Northern blot of rat lung RNA. Lanes 1, 3, 5, and 7: Total RNA (15 μg/lane) from lung tissues of rats exposed for 1, 3, 5, and 7 days to 85% oxygen. Lane C: Total RNA from unexposed animals. Membranes were hybridized with specific cDNA probes for gelatinase A (MMP-2), gelatinase B (MMP-9), and GAPDH as a normalization control.