excitatory neuroeffector transmission in the dog and cat trachea were investigated by use of microelectrode, double sucrose-gap and tension recording methods. 2. In the dog trachea, repetitive stimuli at high frequency (20 Hz) markedly enhanced the amplitude of contraction, the amplitude of contractions evoked by 50 stimuli at 20 Hz relative to that evoked by 5 stimuli being 14.2 +/- 3.8 times (n = 7, +/- s.d.). In the cat, the summation was much less marked, the amplitude of contractions evoked by 50 stimuli relative to that evoked by 5 stimuli being only 2.1 +/- 0.6 times (n = 5, +/- s.d.). Neither VIP antagonist had any effect on the relationship between the number of stimuli at 20 Hz and the relative amplitude of contraction in the dog trachea, but did enhance the amplitude of contractions to 1.1-1.5 times control in the cat trachea. 3. VIP antagonists dose-dependently enhanced the amplitude of excitatory junction potentials (e.j.ps) evoked by a single stimulus in the cat trachea, without changing the resting membrane potential or input membrane resistance of the smooth muscle cells. However, neither antagonist had any effect on the amplitude of the e.j.p. in the dog trachea. 4. Neither VIP antagonist had any effect on the post-junctional response of smooth muscle cells to exogenously applied acetylcholine (ACh; 10(-9)-10(-5) M) in the dog or cat trachea.5. In the cat trachea, VIP (10-11 M) suppressed the ej.p. amplitude to 0.74 +/- 0.09 times the control value (n = 6). However, after pretreatment of the tissue with the VIP antagonists [Ac-Tyr', D-Phe2]-GRF(1-29)- H2 (10-8M) and [4-Cl-D-Phe6, Leu17]-VIP (10-8M), VIP (10-11 M) did not suppress the ej.p. amplitude, indicating that VIP antagonists block the presynaptic inhibitory action of exogenous VIP.6. In parallel with the enhancement of contraction, ej.ps showed marked summation when repetitive field stimulations were applied at high frequency (20 Hz) in the dog trachea. The relationship between the relative amplitude of the ej.p. and number of stimuli at 20 Hz was linear and the slope was 2.2 +/- 0.3 mV/stimulation. VIP antagonists did not affect this relationship. However, in the cat trachea, summation of ej.ps was not at all marked and a linear relationship was not observed with the double sucrose-gap method. Incubation of the cat tracheal tissue with either of the VIP antagonists (10-8 or 10-7M) markedly enhanced the summation of ej.ps evoked by repetitive field stimulation at 20 Hz, and after the treatment a linear relationship between the number of stimuli and the amplitude of ej.ps was observed, the slopes being 0.6 +/- 0.1 (n = 8) and 0.55 +/- 0.1 mV/stimulation (n = 5), respectively.7. These results indicate that both VIP antagonists, [Ac-Tyr', D-Phe2]-GRF(1-29)-NH2 and [4-Cl-DPhe6, Leu17]-VIP, have a prejunctional action accelerating the excitatory neuroeffector transmission, presumably by enhancing transmitter release from the vagus nerves in the cat, but not in the dog trachea.
