Effect of longstanding venous stasis and hypoproteinaemia on lymph flow in the rat tail

Abstract

This study was performed to provide information on the determinants of lymph flow by comparing the effect of venous stasis and hypoproteinaemia in the rat tail. This low‐compliant tissue was chosen in an attempt to induce preferential changes in interstitial pressure or volume. The removal rate (k_AIb) of ¹²⁵1‐labelled human serum albumin (I‐HSA) injected subcutaneously was monitored with external gamma‐counting equipment and used as a measure of lymph flow. Interstitial fluid hydrostatic pressure (P_i) was measured with wick‐in‐needle technique, and interstitial fluid was collected post mortem by dry wicks. Colloid osmotic pressure of plasma (COP_p) and wick fluid (COP_i) was measured with a colloid osmometer. In a separate group of experiments, ⁵¹Cr‐EDTA and [¹²⁵I]HSA were used to measure the interstitial fluid volume. Venous stasis, induced by bilateral ligation of the external tail veins, increased interstitial fluid hydrostatic pressure from 1.7 to 16 mmHg and A/b from 0.030 to 0.063 h^‐1, whereas tail circumference was nearly constant. Interstitial volume averaged 1.17 ml/g dry weight in control animals and 1.27 ml/g during increased venous pressure. Daily injections of aminonucleoside in salt‐loaded rats (0.3yo NaCl as drinking water) reduced colloid osmotic pressure of plasma from 19.1 to 8.5 mmHg and of wick fluid from 11.2 to 2.9 mmHg, while interstitial fluid hydrostatic pressure increased to 5.2 mmHg. The removal rate of ¹²⁵ I‐labelled human serum albumin increased to 0.113 h^‐1, compared to 0.051 h^‐1 in salt‐loaded controls. The interstitial volume showed a marked increase in salt‐loaded hypoproteinaemic rats, 1.75 ml/g dry weight, compared to 1.30 ml/g in salt‐loaded controls. The results indicate that absolute interstitial fluid pressure is not the main determinant of lymph flow and rather support the hypothesis that an increase of interstitial volume promotes lymph formation by pulling open the initial lymphatics through their anchoring filaments.