Reduced oxygen consumption induced by vasopressin in dogs depends on systemic administration

1. Arginine vasopressin reduces whole-body oxygen consumption in conscious dogs. To determine whether this decrease could result from limited oxygen delivery, studies were performed in two groups of chronically instrumented dogs.

2. In the first group (n = 7), vasopressin was infused at a rate of 18.5 pmol min⁻¹ kg⁻¹ while the animals were breathing 10% oxygen. Hypoxaemia alone (arterial partial pressure of oxygen 4.67 kPa) decreased whole-body oxygen delivery by 30%. The fall in whole-body oxygen consumption induced by vasopressin during hypoxaemia was not different from that measured under normoxic conditions, even though whole-body oxygen delivery was more reduced.

3. In a second group of seven dogs, hindquarter blood flow (electromagnetic flowmeter on lower abdominal aorta) and oxygen consumption (blood flow multiplied by arteriovenous oxygen difference) were measured as infusions of vasopressin were given either systemically or into the lower abdominal aorta. Systemic vasopressin infusions at 0.92, 4.6 and 18.5 pmol min⁻¹ kg⁻¹ reduced hindquarter blood flow, oxygen delivery and oxygen consumption, but the decreases in blood flow and oxygen delivery were dose-related whereas that in oxygen consumption was not. Intra-arterial infusions of vasopressin that increased venous concentrations as much as or more than systemic infusion of 0.92 pmol of vasopressin min⁻¹ kg⁻¹ had no effect on oxygen consumption, even though the higher intra-arterial rate reduced blood flow and oxygen delivery as much as the systemic infusion. Thus systemic but not locally administered vasopressin reduced hindquarter oxygen consumption.

4. Small to moderate mechanically induced reductions in hindquarter blood flow resulted in a smaller decrease in oxygen consumption than equivalent blood flow reductions induced by vasopressin. On the contrary, large mechanically induced flow reductions resulted in a larger decrease in oxygen consumption than those induced by vasopressin.

5. These results suggest that the decrease in hindquarter oxygen consumption induced by systemic vasopressin administration cannot be primarily a consequence of oxygen delivery limitation.