Pacemaker potentials in lymphatic smooth muscle of the guinea-pig mesentery

1. Intracellular microelectrode recordings have been made from lymphatic smooth muscle (SM) to investigate pacemaker mechanisms underlying lymphatic pumping. 2. The SM of small lymphangions or that of short segments, cut from lymphangions of any length, behaved similarly. Both preparations exhibited spontaneous transient depolarizations (STDs) and exhibited simplified electrical characteristics approximating those of a spherical cell. 3. STDs were found to underlie activation of action potentials and hence constrictions. 4. The level of STD activity correlated to the pumping activity of lymphangions, the SM from more active chambers exhibiting increased STD activity. 5. Lymphatic SM exhibited STDs with properties similar to the STDs of mesenteric veins. STDs appeared to be of myogenic origin as they were present despite denervation or substantial destruction of the endothelium. 6. Noradrenaline enhanced the size and frequency of STDs. 7. STD activity was abolished by chelation of cytosolic Ca2+. 8. It is proposed that STDs provide a mechanism for pacemaking in the lymphatic SM studied here. Furthermore, it is postulated that STDs are the consequence of Ca(2+)-dependent pulsatile release of an intracellular messenger, probably Ca2+ itself. This mechanism provides a novel means for pacemaking.