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

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Abstract
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%).