Total circulatory capacity in the rat. Effects of epinephrine and vasopressin on compliance and unstressed volume.

In thoracic-spinal-cord-transected rats, mean circulatory pressure (Pmc) was measured during brief circulatory arrest by inflating an indwelling balloon in the right atrium and total blood volume (VB) was measured with 51Cr. The Pmc-VB relationship was determined over the range +/- 30% VB by rapid blood infusion or withdrawal. An infusion of 1.5 microgram/kg per min of epinephrine for 20-40 minutes caused the total circulatory compliance (Ctc) to decrease 18%, the extrapolated unstressed circulatory volume (V'o) and VB to decrease 8.6 and 7.3 ml/kg, respectively, and PMC to increase 41% (2.6 mm Hg). Total circulatory capacity (circulatory volume at a PMC of 8 mm Hg) was decreased 13.7 ml/kg by epinephrine at the above infusion rate. When vasopressin (20 ng/kg per min) and epinephrine (0.3 microgram/kg per min) were infused together for 20-40 minutes, compared to a 0.3 microgram/kg per min infusion of epinephrine alone, the net effect of vasopressin was a 4.8 ml per kg decrease in VB and a 5.9 ml/kg decrease in total circulatory capacity, with insignificant decreases in CTC and V'o. The results indicate that in rats both epinephrine and vasopressin act to decrease blood volume via transcapillary fluid shifts, and that vasopressin has a relatively small overall venoconstricting action.