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 M) 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 M). 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 M nifedipine abolished spontaneous events while tetrodotoxin (TTX; 0.5-1 M) 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 M ؊1 mM) suggesting that I Cl(Ca) plays a significant role in the generation of spontaneous activity in this tissue. Penitrem-A (0.1 M) 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 M) 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.

Lymphatic Vessel Network Structure and Physiology

“…Moreover, pharmacological agents that increase CF through the IP 3 -Ca 2+ pathway also increase the frequency and amplitude of STDs (347, 462, 1104, 1109), while those that decrease CF lower the STD frequency and amplitude (180, 181)}(1109, 1115). CaCI channels have also been shown to have an important role in lymphatic pacemaking (75, 1074, 1112, 1116). Activation of CaCl channels by elevated [Ca 2+ ] i following IP 3 -mediated release from internal stores appears to contribute to STDs (1112).…”

Identification of Interstitial Cajal-Like Cells in the Human Thoracic Duct

“…It is known from studies of animal lymphatic vessels that electrical depolarization precedes contraction [van Helden, 1993;Beckett et al, 2007]. In order for depolarization to synchronize the contraction of multiple SMCs, these cells must be connected to each other via gap junctions to form an electrical syncytium.…”

“…The cellular mechanism responsible for the spontaneous and autoregulatory activity of the lymphatic vasculature has been debated since the very first experimental observations of this activity in animals. Modern studies of the spontaneous and evoked lymph pump activity have speculated as to the presence of pacemaker cells [McCloskey et al, 2002;Beckett et al, 2007;Imtiaz et al, 2007] similar to interstitial cells of Cajal (ICC), a subpopulation of which are critical for coordinating peristalsis in the gastrointestinal tract [Sanders et al, 2006]. Interstitial Cajallike cells (ICLCs), or telocytes as they are also known, are morphologically distinct from smooth muscle cells (SMCs) and while they can be identified by expression of proteins specific to them (such as the receptor tyrosine kinase c-kit), ultrastructural assessment by transmission electron microscopy is the unequivocal method for identifying and distinguishing ICLCs from other interstitial cells, such as fibroblasts [Faussone-Pellegrini and Popescu, 2011].…”

Diminished hERG K+ channel activity facilitates strong human labour contractions but is dysregulated in obese women

“…Many smooth muscle tissues display plateau-like electrical activity, which dictates the amplitude and duration of contraction [22][23][24][25] , and this includes the circular muscle layer of the mouse myometrium 24 . While the level of the plateau in many cases is close to that observed in human myometrium, À 25 to À 30 mV, a striking feature of the AP plateau in human myometrium is the rapidity of repolarization.…”