Relationship between the levels of intracellular cyclic nucleotides and mechanical responses induced by drugs.

The tracheal smooth muscle dissected free from connective tissues was used as a test preparation, since isoprenaline caused a decrease in cyclic GMP in the muscle while it caused an increase in cyclic GMP in the remaining tracheal tissue. Acetylcholine, histamine and papaverine increased both cyclic AMP and cyclic GMP levels in the tracheal muscle. Isoprenaline increased the cyclic AMP level but decreased the cyclic GMP level. However, when tracheal muscle was incubated with isoprenaline in the presence of acetylcholine, isoprenaline did not cause a decrease in cyclic GMP but rather a significant increase in cyclic AMP after only 1 min incubation. These results indicate that the relaxation of the tracheal muscle by isoprenaline is initiated by the increase in cyclic AMP but is not associated with the change in cyclic GMP.