Neither vasoactive intestinal peptide (VIP) nor purine derivatives may mediate non-adrenergic tracheal inhibition

Neurogenic responses in isolated electrical field stimulated guinea-pig tracheas include a prominent relaxation that is only partially inhibited by propranolol. The possibility that either purine derivatives or VIP could be a non-adrenergic inhibitory neurotransmitter was examined. Relaxant characteristics of adenosine, ATP, adenine and VIP in tracheas with different degrees of contraction were compared to those of maximal non-adrenergic nerve stimulation. Spontaneous tone and carbachol (5.5 microM and 55 microM)-contracted tracheas were inhibited 77.5 +/- 5.0% (n = 9), 24.7 +/- 2.8% (n = 11) and 14.5 +/- 5.0% (n = 10), respectively, by electrical nerve stimulation. At the three states of contraction exogenous VIP, but not adenosine, ATP, or adenine, produced relaxations similar to those of electrical nerve stimulation. In carbachol-contracted tracheas with maximally effective concentrations present of adenosine (3 mM), adenine (3 mM) or VIP (23.3 microM), the non-adrenergic neurogenic inhibitory response remained intact. The neurogenic inhibitory response was also unaffected by the presence of theophylline and indomethacin which blocked relaxatory effects of adenosine and ATP, respectively. The results suggests that neither the purine derivatives, nor VIP are involved in the non-adrenergic, neurogenic inhibition in the guinea-pig trachea.