Pacemaking through Ca2+ Stores Interacting as Coupled Oscillators via Membrane Depolarization

This study presents an investigation of pacemaker mechanisms underlying lymphatic vasomotion. We tested the hypothesis that active inositol 1,4,5-trisphosphate receptor (IP3R)-operated Ca2+ stores interact as coupled oscillators to produce near-synchronous Ca2+ release events and associated pacemaker potentials, this driving action potentials and constrictions of lymphatic smooth muscle. Application of endothelin 1 (ET-1), an agonist known to enhance synthesis of IP3, to quiescent lymphatic smooth muscle syncytia first enhanced spontaneous Ca2+ transients and/or intracellular Ca2+ waves. Larger near-synchronous Ca2+ transients then occurred leading to global synchronous Ca2+ transients associated with action potentials and resultant vasomotion. In contrast, blockade of L-type Ca2+ channels with nifedipine prevented ET-1 from inducing near-synchronous Ca2+ transients and resultant action potentials, leaving only asynchronous Ca2+ transients and local Ca2+ waves. These data were well simulated by a model of lymphatic smooth muscle with: 1), oscillatory Ca2+ release from IP3R-operated Ca2+ stores, which causes depolarization; 2), L-type Ca2+ channels; and 3), gap junctions between cells. Stimulation of the stores caused global pacemaker activity through coupled oscillator-based entrainment of the stores. Membrane potential changes and positive feedback by L-type Ca2+ channels to produce more store activity were fundamental to this process providing long-range electrochemical coupling between the Ca2+ store oscillators. We conclude that lymphatic pacemaking is mediated by coupled oscillator-based interactions between active Ca2+ stores. These are weakly coupled by inter- and intracellular diffusion of store activators and strongly coupled by membrane potential. Ca2+ store-based pacemaking is predicted for cellular systems where: 1), oscillatory Ca2+ release induces depolarization; 2), membrane depolarization provides positive feedback to induce further store Ca2+ release; and 3), cells are interconnected. These conditions are met in a surprisingly large number of cellular systems including gastrointestinal, lymphatic, urethral, and vascular tissues, and in heart pacemaker cells.