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 microM-3 mM) produced concentration-dependent, well-sustained contraction. Its -logEC50 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 microM)-induced contraction of human bronchus was epithelium-independent and was not inhibited by
indomethacin (2.8 microM),
zileuton (10 microM), a mixture of
atropine,
mepyramine and
phentolamine (each at 1 microM), or by mast cell degranulation with
compound 48/80. 3.
Vanadate (200 microM)-induced contraction was unaltered by tissue exposure to
verapamil or
nifedipine (each 1 microM) or to a Ca2+-free,
EGTA (0.1 mM)-containing physiological
salt solution (PSS). However, tissue incubation with
ryanodine (10 microM) 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 microM) 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 microM) did not alter
vanadate (200 microM)-induced contraction.
Ouabain (10 microM) abolished the K+-induced relaxation of human bronchus bathed in K+-free PSS. This action was not shared by
vanadate (200 microM). The tissue content of Na+ was increased and the tissue content of K+ was decreased by
ouabain (10 microM). In contrast,
vanadate (200 microM) 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 microM). 5.
Vanadate (200 microM)-induced contractions were markedly reduced by tissue treatment with each of the
protein kinase C (PKC) inhibitors
H-7 (10 microM),
staurosporine (1 microM) and
calphostin C (1 microM).
Genistein (100 microM), an inhibitor of
protein tyrosine kinase, also reduced the response to
vanadate. 6
Vanadate (0.1-3 mM) and ACh (1 microM- 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 microM). 7. In human cultured tracheal smooth muscle cells,
histamine (100 microM) and
vanadate (200 microM) each produced a transient increase in intracellular Ca2+ concentration ([Ca2+]i). 8. Intracellular
microelectrode recording showed that the contractile effect of
vanadate (200 microM) 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.