The effect of long-term intraarterial norepinephrine infusion on transcapillary fluid and protein transport.

The effect of long-term (4 hr) intraarterial norepinephrine (NOR) infusion on vascular resistance (R), transcapillary fluid movement (Jv), and protein transport (Js) was determined in the isolated hindlimb of the dog. The limb was isolated by an occluding tourniquet and perfused at constant pressure with blood from the opposite femoral artery. Tissue volume changes were detected with a Whitney (1953) mercury-in-rubber strain gauge and protein transport assessed by direct monitoring of the rate of accumulation of 125I-albumin in the tissue. Periodically venous pressure (Pv) was elevated 20 mm Hg in order to estimate the capillary filtration coefficient (Kf) and to determine whether or not prolonged NOR exposure modified the microcirculatory response to increased pressure. Infusing NOR at a delivered dose ranging from 0.026 to 0.13 micrograms/ml significantly elevated R for the entire infusion period. Tissue volume and radioactivity declined transiently and then returned to the control level within 15 min and remained at this level for the remainder of the infusion period. These changes were attributed to blood volume shifts. Elevating Pv during NOR infusion further increased R to an extent which exceeded that during control. Jv and Js were elevated to a greater extent than during control with the delta Js exceeding delta Jv. The changes in Jv are probably due to an increase in microvascular surface area as reflected in an elevated Kf. The greater increase in Js is due in part to an increase in convective transport across a greater microvascular surface area. However, since the ratio of Js/Jv increased with Pv elevation following NOR, the increased Js may represent a change in microvascular permeability produced by the increased Pv. It is also possible that due to myogenic activity or a venous-arteriolar reflex, the microvascular pressure profile was shifted toward the venous side where the reflection coefficient is lower.