Various amounts of paraquat (10(-5) to 10(-12) g) in 0.1 ml saline were instilled directly into the left bronchus of male adult rats. Gravimetric, macroscopic, and microscopic studies on the left lobe of the lung showed that 10(-5) g of paraquat produced lung oedema and macroscopic lesions two and 14 days after doing. The pathology of the lung was similar to that seen after systemic poisoning. When 10(-6) g of paraquat was instilled, some animals developed lung oedema and macroscopic lesions. Microscopic examination showed subtle changes in the parenchyma of the lung. With amounts of paraquat equal to or less than 10(-7) g (doses as little as 10(-12) g were used), no changes in the lung were seen. This is contrary to published accounts in which amounts as low as 10(-12) g (1 Pg) were reported to cause acute damage to the rabbit lung. When 3H paraquat was instilled into the left lobe (doses of 10(-5) to 10(-10) g were used), the loss of paraquat from the lung was biphasic. The initial half-life was less than one hour. The secondary phase obeyed first-order kinetics, and the half-life was dependent on the dose of paraquat instilled. This half-life was as short as 11 hours when 10(-5) g paraquat was instilled and was 76 hours after the instillation of 10(-10) g paraquat. The decrease in the half-life of the secondary phase with increasing doses of paraquat is possibly associated with the production of oedema or lung cell damage, or both. After the instillation of 10(-8) g 3H paraquat, the initial half-life was less than 15 minutes, and paraquat was detected in the urine and plasma at that time. This suggests that 50% of the instilled paraquat was rapidly absorbed from the lung into the plasma.
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