1. The effect of oral magnesium aspartate hydrochloride on monocrotaline (MCT)-induced pulmonary arterial hypertension was evaluated in rats.

2. A single subcutaneous injection of MCT, a pyrrolizidine alkaloid of plant origin, induces significant morphological changes in pulmonary vessels, pulmonary arterial hypertension and right ventricular hypertrophy in rats by 3 weeks.

3. Two groups of rats (Mg2+ control and Mg2+ + MCT) were started on oral Mg2+ (15.4 g/l magnesium aspartate hydrochloride dissolved in deionized water) 2 weeks before the MCT injection. The rest were given deionized water. At the start of the experiment, the control groups (deionized water and Mg2+) were given normal saline subcutaneously; the other groups (deionized water and Mg2+) were given MCT (60 mg/kg) subcutaneously.

4. Pulmonary artery pressure, right ventricular hypertrophy, lung pathology, organ weights and serum electrolytes were assessed 3 weeks after a single subcutaneous injection of MCT. Seventy-five per cent of the rats treated with MCT and oral Mg2+ (12 out of 16) showed significant reduction in pulmonary arterial hypertension, arterial pathology and right ventricular hypertrophy.

5. Our data indicate that Mg2+ attenuates experimentally induced pulmonary hypertension, possibly either by modulating the intracellular Ca2+ level and/or by directly affecting the pulmonary endothelial cell–smooth muscle cell complex involved in metabolism and maintenance of pulmonary vascular resistance.

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