PULMONARY MICROSOMES CONTAIN A CA2-TRANSPORT SYSTEM SENSITIVE TO OXIDATIVE STRESS()

A variety of events, including inhalation of atmospheric chemicals, tr auma, and ischemia-reperfusion, may cause generation of reactive oxyge n species in the lung and result in airways constriction. The specific metabolic mechanisms that translate oxygen radical production into ai rways constriction are yet to be identified. In the lung, calcium home ostasis is central to release of bronchoactive and vasoactive chemical mediators and to regulation of smooth muscle cell contractility, i.e. , airway constriction. In the present work, we characterized Ca2+-tran sport in the microsomal fraction of mouse lungs, and determined how re active oxygen species, generated by Fe2+/ascorbate and H2O2/hemoglobin , affected Ca2+ transport. The microsomal fraction of pulmonary tissue accumulated 90 +/- 5 nmol Ca2+/mg protein by an ATP-dependent process in the presence of 15 mM oxalate, and 16 +/- 2 nmol Ca2+ in its absen ce. In the presence of oxalate, the rate of Ca2+ uptake was 50 +/- 5 n mol Ca2+/min per mg protein at pCa 5.9 (37 degrees C). The Ca2+-ATPase activity was 50-60 nmol P-i/min per mg protein (pCa 5.9, 37 degrees C ) in the presence of alamethicin. Inhibitors of mitochondrial H+-ATPas e had no effect on the Ca2+ transport. Half-maximal activation of Ca2 transport was produced by 0.4-0.5 mu M Ca2+. Endoplasmic reticulum Ca 2+-pump (SERC-ATPase) was found to be predominantly responsible for th e Ca2+-accumulating capacity of the pulmonary microsomes. Incubation o f the microsomes in the presence of either Fe2+/ascorbate or H2O2/hemo globin resulted in a time-dependent accumulation of peroxidation produ cts (TBARS) and in inhibition of the Ca2+ transport. The inhibitory ef fect of Fe2+/ascorbate on Ca2+ transport strictly correlated with the inhibition of the Ca2+-ATPase activity. These results are the first to indicate a highly active microsomal Ca2+ transport system in murine l ungs which is sensitive to endogenous oxidation products. The importan ce of this system to pulmonary disorders exacerbated by oxidative chem icals remains to be studied.