Examination of minoxidil-induced acute cardiotoxicity in miniature swine.

Minoxidil, a vasodilating antihypertensive agent, was given orally in doses of 1, 3 or 10 mg/kg to miniature swine on 2 consecutive days. Mean arterial pressure decreased and heart rate increased most consistently after the 10 mg/kg dose. However, all 3 doses of minoxidil induced myocardial hemorrhages and/or left ventricular papillary muscle necrosis within 24 h after the second dose. Necrosis, characterized by hypercontraction of muscle cells and myofibrillar damage, occurred in 1 of 8 pigs given 1 mg/kg, 3 of 13 given 3 mg/kg and 7 of 14 given 10 mg/kg of minoxidil. The pharmacological effects of minoxidil, hypotension and reflex tachycardia, probably led to ischemia and necrosis in left ventricular papillary muscles. Gross hemorrhages involving the left atrium and to a lesser extent the left ventricle were found in 4 of 8 pigs given 1 mg/kg, 9 of 13 given 3 mg/kg and 11 of 14 given 10 mg/kg of minoxidil. The atrial lesions were manifested grossly by diffuse redness and microscopically by interstitial edema, extravasation of erythrocytes and infiltration of areas around small arteries and arterioles with acute and chronic inflammatory cells. The hemmorhagic areas were concentrated along the epicardial surfaces, and to a lesser extent along the endocardial surfaces. Atrial lesions induced by minoxidil preferentially involve the left atrium in pigs and the right atrium in dogs. These differences may be related to the anatomic patterns of coronary circulation in the 2 species.