Effects of increased intracardiac and arterial pressure on plasma vasopressin in humans

The effects of loading cardiopulmonary and sinoaortic baroreceptors on plasma arginine vasopressin (AVP) levels were examined in 18 healthy young adults after a mild water restriction. Cardiopulmonary receptors were acutely loaded by volume expansion which increased central venous pressure (CVP) from 4.7 ± 1.9 to 7.3 ± 2.2 mmHg but did not change mean arterial blood pressure (MAP). Both receptor groups were acutely loaded by applications of lower body positive pressure, which induced a comparable increase in CVP and, in addition, raised MAP from 85 ± 6.7 to 93 ± 6.8 mmHg (P < 0.001). There was evidence of reflex activation as forearm blood flow and heart rate increased during volume expansion while the increase in heart rate was blocked during lower body positive pressure. Neither intervention significantly altered the control AVP level of 4.6 ± 1.4 pg/ml. An investigation of more prolonged stimulation of cardiopulmonary receptors was carried out using 1 hr of head-down tilt. Again, despite increases in CVP (5.1 ± 1.3 to 9.0 ± 1.4 mmHg, P < 0.005), AVP (5.7 ± 2.4 pg/ml) did not decrease. MAP did not change during head-down tilt. There were no changes in osmolality throughout any of the studies. AVP levels in humans are therefore not responsive to moderate isosmotic loading of the cardiopulmonary and sinoaortic baroreceptors under conditions of mild water restriction.