Respirable-sized quartz, a well-established fibrogenic mineral dust, is compared with kaolin in erythrocyte hemolysis assays after treatment with saline dispersion of dipalmitoyl phosphatidylcholine, a primary phospholipid component of pulmonary surfactant. Both dusts are rendered inactive after treatment, but the membranolytic activity is partly to fully restored after treatment with phospholipase A2, an enzyme normally associated with cellular plasma membranes and lysosomes. Phospholipid-coated dusts were incubated for periods of 2-72 h at a series of applied enzyme concentrations, and the adsorbed lipid species and hemolytic activity were quantitated at each time for both dusts. Surfactant was lost more readily from quartz than from kaolin, with consequent more rapid restoration of mineral surface hemolytic activity for quartz. Interactions of surfactant and mineral surface functional groups responsible for the mineral-specific rate differences, and implications for determining the mineral surface bioavailability of silica and silicate dusts, are discussed.
1The Na', K+-pump has been implicated in animal models of airway hyperreactivity. We examined the effects of inhibiting the Na', K+-pump and Na+, Ca2+-exchange on isometric tone of isolated trachealis from humans and other species. 2 In preparations from 5 out of 9 humans, strong spontaneous contractions (36-48 h-'; up to 1.8 g) developed within 25 min. 3 Ouabain (10-7-10-'M) caused an immediate and sustained contraction. This response was not blocked by atropine, diphenhydramine, or cimetidine. 4 Contractions were also elicited when the normal physiological solution was changed to a K+-free solution, a procedure which inhibits the Na+, K+-pump, and in reduced (15 mM) Na+ solution, which inhibits Na+, Ca2+ exchange. 5 In preparations of dog and guinea-pig isolated trachea, ouabain (10-M) caused a multiphasic response; in the rabbit, ouabain was without effect. K+-free solution was without effect in the dog preparations and produced relaxation of the guinea-pig trachea. Guinea-pig tracheae responded to a low Na+ solution with a strong contraction. 6 Our findings indicate that: (a) human airway smooth muscle may be a spontaneously contracting muscle, at least in vitro, (b) a prolonged contraction to ouabain is unique for the human airway smooth muscle among the animals tested, as is the contraction in a K+-free medium, and (c) the contractile response does not involve acetylcholine or histamine release, but may involve a Na+, Ca2+-exchange mechanism. These results suggest that the level of Na+, K+-pump activity could play a role in determining the degree of bronchomotor tone in humans.
The macrophage-like cell line, P388D1, was exposed to dipalmitoyl lecithin (DPL)-coated respirable quartz and kaolin, and the disappearance of the DPL was monitored for up to 9 days. The coating was removed rapidly at first (about 50% in the first 3 days) and then more slowly over the remaining 6 days, until about 30% remained on day 9. The rate of DPL digestion was independent of the type of dust and the amount of coated dust within the cell, indicating the existence of an extracellular phospholipase activity. This extracellular phospholipase activity was partially characterized. It was sensitive to temperatures above 56 degrees C, the presence of EDTA, the action of the proteases trypsin and proteinase K, and pH, being active at pH 7 but not at pH 5. This is consistent with reports in the literature of the existence of an extralysosomal phospholipase which is active at pH 7 and dependent on the presence of divalent metal ions. There was a dust-dependent difference in the extracellular rate of DPL digestion from quartz and kaolin. The coating was removed more slowly from the kaolin than it was from quartz. The removal of the DPL coating seen in the presence of cells was presumably due to both an intracellular and an extracellular phospholipase.
1 We have previously observed a paradoxical relaxant effect of K+ on guinea-pig isolated trachealis after exposure to polyamines. The purpose of the present study was to evaluate whether the relaxation involved a reduction in the entry ofextracellular Ca2+. We therefore investigated the effect of K+ in the presence of Ca2+-entry blocking drugs and in the presence of Ca2+-free solution. 3 A relaxation in response to K+ was also observed in Ca2+-free solution, (with tone induced by methacholine), an effect not blocked by propranolol or ouabain. 4 Tetraethylammonium (30mM) (TEA), which ordinarily evokes contractile responses, induced trachealis relaxation in the presence of verapamil or nifedipine. The relaxation was unaltered by ouabain or propranolol. 5 Tetrodotoxin (10-6 M) (TTX) blocked 65% of the K+-induced relaxation in the presence of nifedipine and 100% of K+-induced relaxation either in a Ca2+-free solution or after polyamine exposure. TTX was without effect on TEA-induced relaxation after Ca2+-entry blocking drugs.6 Atropine (10-6 M) or hexamethonium (10-6 M) did not affect K+-induced relaxation after polyamine exposure. 7 The concentration-response curve for K+-induced contraction in normal modified Krebs-Henseleit solution was shifted to the left by TTX. 8 It is concluded: (a) K+ has a direct effect on the trachealis causing contraction and an indirect effect, mediated by neurotransmitter release, causing relaxation. This latter effect is exposed when the direct effect is inhibited by Ca2+-entry blocking drugs, Ca2+-free solution or polyamine exposure; the indirect effect is non-adrenergic, non-cholinergic and not via ganglionic transmission; (b) the TEAinduced relaxation and a component of the K+ -induced relaxation after Ca2+ blocking drugs cannot be explained by neurotransmitter release; (c) polyamines may act as naturally occurring Ca2+ antagonists.
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