The characteristics of vanadate‐induced contraction of airways smooth muscle are described in isolated preparations of guinea‐pig central and peripheral airways. Vanadate (1–1000 μM) induced sustained contractions of trachea and lung parenchymal strips within 1 min of challenge. It was more potent (P < 0.001) on the lung strip (EC50 = 63 μM) than on the trachea (EC50 =123 μM). The lung strip also developed greater maximum isometric tension (P < 0.001) than the trachea. The efficacy on the lung strip was 2 and the trachea 0.6, relative to the response to acetylcholine (efficacy = 1). Vanadate‐induced contractions of the trachea were not inhibited by atropine, mepyramine, phentolamine or indomethacin, nor after mast cell depletion by compound 48/80, showing that contractions were not mediated via specific receptors or by release of endogenous mediators of tone. Inorganic phosphate specifically inhibited vanadate responses in a dose‐dependent and reversible manner, suggesting a common site of action. Contractions could be elicited in depolarized muscle and after treatment with ouabain plus propranolol, showing that membrane depolarization and inhibition of the Na, K‐ATPase system were not involved in the contractile action of vanadate. Pretreatment of tracheal smooth muscle with verapamil had no influence on contractions elicited by vanadate. After removal of extracellular calcium, vanadate‐induced contractions declined slowly with time, indicating that influx of extracellular calcium was not giving rise to contractions elicited by vanadate. Vanadate markedly increased the rate of calcium efflux from trachealis muscle loaded with 45Ca into both Ca2+‐free and normal Krebs solutions; this is compatible with vanadate mobilizing an intracellular store of Ca2+. Such a store involving sites with Ca‐ATPase activity would be consistent with the action of vanadate in isolated membrane preparations. Membrane‐skinned tracheal fibres contracted by micromolar Ca2+ were relaxed by vanadate in a reversible dose‐related manner, indicating that the contractile action of vanadate was not related to its interaction with proteins at the cross‐bridge level.

The characteristics of vanadate‐induced bronchoconstriction and airways hyperreactivity were observed in spontaneously breathing anaesthetized guinea‐pigs by measurement of airways resistance (Raw) and dynamic lung compliance (Cdyn). Vanadate (0.3–3 mg kg−1 i. v. over 25 min) increased Raw and decreased Cdyn in a reversible, dose‐related manner. This action (1 mg kg−1 vanadate) was not inhibited by atropine (1 mg kg−1 i.v.), propranolol (1 mg kg−1 i.v.) or bilateral vagotomy, suggesting a direct effect on the airways smooth muscle. An aerosol of vanadate (10% w/v in H2O) for 3 min decreased Cdyn by 19.5% (P < 0.05, n = 6) but caused no change in Raw. Histamine (3 μg kg−1 i.v.) caused a bronchoconstriction which was enhanced by vanadate in a dose‐related manner. This hyperreactivity (after 1 mg kg−1 i.v. vanadate) was unchanged after propranolol or bilateral vagotomy, but was partly blocked by atropine (enhancement by vanadate of the Cdyn change to histamine was diminished, P < 0.02, n = 3). Bronchoconstrictor responses to acetylcholine (6 μg kg−1 i.v.) and 5‐hydroxytryptamine (6 μg kg−1 i.v.) were also enhanced by vanadate (1 mg kg−1 i.v.). Hyperreactivity after vanadate to the three bronchoconstrictors tested continued during vanadate infusion and was reversed 45 min after cessation of infusion. Histamine (3 μg kg−1 i.v.) caused a transient tachypnoea which was also enhanced by vanadate (0.3–3 mg kg−1 i.v.), in a dose‐related manner, in association with the increased reactivity of the airways (r = 0.66, n = 11). It is concluded that vanadate‐induced airways hyperreactivity is non‐vagal (efferent) and largely non‐cholinergic in origin and appears to involve an action of vanadate within the lung itself.

Vanadate (Vi, 3-300 microM) reversibly inhibited force development elicited by micromolar Ca2+ in membrane-skinned fibers of smooth muscle from taenia coli and trachea of guinea pig. When relaxed fibers were preincubated with Vi, the contraction to Ca2+ was characterized by a peak response followed by a lower steady-state phase. The peak phase depended on the rate of contraction and the [Vi]and was absent after Vi incubation during a previous contraction. These observations were consistent with Vi binding to a site that was exposed during the cross-bridge cycle but absent in the relaxed state. The actin X myosin X ADP intermediate formed at the active site during the cross-bridge cycle is suggested as the site of action of Vi. A weak antagonism between Pi and Vi was demonstrated during contractions activated by myosin thiophosphorylation. High concentrations of Pi (6-12 mM) were needed to produce a small inhibition (10%) of maximal Ca2+-activated tension. Skinned fibers relaxed slowly after Ca2+ removal, and the absence of an active state suggested that tension was maintained by noncycling cross bridges. Both Vi and Pi increased the rate of tension loss by 10-fold, but Vi was 5-10 times more potent than Pi. It is suggested that Vi and Pi both act on the active site but that Pi has a more efficacious action on slowly cycling than rapidly cycling cross bridges.

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Contractions of guinea-pig isolated tracheal smooth muscle by submaximal capsaicin (0.1 microM) were enhanced by 43% (P less than 0.01) after vanadate (10 microM for 10 min) preincubation. Contractile responses to acetylcholine (0.1-100 microM), histamine (1-100 microM) or substance P (0.01-1 microM) were, in contrast, not affected by prior vanadate exposure. It is suggested that tachykinin release from capsaicin-sensitive afferent nerve endings within the airways was enhanced by vanadate while airway smooth muscle reactivity remained unchanged.