MYOGENIC RESPONSES OF ISOLATED LYMPHATICS - MODULATION BY ENDOTHELIUM

Objective: To characterize the effect of increases of intraluminal pre ssure on the spontaneous diameter oscillations of isolated lymphatics and on the maximum and minimum diameter, in the presence and absence o f endothelium. Methods: Afferent lymphatics were isolated from rat ili ac lymph nodes. After cannulation of both ends, lymphatics were equili brated for 60 minutes at a pressure of 6 cm H2O, and then changes in d iameter oscillations to increases in perfusion pressure (from 2-10 cm H2O) cc ere measured. The endothelium was removed by intraluminal infu sion of air. Changes in diameter were expressed as a percentage of the corresponding passive diameter obtained in Ca2+-free solution.Results : Isolated lymphatics showed phasic spontaneous constriction and dilat ion (similar to 50 to similar to 150 mu m). Removal of extracellular C a2+ abolished spontaneous diameter oscillations-and the passive diamet er increased from 183.0 +/- 5.3 to 205.1 +/- 8.8 mu m as pressure incr eased from 2 to 10 cm H2O. In response to increases in pressure, the n ormalized minimum diameter (Dmin) increased (from 30.1% +/- 1.1% to 54 .8% +/- 1.9%) while the normalized maximum diameter (Dmar) of lymphati cs did not change. With increasing pressure tile amplitude of vasomoti on (Dmax-Dmin) decreased, whereas the oscillation frequency increased. Removal of endothelium significantly reduced Dmax; hence, tile osa ci llation amplitude decreased while the frequency increased. Also, endot helium removal elicited a significant change in the slope of the press ure-Dmax curve (from 0.01 +/- 0.4 to -1.43 +/- 0.34). Conclusions: Inc reases in intraluminal pressure greatly affect the spontaneous vasomot ion of lymphatics and activate the myogenic mechanism intrinsic to the smooth muscle. In addition, endothelial factors are important to main tain adequate lymphatic vasomotion. These findings suggest that intral uminal pressure and endothelial factors can be important contributors to the tone and pumping activity of lymphatics in vivo.