Nernst-Planck-Gaussian finite element modelling of Ca(2+) electrodiffusion in amphibian striated muscle transverse tubule-sarcoplasmic reticular triadic junctional domains.

INTRODUCTION: Intracellular Ca(2+) signalling regulates membrane permeabilities, enzyme activity, and gene transcription amongst other functions. Large transmembrane Ca(2+) electrochemical gradients and low diffusibility between cell compartments potentially generate short-lived, localised, high-[Ca(2+)] microdomains. The highest concentration domains likely form between closely apposed membranes, as at amphibian skeletal muscle transverse tubule-sarcoplasmic reticular (T-SR, triad) junctions. MATERIALS AND METHODS: Finite element computational analysis characterised the formation and steady state and kinetic properties of the Ca(2+) microdomains using established empirical physiological and anatomical values. It progressively incorporated Fick diffusion and Nernst-Planck electrodiffusion gradients, K(+), Cl(-), and Donnan protein, and calmodulin (CaM)-mediated Ca(2+) buffering. It solved for temporal-spatial patterns of free and buffered Ca(2+), Gaussian charge differences, and membrane potential changes, following Ca(2+) release into the T-SR junction. RESULTS: Computational runs using established low and high Ca(2+) diffusibility (D (Ca2+)) limits both showed that voltages arising from intracytosolic total [Ca(2+)] gradients and the counterions little affected microdomain formation, although elevated D (Ca2+) reduced attained [Ca(2+)] and facilitated its kinetics. Contrastingly, adopting known cytosolic CaM concentrations and CaM-Ca(2+) affinities markedly increased steady-state free ([Ca(2+)](free)) and total ([Ca(2+)]), albeit slowing microdomain formation, all to extents reduced by high D (Ca2+). However, both low and high D (Ca2+) yielded predictions of similar, physiologically effective, [Ca(2+)-CaM]. This Ca(2+) trapping by the relatively immobile CaM particularly increased [Ca(2+)] at the junction centre. [Ca(2+)](free), [Ca(2+)-CaM], [Ca(2+)], and microdomain kinetics all depended on both CaM-Ca(2+) affinity and D (Ca2+.) These changes accompanied only small Gaussian (∼6 mV) and surface charge (∼1 mV) effects on tubular transmembrane potential at either D (Ca2+). CONCLUSION: These physical predictions of T-SR Ca(2+) microdomain formation and properties are compatible with the microdomain roles in Ca(2+) and Ca(2+)-CaM-mediated signalling but limited the effects on tubular transmembrane potentials. CaM emerges as a potential major regulator of both the kinetics and the extent of microdomain formation. These possible cellular Ca(2+) signalling roles are discussed in relation to possible feedback modulation processes sensitive to the μM domain but not nM bulk cytosolic, [Ca(2+)](free), and [Ca(2+)-CaM], including ryanodine receptor-mediated SR Ca(2+) release; Na(+), K(+), and Cl(-) channel-mediated membrane excitation and stabilisation; and Na(+)/Ca(2+) exchange transport.