Finite element analysis predicts Ca 2+ microdomains within tubular-sarcoplasmic reticular junctions of amphibian skeletal muscle
Abstract: A finite element analysis modelled diffusional generation of steady-state Ca2+ microdomains within skeletal muscle transverse (T)-tubular-sarcoplasmic reticular (SR) junctions, sites of ryanodine receptor (RyR)-mediated SR Ca2+ release. It used established quantifications of sarcomere and T-SR anatomy (radial diameter d=220nm; axial distance w=12nm). Its boundary SR Ca2+ influx densities,Jinflux, reflected step impositions of influxes, Φinflux=Jinfluxπd24, deduced from previously measured Ca2+ signals following muscle fibre depolarization. Predicted steady-state T-SR junctional edge [Ca2+], [Ca2+]edge, matched reported corresponding experimental cytosolic [Ca2+] elevations given diffusional boundary effluxΦefflux=D[Ca2+]edgeλ(πdw), established cytosolic Ca2+ diffusion coefficients (D=4×107nm2/s) and exit length λ=9.2nm. Dependences of predicted [Ca2+]edge upon Jinflux then matched those of experimental [Ca2+] upon Ca2+ release through their entire test voltage range. The resulting model consistently predicted elevated steady-state T-SR junctional ~ µM-[Ca2+] elevations radially declining from maxima at the T-SR junction centre along the entire axial T-SR distance. These [Ca2+] heterogeneities persisted through 104- and fivefold, variations in D and w around, and fivefold reductions in d below, control values, and through reported resting muscle cytosolic [Ca2+] values, whilst preserving the flux conservation (Φinflux=Φefflux) condition, Ca2+edge=λdJinflux4Dw. Skeletal muscle thus potentially forms physiologically significant ~ µM-[Ca2+] T-SR microdomains that could regulate cytosolic and membrane signalling molecules including calmodulin and RyR, These findings directly fulfil recent experimental predictions invoking such Ca2+ microdomains in observed regulatory effects upon Na+ channel function, in a mechanism potentially occurring in similar restricted intracellular spaces in other cell types.
Wellcome Trust (105727/Z/14/Z)
British Heart Foundation (PG/14/79/31102)