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 vanadate on human isolated bronchus. Vanadate (0.1 mu M-3 mM) produced concentration-dependent, well-sustained contraction. Its -logEC(50) was 3.74 +/- 0.05 (mean +/- s.e. mean) and its maximal effect was equivalent to 97.5 +/- 4.2% of the response to acetylcholine (ACh, 1 mM). 2 Vanadate (200 mu M)-induced contraction of human bronchus was epithelium-independent and was not inhibited by indomethacin (2.8 mu M), zileuton (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 Vanadate (200 mu M)-induced contraction was unaltered by tissue exposure to verapamil or nifedipine (each 1 mu M) or to a Ca2+-free, EGTA (0.1 mM)-containing physiological salt solution (PSS). However, tissue incubation with ryanodine (10 mu M) in Ca2+-free, EGTA (0.1 mM)-containing PSS reduced vanadate-induced contraction. A series of vanadate challenges was made in tissues exposed to Ca2+-free EGTA (0.1 mM)-containing PSS with the object of depleting intracellular Ca2+ stores. In such tissues cyclopiazonic acid (CPA; 10 mu M) prevented Ca2+-induced recovery 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 alter vanadate (200 mu M)-induced contraction. Ouabain (10 mu M) abolished the K+-induced relaxation of human bronchus bathed in K+-free PSS. This 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 ouabain (10 mu M). In contrast, vanadate (200 mu M) did not alter the tissue content of these ions. Tissue incubation in a Na+-deficient (25 mM) PSS or in PSS containing amiloride (0.1 mM) markedly inhibited the spasmogenic effect of vanadate (200 mu M). 5 Vanadate (200 mu M)-induced contractions were markedly reduced by tissue treatment with each of the 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 protein tyrosine kinase, also reduced the response to vanadate. 6 Vanadate (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+-free medium either alone or in combination with ryanodine (10 mu M). 7 In human cultured tracheal smooth muscle cells, histamine (100 mu M) and vanadate (200 mu M) each produced a transient increase in intracellular Ca2+ concentration ([Ca2+](i)). 8 Intracellular microelectrode recording showed that the contractile effect of vanadate (200 mu M) in human bronchus was associated with cellular depolarization. 9 It is concluded that vanadate acts directly on human bronchial smooth muscle, promoting the release of Ca2+ from an intracellular store. The Ca2+ release mechanism involves both the production of inositol phosphate second 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.