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.