Whole body and regional hemodynamic effects of minoxidil in the conscious dog

Microsphere estimates of whole body hemodynamics and tissue blood flow were made in conscious and pentobarbital-anesthetized dogs treated orally with the peripheral vasodilator minoxidil. Under both circumstances, 1.0-30 mg/kg minoxidil significantly reduced mean arterial pressure 21-41% and total peripheral resistance 52-75% 4 h after administration. Dose-dependent increases in heart rate and cardiac output were evident under conscious conditions, with both parameters approximately doubling at 1.0 mg/kg. The near maximal vasodilation achieved with this dose of minoxidil was due to diminished vascular resistance in all major tissue beds. The enhanced cardiac output was associated with significant 50-87% increases in blood flow to the skin, skeletal muscle, bone, stomach, large intestine, and pancreas. Far more dramatic six- to 10-fold increases in regional myocardial blood flow were seen at this dose, which appeared to be only partially dependent on increased cardiac work. Comparable blood flow patterns were seen with acute minoxidil at doses greater than 1.0 mg/kg, and with chronic minoxidil at 1.0 and 30 mg/kg/day. These experiments establish minoxidil's relative vasodilation in the major tissue beds of the dog which contributes to its hypotensive activity.