Adrenergetic modulation of the intrinsic myogenic activity of peripheral diaphragmatic lymphatics.

The diaphragmatic lymphatic system drains fluids and solutes from diaphragmatic interstitium and from the pleural and peritoneal cavities. Initial lymphatics, devoid of smooth muscle cells (SMCs) in their walls, join into long linear vessels or complex loops, formed at the confluence of linear vessels. Both linear vessels and loops, visible below the pleural and peritoneal mesothelia over the diaphragmatic dome, may be segmented in functional units, called lymphangions, separated by intraluminal valves, which ensure unidirectional lymph flow. Lymph progression within the diaphragmatic lymphatic vessels is due to an hydraulic pressure gradient of between adjacent lymphatic segments whose generation and maintenance depends upon a system of extrinsic and intrinsic pumps. Extrinsic pumping is mainly due to the movement of the surrounding tissues which causes the contraction/expansion of the vessel, while the intrinsic pumping mechanism is due to the rhythmic contraction of the smooth muscle cells surrounding the wall of the lymphangions. Extrinsic pumping prevails in lymphatics of the medial diaphragm, while lymphatic loops located at the extreme diaphragmatic periphery do require an intrinsic pumping mechanism to propel lymph centripetally. Lymph propulsion within the most peripheral diaphragmatic lymphatics depends upon tissue displacements and contraction of smooth muscle cells that surround the collecting lymphatics. The aim of the present work was to investigate, in actively contracting sites of peripheral diaphragmatic lymphatic vessels, the contribution of single strokes and valves opening/closing dynamics to lymph propulsion, and to analyze how this phenomenon is modulated by epinephrine. Anaesthetized rats received an intraperitoneal injection of a mixture of FITC-conjugated dextrans and TRITC-labeled microspheres (0.1-1 m diameter). After passive lymphatic vessels loading, microspheres movement were video recorded ex-vivo in excised pieces of diaphragm, kept superfused with warmed oxygenated Tyrode’s solution in a flow chamber on the stage of an upright microscope. Instantaneous and mean microsphere velocities were derived from microsphere trajectories along with vessel diameter changes due to spontaneous active strokes. Data obtained show that active strokes exert a distance-dependent effect on microspheres progression from the contracting site. Their velocity profile results parabolic with a peak velocity of about 96 μm/sec. In the presence of intraluminal valves, microspheres show an oscillatory trajectory on the proximal side and monotonic outward directed flow on the distal side of the valve. Epinephrine administration has opposite effects in linear vessels and lymphatic loops: in particular, epinephrine determines an increase in contraction frequency of about 3 bpm and a greater distance traveled by microspheres in loops and an impairment of spontaneous activity in linear vessels.