Length-Dependence of Lymphatic Phasic Contractile Activity Under Isometric and Isobaric Conditions

Objective: Experiments were designed to determine if the effect of preload on the spontaneous contractile activity of rat mesenteric lymphatics would be different under isometric and isobaric conditions. Methods: Isobaric protocols on cannulated, pressurized rat mesenteric lymphatics (∼ 120 μ m, ID) measured the effects of lumenal pressure on contraction amplitude, frequency, and rate of diameter change (dD/dt). Analogous protocols were conducted using a wire myograph to determine the effects of passive force (preload) on the characteristics of spontaneous, isometric force transients. Servo-control systems allowed responses to ramp or step increases in preload/pressure to be tested. Results: Under isobaric conditions, contraction amplitude was maximal at the lowest pressure tested (1 cm H2O), and progressively declined with pressure elevation to 10 cm H2O. Changes in dD/dt followed a similar pattern. In contrast, contraction frequency progressively increased with pressure. Under isometric conditions, the amplitude of spontaneous force transients was maximal at preloads of 0.3-0.4 mN, with dF/dt following a similar pattern; the average peak amplitude was ∼ 15% of the maximal active contractile force developed during agonist stimulation. The frequency of the transients increased with preload from 0.05 to 0.3 mN, and remained constant at higher preloads. The amplitude-frequency product, an estimate of lymphatic pumping ability, reached a maximum value at 0.4 mN in isometric vessels and at 6 cm H2O in isobaric vessels. Conclusions: Isometric and isobaric methods yielded qualitatively similar indices of spontaneous contractile activity. However, the ranges of amplitude and frequency changes were much greater under isobaric conditions (3- to 5-fold) than under isometric conditions (50-80%).