APPS November 2002 Meeting Abstract 1149

Lymphatic vasomotion arises through Ca²⁺ release from intracellular IP₃ receptor-operated stores, the chemical to electrical transformation occurring through opening of excitatory chloride channels. The resultant potentials, referred to as spontaneous transient depolarizing potentials (STDs) synchronise and when of sufficient magnitude, subserve as pacemaker potentials to generate action potentials and resultant vasomotion¹. However the mechanisms underlying synchronisation of STDs are not known. Mesenteric lymphatics were isolated from young guinea-pigs killed by an overdose of the inhalation anaesthetic halothane followed by decapitation. Intracellular microelectrode recordings were made from the smooth muscle of short vessel segments (length <300 µm, diameter <150 µm). Ca²⁺ imaging experiments were made on lymphatic chambers mounted on a myograph with the smooth muscle loaded using the Ca²⁺ indicator Oregon green/AM (5 µM) and viewed with a confocal microscope. Tissues in control conditions could be either quiescent or exhibit vasomotion. Electrophysiological recording and Ca²⁺ imaging of quiescent tissues revealed asynchronous STDs or asynchronous Ca²⁺ oscillations, each occurring in single smooth muscle fibres and at times travelling as waves along the cell. Application of endothelin-1 (ET-1) at 1 nM caused generation of pacemaker potentials, action potentials and vasomotion. Imaging revealed that ET-1 enhanced asynchronous Ca²⁺ oscillations across the smooth muscle cells this activity then tending to synchronise leading to vasomotion, a behaviour paralleling that observed in spontaneously active tissues. Ca²⁺ free solution, thapsigargin and 2-aminoethoxyphenyl borate abolished both spontaneous and ET-1-induced vasomotion through inhibition of pacemaker potentials and underlying STDs. In contrast, nifedipine (1 µM) inhibited synchronous Ca²⁺ release and pacemaker potentials/vasomotion but did not block the underlying STDs. This inhibition could be reversed by further increasing [ET-1]. However, very high [ET-1] (e.g. ≥100 nM) produced asynchronous Ca²⁺ oscillations and vasospasm.