Anisotropy and temperature dependence of myoglobin translational diffusion in myocardium: Implication for oxygen transport and cellular architecture

Pulsed field gradient NMR methods have determined the temperature-dependent diffusion of myoglobin (Mb) in perfused rat myocardium. Mb diffuses with an averaged translational diffusion coefficient (D_Mb) of 4.24-8.37 × 10^-7 cm²/s from 22°C to 40°C and shows no orientation preference over a root mean-square displacement of 2.5-3.5 μm. The D_Mb agrees with the value predicted by rotational diffusion measurements. Based on the D_Mb, the equipoise diffusion PO ₂, the PO₂ in which Mb-facilitated and free O₂ diffusion contribute equally to the O₂ flux, varies from 2.72 to 0.15 in myocardium and from 7.27 to 4.24 mmHg in skeletal muscle. Given the basal PO₂ of ∼10 mmHg, the Mb contribution to O₂ transport appears insignificant in myocardium. In skeletal muscle, Mb-facilitated diffusion begins to contribute significantly only when the PO₂ approaches the P50. In marine mammals, the high Mb concentration confers a predominant role for Mb in intracellular O₂ transport under all physiological conditions. The Q₁₀ of the D_Mb ranges from 1.3 to 1.6. The Mb diffusion data indicate that the postulated gel network in the cell must have a minimum percolation cutoff size exceeding 17.5 Å and does not impose tortuosity within the diffusion root mean-square displacement. Moreover, the similar Q₁₀ for the D_Mb of solution versus cell Mb suggests that any temperature-dependent alteration of the postulated cell matrix does not significantly affect protein mobility.