Effects of cromakalim on neurally‐mediated responses of guinea‐pig tracheal smooth muscle

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Abstract
1 The ability of cromakalim to modulate several different types of neuroeffector transmission has been assessed in guinea-pig isolated trachea. 2 In trachea treated with propranolol (10−6m) and indomethacin (2.8 × 10−6m), stimulation of the extrinsic vagal nerves evoked contractions which were blocked by hexamethonium (5 × 10−4m) or by tetrodotoxin (TTX; 10−6m). Cromakalim (10−5m) caused a two fold rightward shift of the frequency-response curve. 3 In carinal trachea treated with propranolol and indomethacin, transmural stimulation evoked an initial, rapid contraction followed by a more sustained secondary contraction. The initial, rapid contractile response was virtually ablated by atropine (10−6m) or by TTX but was resistant to hexamethonium. Cromakalim (10−8–10−5m) caused a concentration-dependent rightward shift of the frequency-response curve for the initial contraction. 4 In carinal trachea treated with atropine, propranolol and indomethacin, transmural stimulation evoked only the secondary (non-adrenergic, non-cholinergic (NANC)) contractile responses. These were markedly reduced by TTX but were resistant to hexamethonium. Cromakalim (10−8–10−5m) suppressed the NANC contractile responses in a concentration-dependent manner. This action could be offset by glibenclamide (10−6m). 5 In trachea treated with atropine, histamine (10−4m), propranolol and indomethacin, transmural stimulation evoked NANC relaxant responses. Cromakalim (up to 10−5m) was without effect on the frequency-response curve for the stimulation of NANC inhibitory nerves. 6 Tested on trachea bathed by drug-free Krebs solution, cromakalim (10−7–10−5m) caused concentration-dependent suppression of tracheal tone. In trachea treated with propranolol and indomethacin, cromakalim (10−7–10−5m) caused concentration-dependent antagonism of acetylcholine (ACh). In trachea treated with atropine, propranolol and indomethacin, cromakalim (up to 10−5m) failed to antagonize effects of either histamine or substance P. 7 It is concluded that cromakalim can inhibit cholinergic (excitatory) neuroeffector transmission in the trachea but only at a concentration having demonstrable inhibitory activity against the action of exogenous ACh and the spontaneous tone of the airways smooth muscle. In contrast, cromakalim may depress NANC excitatory (putative peptidergic) neuroeffector transmission at a concentration below that exerting inhibitory activity on airways smooth muscle. Cromakalim does not concurrently depress NANC inhibitory neuroeffector transmission. Depression of NANC excitatory neuroeffector transmission could explain the ability of cromakalim to suppress airway hyperreactivity or bronchial asthma at doses lacking direct relaxant effect on airways smooth muscle.