Distinctive pharmacological profile of a nonadrenergic inhibitory system in bullfrog lung

1 Bullfrog hemilungs, pretreated with atropine, are markedly relaxed on addition of carbachol. Since the relaxant effect is inhibited by tetrodotoxin or hexamethonium, it is neurally mediated and involves stimulation of nicotinic receptors with release of an unknown inhibitory transmitter.2 Carbachol-induced relaxation is nonadrenergic since: (a) it considerably exceeds the maximal effects of isoprenaline or the effect of 10(-3)M adrenaline or noradrenaline; (b) it elicits marked further relaxation in preparations already relaxed by high concentrations of catecholamines; (c) it is not attenuated by low concentrations of propranolol (10(-6) and 3 x 10(-6)M) that competitively antagonize isoprenaline-induced relaxation.3 Carbachol-induced relaxation has multiple distinguishing characteristics, which serve as a fingerprint for the unknown inhibitory transmitter. These include an exceptionally rapid onset of action, a ceiling effect at 50% of maximal relaxation, and minimal retardation by concentrations of procaine that block or markedly retard relaxant responses to all other agonists.4 This distinctive pharmacological profile cannot be reproduced by addition of exogenous catecholamines, 5-hydroxytryptamine, adenosine triphosphate (ATP) or adenosine, or by addition of ATP or adenosine following pretreatment with indomethacin. Furthermore, addition of carbachol to preparations previously relaxed with 10(-3)M concentrations of these agents produced marked, additional relaxation.5 Maximally effective concentrations of vasoactive intestinal peptide produced a barely detectable relaxant response equivalent to 8% of maximal relaxation. The response was totally prevented by pretreatment with procaine.6 Carbachol-induced relaxation was not impaired by pretreatment with 10(-4)M indomethacin.7 Carbachol-induced relaxation of bullfrog lung therefore involves a postganglionic inhibitory transmitter that in nonadrenergic, non-5-hydroxytryptaminergic, and nonpurinergic, and whose effects are not dependent on prostaglandin synthesis. Although a peptide may function as the inhibitory transmitter, it is not vasoactive intestinal peptide.