Cardiovascular and endocrine responses to head-up tilt and vasopressin infusion in humans.

The effects of passive tilt on arterial and central venous pressures (MAP and CVP), heart rate (HR), arterial concentrations of vasopressin (AVP) and aldosterone (ALDO), osmolality, and hematocrit were studied in eight healthy young men. Tilting was performed at 30 degrees/min to 20 or 40 degrees and maintained for 45 min. The 20 degrees tilt did not change MAP while CVP fell by 5.9 mmHg (P less than 0.01) and HR increased by 5 beats/min (51-56 beats/min, P less than 0.05). AVP and ALDO concentrations were unchanged. At 40 degrees, MAP increased by 5.7 mmHg (86.0-91.7 mmHg, P less than 0.01), CVP decreased by 7.4 mmHg (P less than 0.01), HR rose by 8 beats/min (55-63 beats/min, P less than 0.01), and pulse pressure fell by 6 mmHg (P less than 0.05). ALDO increased fivefold (P less than 0.05), but AVP did not change. Infusions of AVP at 0.25 or 1.0 ng X min-1 X kg body wt-1 elevated plasma AVP to 11 +/- 1 and 52 +/- 8 pg/ml. Only the larger dose caused a small increase in MAP while CVP and HR remained unchanged; however, washout of a subcutaneous depot of Xe was reduced greater than 60% even by the lower dose. It is concluded that small-angle nonhypotensive passive tilt does not affect AVP measurably despite substantial reductions of CVP and a marked increase in ALDO. Elevations of AVP elicit marked subcutaneous vasoconstriction also in the absence of changes in CVP, HR, and MAP.