Lung epinephrine synthesis

We studied in vitro and in vivo epinephrine (E) synthesis by rat lung. Nine days after removal of the adrenal medullas, circulating E was reduced to 7% of levels found in sham-operated rats but 30% of lung E remained. Treatment of demedullated rats with 6 hydroxydopamine plus reserpine did not further reduce lung E. In the presence of S-[3H]adenosylmethionine lung homogenates readily N-methylated norepinephrine (NE) to form [3H]E. The rate of E synthesis by lung homogenates was progressively more rapid with increasing NE up to a concentration of 3 mM, above which it declined. The rate of E formation was optimal at an incubation pH of 8 and at temperatures of approximately 55 degrees C. We compared the E-forming enzyme(s) of lung homogenates with those of adrenal and cardiac ventricle. The adrenal contains mainly phenylethanolamine N-methyltransferase (PNMT), which is readily inhibited by SKF 29661 and methylates dopamine (DA) very poorly. Cardiac ventricles contain mainly nonspecific N-methyltransferase (NMT), which is poorly inhibited by SKF 29661 and readily methylates both DA and NE. Lung homogenates were inhibited by SKF 29661 about half as well as adrenal but more than ventricle. We used the rate of E formation from NE as an index of PNMT-like activity and deoxyepinephrine synthesis from DA as an index of NMT-like activity. PNMT and NMT activity in rat lung homogenates were not correlated with each other, displayed different responses to change in temperature, and were affected differently by glucocorticoids