Increased vascular responsiveness in lungs of rats with pulmonary hypertension induced by monocrotaline pyrrole.

Monocrotaline (MCT) is a natural product that causes pulmonary hypertension in rats after metabolic activation to a pyrrole form (MCTP). To examine the vascular reactivity of the pulmonary bed, blood-perfused isolated lungs from MCTP- or vehicle-treated rats were challenged with angiotensin II (AII) and 5-hydroxytryptamine (5HT), and the resultant increases in perfusion pressure were measured. Fourteen days after a single exposure, the pressor responses to AII (0 25 or 0.50 microgram) and 5 HT (12.5 to 50 microgram) were approximately 3 times as great in isolated lungs of MCTP-treated rats as in those of control rats. When examined 7 days after exposure, the response to 25 micrograms 5HT but not to 0.25 microgram AII was enhanced by MCTP; MCT is known to decrease 5HT uptake by pulmonary capillary endothelial cells. Imipramine, a 5HT uptake inhibitor, did not alter the vascular responses to 25 micrograms 5HT in lungs from either control or MCTP-treated rats. The increased responsiveness of the pulmonary vasculature to AII and 5HT after MCTP exposure could play a role in the development and/or maintenance of pulmonary hypertension in this rat model.

Injury to the isolated, perfused lung by exposure in vitro to monocrotaline pyrrole.

Monocrotaline pyrrole (MCTP) is a reactive metabolite of the pyrrolizidine alkaloid monocrotaline. It causes pulmonary lesions associated with pulmonary hypertension and right ventricular hypertrophy. Conditions of exposure to MCTP that result in early lung injury were examined in isolated rat lungs perfused with buffered medium containing 4% bovine serum albumin. When a high, acutely lethal dose (8.1 mg) of chemically synthesized MCTP was added to the 50-ml perfusion medium reservoir, no effects on the perfused lung occurred. However, when the same quantity of MCTP was injected directly into the pulmonary arterial (PA) cannula, the lungs accumulated considerable fluid within 1 h, and this was accompanied by elevated perfusion pressure and elevated lactate dehydrogenase (LDH) activity in the perfusion medium. A lower dose (1.2 mg) of MCTP that is pulmonary hypertensive in vivo also produced edema and elevated perfusion pressure when injected into the PA cannula. Removal of perfused 5-hydroxytryptamine, a function of pulmonary endothelium, was unaltered compared to vehicle-treated control lungs, as was perfusate LDH activity. These results indicate that injury to isolated lungs occurs soon after direct exposure to MCTP, even at a moderate dose that produces primarily delayed effects in vivo.

Alteration of monocrotaline pyrrole-induced cardiopulmonary effects in rats by hydrallazine, dexamethasone or sulphinpyrazone.

The effects of intraperitoneal hydrallazine, dexamethasone, or sulphinpyrazone on the toxicity of monocrotaline pyrrole (MCTP) were examined in rats 14 days after injection of MCTP (5 mg kg-1, i.v.). MCTP alone caused increases in lung weight, and of both lactate dehydrogenase activity and protein concentration in bronchopulmonary lavage fluid. Right ventricular hypertrophy also occurred. Hydrallazine (3 mg kg-1, daily), a vasodilator and platelet prostaglandin synthesis inhibitor, reduced the degree of right ventricular hypertrophy and the elevation in the concentration of protein in lavage fluid. Dexamethasone (27 micrograms kg-1, daily), an anti-inflammatory agent and inhibitor of phospholipase, also reduced the right ventricular hypertrophy and the increased protein concentration in lavage fluid caused by MCTP. Sulphinpyrazone (100 mg kg-1, twice daily), an inhibitor of platelet prostaglandin biosynthesis, prevented right ventricular hypertrophy in the MCTP treated rats without affecting any of the indices of lung injury. These results provide further support for the hypothesis that platelets and vasoconstrictor agents play a role in the development of MCTP-induced pulmonary hypertension.

Effects of thrombocytopenia on monocrotaline pyrrole-induced pulmonary hypertension.

Monocrotaline pyrrole (MCTP) causes lung injury, pulmonary hypertension, and right ventricular hypertrophy in rats. To determine if platelets are involved in the cardiopulmonary effects of MCTP, the response to MCTP was determined in thrombocytopenic rats. Blood platelet count was reduced to 10-20% of normal for 48 h by ip administration of an antirat platelet serum (PAS) prepared in the goat. Rats were treated iv with either MCTP 5 mg/kg or dimethylformamide vehicle and with either PAS or preimmune serum. Fourteen days after MCTP, right ventricular hypertrophy and several indexes of lung injury were measured. MCTP treatment produced right ventricular hypertrophy, increased lactate dehydrogenase activity and protein concentration in bronchopulmonary lavage fluid, increased perfusion pressure in isolated lungs, and decreased pulmonary clearance and metabolism of perfused 5-hydroxytryptamine. Thrombocytopenia did not influence the changes in these indexes of lung injury produced by MCTP in this protocol. When PAS was given 12 h before MCTP, it did not affect right ventricular hypertrophy, but when PAS treatment was begun 3 or 6 days after MCTP, right ventricular hypertrophy was decreased by 19 or 41%, respectively. These results suggest that platelets help to mediate the development of pulmonary hypertension and the hypertrophic response of the right heart following MCTP administration.

Monocrotaline pyrrole-induced pulmonary hypertension in fawn-hooded rats with platelet storage pool deficiency: 5-hydroxytryptamine uptake by isolated, perfused lungs.

