THE SPASMOGENIC EFFECTS OF VANADATE IN HUMAN ISOLATED BRONCHUS

1 Inhalation of vanadium compounds, particularly vanadate, is a cause of occupational bronchial asthma. We have now studied the action of va nadate on human isolated bronchus. Vanadate (0.1 mu M-3 mM) produced c oncentration-dependent, well-sustained contraction. Its -logEC(50) was 3.74 +/- 0.05 (mean +/- s.e. mean) and its maximal effect was equival ent to 97.5 +/- 4.2% of the response to acetylcholine (ACh, 1 mM). 2 V anadate (200 mu M)-induced contraction of human bronchus was epitheliu m-independent and was not inhibited by indomethacin (2.8 mu M), zileut on (10 mu M), a mixture of atropine, mepyramine and phentolamine (each at 1 mu M), or by mast cell degranulation with compound 48/80. 3 Vana date (200 mu M)-induced contraction was unaltered by tissue exposure t o verapamil or nifedipine (each 1 mu M) or to a Ca2+-free, EGTA (0.1 m M)-containing physiological salt solution (PSS). However, tissue incub ation with ryanodine (10 mu M) in Ca2+-free, EGTA (0.1 mM)-containing PSS reduced vanadate-induced contraction. A series of vanadate challen ges was made in tissues exposed to Ca2+-free EGTA (0.1 mM)-containing PSS with the object of depleting intracellular Ca2+ stores. In such ti ssues cyclopiazonic acid (CPA; 10 mu M) prevented Ca2+-induced recover y of vanadate-induced contraction. 4 Tissue incubation in K+-rich (80 mM) PSS, K+-free PSS, or PSS containing ouabain (10 mu M) did not alte r vanadate (200 mu M)-induced contraction. Ouabain (10 mu M) abolished the K+-induced relaxation of human bronchus bathed in K+-free PSS. Th is action was not shared by vanadate (200 mu M). The tissue content of Na+ was increased and the tissue content of K+ was decreased by ouaba in (10 mu M). In contrast, vanadate (200 mu M) did not alter the tissu e content of these ions. Tissue incubation in a Na+-deficient (25 mM) PSS or in PSS containing amiloride (0.1 mM) markedly inhibited the spa smogenic effect of vanadate (200 mu M). 5 Vanadate (200 mu M)-induced contractions were markedly reduced by tissue treatment with each of th e protein kinase C (PKC) inhibitors H-7 (10 mu M), staurosporine (1 mu M) and calphostin C (1 mu M). Genistein (100 mu M), an inhibitor of p rotein tyrosine kinase, also reduced the response to vanadate. 6 Vanad ate (0.1-3 mM) and ACh (1 mu M-3 mM) each increased inositol phosphate accumulation in bronchus. Such responses were unaffected by a Ca2+-fr ee medium either alone or in combination with ryanodine (10 mu M). 7 I n human cultured tracheal smooth muscle cells, histamine (100 mu M) an d vanadate (200 mu M) each produced a transient increase in intracellu lar Ca2+ concentration ([Ca2+](i)). 8 Intracellular microelectrode rec ording showed that the contractile effect of vanadate (200 mu M) in hu man bronchus was associated with cellular depolarization. 9 It is conc luded that vanadate acts directly on human bronchial smooth muscle, pr omoting the release of Ca2+ from an intracellular store. The Ca2+ rele ase mechanism involves both the production of inositol phosphate secon d messengers and inhibition of Ca-ATPase. The activation of PKC plays an important role in mediating vanadate-induced contraction at values of [Ca2+](i) that are close to basal.