Factors Controlling Nitrosamine Formation in the Lung: A Unique Uptake System

Abstract

This study seeks to characterize the uptake of hydroxyproline into the rat lung from the vascular system using the isolated ventilated perfused lung (IVPL) and further to determine whether this amine is trapped sufficiently to act as a reactant with NO₂ in the production of N-nitrosohydroxyproline. Using blue dextran as an extracellular marker, hydroxyproline was found to be retained by the IVPL. The rate of hydroxyproline absorption by the lung from the perfusate did not increase linearly with hydroxyproline concentration but saturated at higher concentrations. This carrier-mediated hydroxyproline absorption had a V_max of 968 nmol/min.g dry weight tissue and a K_m of 217 μM. Similar to the 5-hydroxytryptamine uptake system, the uptake of hydroxyproline was inhibited in a sodium-free medium (66%). However, 5-hydroxytryptamine (or putrescine) did not inhibit uptake of hydroxyproline. Uptake of hydroxyproline was not sensitive to sodium cyanide or to carbonyl cyanide 4-(trifluoromethoxy)-phenylhydrazone. While this work shows that the lung preferentially sequesters hydroxyproline, published nitrosation rates of morpholine by NO₂ suggest that only about 0.0001% hydroxyproline would be converted to nitrosohydroxyproline in the IVPL system. This uptake system represents a unique pulmonary carrier-mediated process in that it is unlike the 5-hydroxytryptamine or diamine pulmonary uptake systems previously described; it is a sodium-facilitated process, and the carrier-mediated uptake is not energy dependent. A rate of nitrosation sufficient to monitor by reaction of NO₂ with radiolabeled hydroxyproline is unlikely to be obtained.