Resistance and volume changes caused by nitroprusside in the dog

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Abstract
Changes in vascular volume caused by a pharmacologic agent are frequently inferred rather than directly measured. The effects of nitroprusside were investigated in 8 dogs divided into 2 groups: control and splenectomized. The dogs were anesthetized using pentobarbital, and a veno-right atrial bypass preparation whose controlled cardiac output and external reservoir allowed measurement of both changes in vascular resistance and changes in vascular volume was surgically prepared. In both groups, blood pressure (mean .+-. SD) decreased at each successive level of nitroprusside: 114 .+-. 24 mmHg (base line), 101 .+-. 19 mmHg (45 .mu.g/min), 90 .+-. 16 mmHg (90 .mu.g/min), 81 .+-. 17 mmHg (180 .mu.g/min), 68 .+-. 18 mmHg (360 .mu.g/min). Nitroprusside caused a large and similar decrease in vascular resistance in both groups. In the control group, vascular volume increased above base line 5.5 .+-. 2.7, 8.3 .+-. 3.2, 11.6 .+-. 2.9, and 14.7 .+-. 3.5 ml/kg at each successive level of nitroprusside infusion, whereas in the splenectomized group vascular volume increased above base line 0.9 .+-. 0.3, 2.5 .+-. 1.0, 3.3 .+-. 1.1, and 4.0 .+-. 1.3 ml/kg at each successive level of nitroprusside infusion, but increased significantly less than the control group. Nitroprusside apparently decreases vascular resistance and increases vascular volume; the spleen is the major site of changes in vascular volume caused by nitroprusside.

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