Arginine vasopressin increases apparent whole body capacity in anesthetized cats

The effects of arginine vasopressin (AVP) on capacitance function were assessed in anesthetized cats by draining blood from the superior and inferior venae cavae into an external reservoir and then returning blood to the right atrium at a constant rate. Under these conditions, changes in reservoir volume were assumed to reflect reciprocal changes in whole body capacity. Intravenous infusions of AVP (1, 10, and 100 ng.kg-1.min-1) that elevated plasma AVP concentrations by approximately 30, 400, and 4,000 fmol/ml were associated with concentration-dependent increases in whole body capacity that ranged between 1.6 and 8 ml/kg. In contrast to AVP, intravenous infusions of angiotensin II (5, 10, and 50 ng.kg-1.min-1) had relatively little influence on capacity, whereas norepinephrine administration (300, 1,000, and 3,000 ng.kg-1.min-1) was associated with dose-dependent decreases in capacity of 4.5-10.3 ml/kg. Virtually the entire increase in whole body capacity to AVP can be accounted for by summation of the predicted contributions of an active reflex venodilatatory component and those of a passive component (arterial and cardiopulmonary compartments). Because systemic compliance (delta reservoir volume/delta venous pressure) was not changed by AVP administration, the contribution of a reflex venodilatatory component must be the result of increases in unstressed vascular volume (contained volume at 0 transmural pressure) as opposed to changes in compliance. These results may explain why AVP decreases cardiac output to a greater extent than either angiotensin II or sympathomimetics and, thus, why AVP is a weaker pressor agent in animals with intact autonomic function.