Effects of tachykinins and capsaicin on the mechanical and electrical activity of the guinea-pig isolated trachea.

1. The effects of tachykinins and capsaicin were studied by means of intracellular membrane potential and isometric tension recordings in the isolated trachea of the guinea-pig. 2. The basal membrane potential averaged -51 mV, and most preparations demonstrated spontaneous slow waves. Tetraethylammonium (TEA), a potassium channel blocker (8 x 10(-3) M), depolarized the membrane potential to -44 mV and induced a rhythmic activity. 3. In control solution, substance P (10(-8)-10(-6) M), [Nle10]-neurokinin A(4-10) (10(-8)-10(-6) M) and capsaicin (10(-7)-10(-6) M) induced concentration-dependent depolarizations which were statistically significant at the highest concentration tested (depolarization by 10(-6) M: 8, 11 and 16 mV for the NK1 agonist, the NK2 agonist and capsaicin, respectively). 4. In the presence of TEA (8 x 10(-3) M), the three substances induced depolarizations which were statistically significant at the highest concentration tested for substance P (10(-6) M) and at 10(-7) and 10(-6) M for both [Nle10]-neurokinin A(4-10) and capsaicin (depolarization by 10(-6) M: 11, 17 and 10 mV for substance P, [Nle10]neurokinin A(4-10) and capsaicin, respectively). 5. In the presence or absence of tetraethylammonium, [MePhe7]-neurokinin B (10(-8)-10(-6) M) did not induce any significant changes in membrane potential. 6. The depolarizing effects of substance P (10(-6) M) and [Nle10]-neurokinin A(4-10) (10(-6) M) were blocked only by the specific antagonists for NK1 and NK2 receptors, SR 140333 (10(-7) M) and SR 48968 (10(-7) M), respectively. The effects of capsaicin (10(-6) M) were partially inhibited by each antagonist and fully blocked by their combination. 7. Substance P (10(-9) to 10(-4) M), [Nle10]-neurokinin A(4-10) (10(-10) to 10(-5) M), [MePhe7]-neurokinin B and capsaicin (10(-7) to 10(-5) M) evoked concentration-dependent contractions. 8. The contractions to substance P were significantly inhibited by SR 140333 (10(-8) to 10(-6) M) but unaffected by SR 48968 (10(-8) to 10(-6) M). Furthermore, the response to [Nle10]-neurokinin A(4-10) was significantly inhibited by SR 48968 and unaffected by SR 140333 at the same concentrations. Although SR 48968 (10(-7) M) alone did not influence the effects of substance P, it potentiated the inhibitory effect of SR 140333 (10(-7) M). A similar synergetic effect of these two compounds was observed in the inhibition of the contractile response to [Nle10]-neurokinin A(4-10). 9. Neither SR 140333 (10(-7) M) nor SR 48968 (10(-7) M) alone influenced the contractions to [MePhe7]-neurokinin B and capsaicin. However, the combination of the two antagonists abolished the contractions to either peptide. 10. These results demonstrate that the stimulation of both NK1 and NK2 tachykinin-receptors induced contraction and depolarization of the guinea-pig tracheal smooth muscle and that both receptors were stimulated during the endogenous release of tachykinins by capsaicin. There was no evidence for a major role of NK3 receptors in the contractile and electrical activity of the guinea-pig isolated trachea.