Shortening velocities of rat mesenteric lymphatics during spontaneous and agonist‐induced contractions

Determination of the maximum shortening velocity (Vmax) for lymphatic smooth muscle is critical to the analysis of lymphatic contractile properties. Rat mesenteric lymphatics were subjected to isotonic release protocols during stimulated or spontaneous contractions using a modified wire myograph. Maximal activation was evoked using Substance P (SP) and K + . Upon activation at optimal preload, lymphatics exhibited time‐dependent force (F) development, with peak active F = 0.34±0.02 mN and subsequent lower plateau F = 0.21±0.02 mN at ~90 sec. At the peak, Vmax = 0.48±0.04 lengths/sec, whereas Vmax at the plateau declined to 0.26±0.03 length/sec. In cannulated lymphatics under isobaric conditions, shortening velocity (V) was estimated from dD/dt during spontaneous contractions. Paradoxically, V was ~10‐fold higher than in maximally activated vessels. Further, V declined with progressive activation by SP. Using the myograph, releases were then triggered at a predetermined F during the upstroke of spontaneous isometric contractions. Vmax so obtained was in closer agreement with isobaric V. The results suggest that lymphatic activation is associated with a profound slowing of V and are consistent with lymphatic smooth muscle being a biochemical hybrid of smooth and striated muscle that exhibits different mechanical properties in the phasic and tonic components of contraction. Supported by NIH HL‐075199.