Effect of outflow pressure on lymphatic pumping in vitro

The objective of this study was to determine how lymphatic vessels responded to outflow pressure changes in vitro. Bovine mesenteric lymphatics were suspended in an organ bath preparation with both inflow and outflow ends cannulated. Input to the duct was provided from a reservoir filled with Krebs solution. To initiate pumping, a transmural pressure was applied to the ducts by elevating the fluid reservoir and outflow catheters and making their heights equal to one another. The outflow catheter was then elevated above the liquid in the reservoir in 2-cmH2O increments, and pumping activity was monitored for 10 min at each outflow pressure. Outflow pressures were calculated as the product of the flow rate and outflow cannula resistance plus the height of the tip of the outflow catheter above the liquid in the organ bath. At low transmural pressures (2-4 cmH2O), elevations in outflow pressure often had little effect on flow rates until high outflow pressures had been attained. In contrast, elevations in outflow pressures resulted in an increasingly rapid decline in flow rates as transmural pressures were incrementally increased. The mean power (in mu W) required to produce the observed flow rate was estimated at each outflow pressure as the product of the flow rate and the pressure across the lymphatic vessel. The ability of the lymphatics to generate sustained or enhanced power output in response to an increasing outflow pressure challenge was most pronounced at lower transmural pressures. As transmural pressures were increased, the range of outflow pressures that stimulated increased power production was diminished. We conclude that elevations in outflow pressure in an in vitro preparation result in a nonlinear decline in flow rates. This nonlinearity is due to an active lymphatic pump mechanism.