1. Ten healthy volunteers received intravenous infusions of arginine vasopressin (AVP) at 0.1 m-unit min−1 kg−1 and 5% d-glucose on separate days. AVP caused a small fall in forearm blood flow and small rises in mean arterial pressure and systemic vascular resistance. Cardiac output was unaffected.

2. When subjects were tilted to 50° the fall in forearm blood flow was much greater, mean fall being 44.8% with AVP compared with 18.2% with d-glucose. Cardiac output also fell significantly more with AVP, and diastolic pressure, mean arterial pressure and systemic vascular resistance rose significantly more on tilting during AVP infusion than with d-glucose.

3. Six of the same volunteers were given sequential infusions of ‘low dose’ (0.0125 m-unit min−1 kg−1) and ‘high dose’ (0.3 m-unit min−1 kg−1) AVP on a third occasion. Tilting still produced a mean fall in forearm blood flow of 41.2% during low dose infusion, despite a mean plasma AVP level of only 1.9 pg/ml, which is well within the physiological range. When the AVP concentration was increased 24-fold to the high dose, forearm blood flow fell only a further 8.8%. The low dose infusion was also associated with a marked fall in cardiac output on tilting and a rise in systemic vascular resistance.

4. We conclude that AVP has profound haemodynamic effects in man at physiological concentrations. Although these effects are modest in the supine position, they become marked on tilting, suggesting a possible role for AVP in the postural control of blood pressure.

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