OBJECTIVE:
To evaluate the age-related changes in pumping of mesenteric lymphatic vessels in 9- and 24-month-old male Fisher-344 rats.

METHODS:
Lymphatic diameters, contraction amplitude, contraction frequency, and fractional pump flow were determined in isolated MLV before and after l-NAME application.

RESULTS:
The data demonstrate a severe weakening of the lymphatic pump in aged MLV including diminished lymphatic contraction amplitude, contraction frequency, and as a result, lymphatic pump activity. The data also suggest that the imposed flow gradient-generated shear-dependent relaxation does not exist in aged rat MLV, and the sensitivity of both adult and aged MLV to such shear cannot be eliminated by nitric oxide (NO) synthases blockade.

CONCLUSIONS:
These data provide new evidence of lymphatic regional heterogeneity for both adult and aged MLV. In MLV, a constant interplay between the tonic and phasic components of the myogenic response and the shear-dependent release of NO predominantly determine the level of contractile activity; the existence of another shear-dependent, but NO-independent regulatory mechanism is probably present. Aging remarkably weakens MLV contractility, which would predispose this lymphatic network to lower total lymph flow in resting conditions and limit the ability to respond to an edemagenic challenge in the elderly.