Spontaneous electrical activity in sheep mesenteric lymphatics

Abstract

Background: It has recently become apparent that the lymph pump is an electrical entity that rivals the heart in complexity. Many interesting currents have been demonstrated by voltage clamping isolated lymphatic smooth muscle cells, but until now the role of these currents in the intact syncitium has not been studied. Methods and Results: Intracellular microelectrode recordings were made from smooth muscle of sheep mesenteric lymphatics to investigate the electrophysiological basis of lymphatic pumping. Approximately 50% of the vessels exhibited spontaneous electrical activity, varying from regular oscillations in membrane potential to spike complexes. Spike complexes generally consisted of one or more action potentials superimposed on a slower depolarization or ‘plateau’ phase and were often preceded by a slow diastolic depolarization or ‘pre-potential’. Norepinephrine (5 μM) induced depolarizing events in quiescent preparations. Both agonist-induced oscillations and spike complexes were attenuated or completely abolished by 2-aminoethoxydiphenyl borate (2-APB); 10–100 μM). Cesium (1 mM) reduced the frequency of spontaneous firing by approximately 30% by flattening the pre-potential phase. In addition to having a negative inotropic effect, 10 mM Cs+ also caused gradual membrane depolarization and prolonged the plateau. 1 μM nifedipine abolished spontaneous events while tetrodotoxin (TTX; 0.5–1 μM) decreased the amplitude and maximum dV/dt of the spike upstroke or stopped activity completely. Spontaneously active segments of lymphatic vessel were inhibited by the chloride channel blocker, anthracene-9-carboxylic acid (9-AC; 250 μM 1 mM) suggesting that ICl(Ca) plays a significant role in the generation of spontaneous activity in this tissue. Penitrem-A (0.1 μM) did not affect resting membrane potential but increased action potential amplitude and prolonged the plateau, suggesting that calcium-activated potassium current does not make a significant contribution to resting membrane conductance but is important in membrane repolarization following calcium influx during the action potential. In contrast 4-aminopyridine (4-AP; 5 μM) caused significant membrane depolarization, suggesting the existence of an active 4-AP-sensitive current at rest. Conclusions: These results demonstrate that the currents found in isolated voltage-clamped cells from sheep mesenteric lymphatics do play a significant role in the shaping of spontaneous electrical activity of the intact syncitium.