Phasic contractions of rat mesenteric lymphatics increase basal and phasic nitric oxide generation in vivo.

Multiple investigators have shown interdependence of lymphatic contractions on nitric oxide (NO) activity by pharmacological and traumatic suppression of endothelial NO synthase (eNOS). We demonstrated that lymphatic diastolic relaxation is particularly sensitive to NO from the lymphatic endothelium. The predicted mechanism is shear forces produced by the lymph flow during phasic pumping, activating eNOS in the lymphatic endothelium to produce NO. We measured [NO] during phasic contractions using microelectrodes on in situ mesenteric lymphatics in anesthetized rats under basal conditions and with an intravenous saline bolus (0.5 ml/100 g) or infusion (0.5 ml x 100 g(-1) x h(-1)). Under basal conditions, [NO] measured on the tubular portions of the lymphatics was approximately 200-250 nM, slightly higher than in the adjacent adipocyte microvasculature, whereas [NO] measured on the lymphatic bulb surface was approximately 400 nM. Immunohistochemistry of eNOS in isolated lympathics indicated a much greater expression in the lymph valves and surrounding bulb area than in the tubular regions. During phasic lymphatic contractions, the valve and tubular [NO] increased with each contraction, and during intravenous saline infusion, [NO] increased in proportion to the contraction frequency and, presumably, lymph flow. The partial blockade of eNOS over approximately 1 cm length with N(omega)-nitro-L-arginine methyl ester lowered the [NO]. These in vivo data document for the first time that both valvular and tubular lymphatic segments increase NO generation during each phasic contraction and that [NO] summated with increased contraction frequency. The combined data predict regional variations in eNOS and [NO] in the tubular and valve areas, plus the summated NO responses dependent on contraction frequency provide for a complex relaxation mechanism involving NO.