1. The aim of this study was to compare the ability of four different bronchodilators (atrial natriuretic peptide, salbutamol, sodium nitroprusside and isosorbide dinitrate) to reverse and also to protect against contractions evoked by the spasmogens endothelin-1 and methacholine in human isolated bronchial rings. 2. Contractions evoked by either endothelin-1 or methacholine were reversed by atrial natriuretic peptide (10 −9 −10 −6 mol/l), salbutamol (10 −9 −10 −5 mol/l), sodium nitroprusside (10 −9 −10 −5 mol/l) and isosorbide dinitrate (10 −7 −10 −4 mol/l). 3. Sodium nitroprusside produced a significantly ( P < 0.05 for data points) greater mean maximal inhibition of endothelin-1-induced tone than methacholine-induced tone; however, the other dilators were equally effective at reversing either endothelin-1- or methacholine-induced contractions. Comparing sodium nitroprusside and salbutamol, sodium nitroprusside was significantly ( P < 0.05 for data set) less effective than salbutamol at reversing either endothelin-1- or methacholine-induced contractions. 4. To compare the ability of these bronchodilator substances to protect against spasmogen challenge, cumulative concentration—response curves to either endothelin-1 (10 −10 −3 × 10 −7 mol/l) or methacholine (10 −9 −3 × 10 −4 mol/l) were constructed in the presence and absence of each bronchodilator. Atrial natriuretic peptide, at a concentration of 10 −6 mol/l, significantly attenuated contractions evoked by methacholine. In contrast, contractions evoked by endothelin-1 were enhanced by atrial natriuretic peptide at concentrations of 3 × 10 −7 and 10 −6 mol/l. Preincubation of salbutamol at a concentration of 10 −6 mol/l significantly attenuated methacholine-induced contractions, but responses to endothelin-1 were not altered by preincubation of salbutamol at concentrations of 3 × 10 −7 , 10 −6 and 3 × 10 −6 mol/l. Sodium nitroprusside (10 −6 mol/l) and isosorbide dinitrate (3 × 10 −5 mol/l) did not alter responses evoked by subsequent addition of either endothelin-1 or methacholine. At a concentration of 10 −4 mol/l, however, isosorbide dinitrate significantly attenuated endothelin-1-evoked contractions. 5. These results show that drugs which reverse agonist-induced tone in isolated bronchial rings may not necessarily protect against subsequent challenge with this agonist. This suggests that the pharmacology of relaxation may be dissimilar to that of protection.

1. We have previously shown that atrial natriuretic peptide causes bronchodilatation and reduces bronchial reactivity when administered intravenously or by inhalation to asthmatic patients. We wished to determine the direct effect of exogenously applied atrial natriuretic peptide on isolated airway and the role of proteases important in atrial natriuretic peptide degradation in other organ systems. 2. The ability of atrial natriuretic peptide (α-human atrial natriuretic peptide 28-amino acid) to relax precontracted tissues and to protect against methacholine-induced contraction was studied in human and bovine tissue. The role of neutral endopeptidase-24.11 and other proteases in regulating the effect of atrial natriuretic peptide on bronchial smooth muscle was also examined by studying the influence of phosphoramidon, a protease inhibitor, whose actions include the inhibition of neutral endopeptidase-24.11, and the protease inhibitors leupeptin, aprotinin and soybean trypsin inhibitor on the airway response to atrial natriuretic peptide. 3. In human and bovine tissue atrial natriuretic peptide (10 −6 mol/l) caused a slight relaxation of methacholine-contracted tissue [mean (SEM) percentage inhibition of contraction of 13.2 (3.02)% and 9.41 (2.63)% respectively] and evoked a signficant rightward shift of the cumulative concentration–-response curve to methacholine [p D 2 5.15 (0.23) and 4.85 (0.1) compared with control values of 6.14 (0.1) and 5.85 (0.16), respectively]. 4. Phosphoramidon potentiated atrial natriuretic peptide-induced relaxation of methacholine-induced tone and the ability of atrial natriuretic peptide to protect against methacholine-induced contraction. The combination of leupeptine, aprotinin and soybean trypsin inhibitor did not significantly alter the bronchial response to atrial natriuretic peptide in either human or bovine tissues. 5. We conclude that in human and bovine tissue, atrial natriuretic peptide confers protection against methacholine-induced contraction and can relax induced tone and that inhibition of phosphoramidon-sensitive protease increases these effects of atrial natriuretic peptide on airway smooth muscle.

1. The effects of varying [Ca 2+ ] o on the contraction of smooth muscle by different α-adrenoceptor agonists were examined on rat isolated anococcygeus muscle. Agonists were tested in the presence of various [Ca 2+ ] o or ‘Ca 2+ -re-addition curves’ were constructed. In some experiments the [Ca 2+ ] free was buffered with EGTA and nitrilotriacetic acid. The components of the response which were revealed were further analysed by using drugs which modify Ca 2+ mobilization. 2. Three separate elements in the contractile response were identified: (i) an initial transient contraction, due to intracellular Ca 2+ release could be isolated with [Ca 2+ ] o between 1 nmol/l and 3 μmol/l (this could be obtained only with noradrenaline, phenylephrine and amidephrine); (ii) a nifedipine-sensitive response requiring [Ca 2+ ] o of 3 μmol/l or more; (iii) a nifedipine-resistant response requiring [Ca 2+ ] o of 100 μmol/l or more. Presumably (ii) and (iii) involve the entry of Ca 2+ o : they could be obtained with all agonists tested, including these above, methoxamine, indanidine and xylazine. 3. The results are discussed in relation to the possibility of distinct types of response and their relationship to subgroups of receptors or agonists. It is concluded that there is a continuous spectrum of activity across the agonist range and that this is likely to correlate with ‘efficacy’ at a single α 1 receptor type.