Platelets are believed to be involved in the development of monocrotaline pyrrole (MCTP)-induced pulmonary hypertension. To help identify the role of the platelet, the cardiopulmonary toxicity of MCTP was examined in fawn-hooded (FH) rats, a strain with a platelet function defect. Both Sprague-Dawley (SD) and FH rats developed right ventricular hypertrophy and increased lung weights and exhibited decreased biogenic amine removal by isolated, perfused lung preparations after MCTP treatment. The responses of the FH rats were not significantly different from those of the SD rats, suggesting that platelet uptake and release of 5-hydroxytryptamine (5HT) are not the platelet functions involved in MCTP-induced pulmonary hypertension. The FH rats had an interesting strain-related difference from SD rats; isolated lungs from FH rats removed and metabolized a greater proportion of perfused 5HT than the SD rats.

Aggregation of platelets from monocrotaline pyrrole-treated rats.

Aggregation responses of platelet-rich plasma (PRP) from monocrotaline pyrrole (MCTP)-treated rats were examined to help elucidate the role of platelets in MCTP-induced pulmonary hypertension. PRP from rats treated one or four days earlier with MCTP (5 mg/kg, i.v.) or vehicle exhibited the same slope and maximum aggregation to ADP (1 X 10(-5) M, 2 X 10(-6) M, 1 X 10(-6) M), 74 micrograms/ml dog collagen, or 500 micrograms/ml arachidonic acid. PRP collected 7 days after MCTP treatment had a 15% decrease in both slope and maximum aggregation to 1 X 10(-5) M ADP and a 25% decrease in response to 2 X 10(-6) M ADP relative to the control group. Fourteen days after MCTP treatment, the responses to all of the aggregating agents were decreased 18-71% from control values. These results indicate that MCTP treatment alters the responses of PRP to aggregating agents.

Pulmonary hypertension and ECG changes from monocrotaline pyrrole in the rat.

Chemically synthesized monocrotaline pyrrole (MCTP) was administered to adult male rats at a dose of 5 mg/kg in the tail vein. Controls received an equivalent volume of dimethylformamide vehicle. Rats were killed at 3, 5, 7, 10, and 14 days after treatment. Bronchopulmonary lavage fluid lactate dehydrogenase activity and lung weight were significantly elevated at 4 and 7 days, respectively, after MCTP, indicating that pulmonary damage had occurred. White blood cell count was elevated 7 days after treatment. Mean pulmonary arterial pressure was also first elevated in treated (22 +/- 3 mmHg) compared with control (16 +/- 1 mmHg) animals 7 days after treatment. Right ventricle-to-left ventricle plus septum weight ratios were significantly increased in treated (0.429 +/- 0.015) vs. control (0.320 +/- 0.015) animals 14 days after treatment. Development of right heart enlargement correlated with a shift in the QRS complex mean electrical axis in the frontal plane of the electrocardiogram. These results indicate that MCTP produces effects similar to that caused by monocrotaline, that pulmonary arterial pressure increases from control levels between 5 and 7 days after treatment, and that measurement of mean electrical axis of the electrocardiogram may be a useful, noninvasive method to monitor MCTP-induced cardiac changes in vivo.

Effects of monocrotaline and monocrotaline pyrrole on 5-hydroxytryptamine and paraquat uptake by lung slices.

Treatment of rats with monocrotaline (MCT) or its reactive pyrrole metabolite, dehydromonocrotaline (MCTP), injures pulmonary capillary endothelium and other lung cell types. The mechanism by which pulmonary damage occurs is unknown. To investigate the possibility that the metabolites of MCT can injure lung cells directly, slices of lung from male Sprague-Dawley rats were incubated with chemically synthesized MCTP (0.1-1.0 mM). The ability of the slices to accumulate 5-hydroxytryptamine (5HT) was then examined. After 5 hours exposure to MCTP in vitro, there was no change in either 5HT or PQ uptake at any of the MCTP concentrations tested. Similar results were obtained when lung slices were incubated with liver slices and MCT to generate MCT metabolites in vitro. In contrast, treatment in vivo with MCTP resulted in a 35% decrease in the uptake of 5HT and a 15% decrease in the uptake of PQ by lung slices 14 days after treatment. These results suggest that damage does not occur by direct interaction of MCTP with the lung tissue or that functional injury is slow to develop. Alternatively, the damage in vivo may be mediated by factors not present in the sliced tissue.

Pneumotoxicity and thrombocytopenia after single injection of monocrotaline.

Adult Sprague-Dawley rats were treated once with 105 mg/kg monocrotaline (MCT) subcutaneously or an equivalent volume of isotonic saline and examined 2, 5, 10, and 14 days later. The earliest changes observed were in the platelet count, which was decreased in the MCT animals at 2, 5, and 10 days postinjection. Clearance of perfused 5-hydroxytryptamine, a function of pulmonary vascular endothelium, was unaltered in isolated lungs of treated rats until 5 days after dosing but decreased progressively thereafter in the MCT animals and was 24% less than controls by 14 days. The magnitude of this effect was dose related. Inflow perfusion pressure was elevated in perfused lungs of MCT-treated animals at day 14. Right heart hypertrophy, measured as an increase in the ratio of right ventricle to left ventricle plus septum weights, was not evident until 14 days after treatment. A larger dose of MCT (130 mg/kg) resulted in significant mortality, whereas a lower dose (60 mg/kg) did not result in right ventricular hypertrophy 2 wk after treatment. The treatment regimen described has advantages over administration of MCT by ingestion and may prove suitable for investigations of the mechanism by which MCT results in pulmonary hypertension.