TRANSPORT IN LYMPHATIC CAPILLARIES .2. MICROSCOPIC VELOCITY-MEASUREMENT WITH FLUORESCENCE PHOTOBLEACHING

Despite its relevance to the physiology of lymph formation and propuls ion, the instantaneous flow velocity in single lymphatic capillaries h as not been measured to date. The method of fluorescence recovery afte r photobleaching (FRAP) was adapted for this purpose and used to chara cterize flow in the lymphatic capillaries in tail skin of anesthetized mice during a constant-pressure intradermal injection of fluorescein isothiocyanate-dextran (mol wt 2 x 10(6)). The median lymph flow veloc ity was 4.7 mu m/s, and the velocity magnitude ranged from 0 to 29 mu m/s. The direction of flow was generally proximal, but stasis and back flow toward the site of injection was also detected. Evidence for osci llatory flow was detected in some FRAP experiments, and in separate ex periments a periodicity of similar to 120 min(-1), directly correlated to respiration frequency, was measured by tracking the motion of fluo rescent latex microspheres (1 mu m diam) introduced into the lymphatic capillary network. The velocity magnitude showed a correlation with d uration of infusion but not with distance from injection site. It is s peculated that the temporal decay of mean velocity magnitude could be related to the relaxation of local pressure gradients as partially col lapsed vessels expand during the infusion.