Distinctive pharmacological profile of a nonadrenergic inhibitory system in bullfrog lung

Abstract

1Bullfrog 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 10-3M 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 10-6M) 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 10-3M 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 10-4M 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.

Mechanisms of intrinsic tone in bullfrog lung: relaxant effects of indomethacin, ouabain and potassium

“…Ouabain (10-4 and 10-5 M) caused a transient contraction followed by a marked, progressive relaxation (P <0.01) which peaked in 4 to 16 h. Since the resting length of untreated hemilungs is already near maximal contraction (Downes & Taylor, 1983), the ouabaininduced contraction was of much less magnitude than the eventual relaxation. Ouabain produced qualitatively similar effects in hemilungs obtained from both large (250-500 g), and small (100-150 g) bullfrogs, but the relaxant effect was significantly greater in the smaller frogs.…”

“…Raising the concentration to 10-4 M produced greater relaxation (23% of Imax), but also markedly potentiated the effect of isoprenaline. Normally, the maximal effect of isoprenaline and nes & Taylor, 1983). However, the relaxant effect, as well as the increase in tissue cyclic AMP content, can be markedly potentiated by addition of papaverine , an inhibitor of cyclic nucleotide phosphodiesterase.…”

“…In contrast, indomethacin does not relax human bronchial preparations (Brink et al, 1980;Hutas et al, 1981), suggesting 'that the regulatory role of prostanoids in human airway muscle may be less than in other species' (Brink et al, 1980). Bullfrog lung is of particular interest with respect to intrinsic tone, in that the resting length of the excised lung is near maximal contraction (Downes & Taylor, 1983). This high level of intrinsic tone does not reflect the activity of intrapulmonary nerves since it is unaffected by atropine, ganglionic blocking agents, or tetrodotoxin (Downes & Taylor, 1983), and is enhanced rather than inhibited by local anaesthetics (Dow-nes & Taylor, 1982).…”

“…Bullfrog lung is of particular interest with respect to intrinsic tone, in that the resting length of the excised lung is near maximal contraction (Downes & Taylor, 1983). This high level of intrinsic tone does not reflect the activity of intrapulmonary nerves since it is unaffected by atropine, ganglionic blocking agents, or tetrodotoxin (Downes & Taylor, 1983), and is enhanced rather than inhibited by local anaesthetics (Dow-nes & Taylor, 1982). The present study tests the effects of indomethacin and ouabain, and shows that a ouabain-sensitive mechanism is far more important than prostaglandin synthesis to the maintenance of intrinsic tone in this preparation.…”