Cardiocirculatory responses to AII and AVP in conscious rats

Cardiac and peripheral circulatory responses to changes in afterload with angiotensin II (AII) and vasopressin (AVP) were investigated in ganglion-blocked (hexamethonium) conscious rats. Cardiac output (CO) was measured by thermodilution. Both hormones were infused at a dose adjusted to increase mean arterial pressure 70% above baseline. AVP (11.4 +/- 2.2 ng/kg/min, n = 6) decreased CO from 43.4 +/- 2.3 to 34.1 +/- 2.9 ml/min/100 g (p less than 0.001), whereas AII (33.4 +/- 7.4 ng/kg/min, n = 7) increased CO from 38.7 +/- 2.6 to 44.9 +/- 3.4 ml/min/100 g (p less than 0.01). Heart rate did not change with the increase in afterload with either vasoconstrictor. To study whether the different effects of AII and AVP on CO may be explained by their different actions on the venous system, changes in venous tone were evaluated by measuring mean circulatory filling pressure (MCFP) and determining the pressure gradient for venous return (PGVR). AVP changed neither MCFP nor PGVR, whereas AII increased both these parameters, 20.7 +/- 2.8% (p less than 0.01) and 20.3 +/- 6.4% (p less than 0.01), respectively, above control. We also examined the effects of AII and AVP on ventricular dynamics: left ventricular systolic pressure and left ventricular dP/dtmax increased as aortic pressure was increased in a similar manner with both vasoconstrictors. However, AVP induced a greater increase in left ventricular end diastolic pressure than AII. Our results indicate that AII induces an increase in preload by its effect on venous tone, which is adequate to increase cardiac output. The decrease in cardiac output induced by increasing afterload with AVP can be explained by two mechanisms: an inadequate venous return and a failure of the left ventricle to overcome the increased afterload.