Polyamine synthesis blockade in monocrotaline-induced pneumotoxicity

Based on the documented regulatory role of polyamines in cell growth and differentiation, we have proposed that these organic cations are involved with the development of monocrotaline (MCT)-induced hypertensive pulmonary vascular disease. Two lines of evidence support this hypothesis: (1) MCT causes progressive increases in lung polyamine contents which are temporarily related to the development of cardiopulmonary abnormalities, and (2) blockade of polyamine synthesis with the site-selective enzyme-activated inhibitor, alpha-difluoromethylornithine (DFMO), attenuates development of medial arterial thickening, increased pulmonary arterial pressure, and right ventricular hypertrophy. To evaluate the mechanism of DFMO protection, the present study assessed when, during the course of MCT-induced pneumotoxicity, DFMO exerts its salutary effects, and determined if the protection afforded by DFMO could be reversed through supplementation with exogenous polyamines. To address the first issue, rats were treated with 30 mg/kg MCT and, 10 days after administration when lung polyamine contents were augmented and when pulmonary edema was evident, DFMO treatment was initiated as a 2% solution in the drinking water. In animals receiving MCT only, lung polyamine contents were elevated and right ventricular hypertrophy was evident at both 20 and 35 days after treatment. DFMO treatment initiated at day 10 attenuated the increases in putrescine and spermidine but not spermine and reduced the degree of right ventricular hypertrophy at both the 20- and 35-day time points. To determine if the blockade by DFMO could be reversed by supplementation with exogenous polyamines, animals were treated simultaneously with MCT and DFMO as described above and the immediate precursor to the polyamines, ornithine, was added to the drinking water as a 2% solution. Relative to animals receiving MCT and DFMO, ornithine supplementation increased lung polyamine contents to levels normally associated with MCT treatment only. Ornithine also reversed the protection against right ventricular hypertrophy normally afforded by DFMO. These observations indicate that the salutary effects of DFMO in MCT-induced pulmonary hypertension cannot be ascribed solely to interference in the early events after MCT treatment and that restoration of lung polyamine contents to high levels by supplementation with exogenous ornithine reverses DFMO protection against sustained pulmonary hypertension. It is concluded, therefore, that polyamines play a central role in delayed responses of lung cells underlying the development of MCT-induced sustained pulmonary hypertension.