Objective: It was the objective of this study to investigate whether a facilitatory role of vasopressin (AVP) on sympathetic nerve activity can be demonstrated in humans at the peripheral level.
Methods: Eight subjects (32 +/- 2.3 years) participated in this study. Forearm blood flow (FABF) was measured using the venous occlusion plethysmography model. Each session was performed in the presence of a continuous infusion (into the brachial artery) of AVP in sub-pressor dosage of 0.008 ng/kg per min, or NaCl 0.9%. Using lower-body negative pressure (LBNP) (-10, -20 and -30 mmHg) the combined pre- and postsynaptic action of AVP on the sympathetic nervous system was investigated. This was followed by a second protocol in which the possible postsynaptic effects of AVP were evaluated with intra-arterial infused norepinephrine (NE).
Results: The baseline FABF was 5.2 +/- 0.6 ml/100 ml per min. After infusion of AVP (0.008 ng/kg per min), the FABF remained unchanged at a flow of 5.5 +/- 0.6 ml/100 ml per min (P = 0.26). LBNP caused a pressure-dependent decrease in FABF (25.6 +/- 4.4, 29.0 +/- 6.1 and 38.6 +/- 6.9%, for -10, -20 and -30 mmHg, respectively). AVP significantly enhanced the FABF responses to lower-body negative pressures (38.0 +/- 8.6, 49.3 +/- 5.1 and 58.9 +/- 6.3%, respectively (P = 0.014). NE caused a dose-dependent vasoconstriction by 3.1 +/- 4.6, 17.0 +/- 4.3 and 23.2 +/- 4.9%, at dosages of 10, 20 and 40 pg/min, respectively, unaffected by AVP (P = 0.91).
Conclusions: We conclude that AVP can facilitate vasoconstriction mediated by the peripheral sympathetic nervous system at the presynaptic level in humans.