Radial and longitudinal diffusion of myoglobin in single living heart and skeletal muscle cells

Abstract

We have used a fluorescence recovery after photobleaching (FRAP) technique to measure radial diffusion of myoglobin and other proteins in single skeletal and cardiac muscle cells. We compare the radial diffusivities, D _r (i.e., diffusion perpendicular to the long fiber axis), with longitudinal ones, D _l (i.e., parallel to the long fiber axis), both measured by the same technique, for myoglobin (17 kDa), lactalbumin (14 kDa), and ovalbumin (45 kDa). At 22°C, D _l for myoglobin is 1.2 × 10⁻⁷ cm²/s in soleus fibers and 1.1 × 10⁻⁷ cm²/s in cardiomyocytes. D _l for lactalbumin is similar in both cell types. D _r for myoglobin is 1.2 × 10⁻⁷ cm²/s in soleus fibers and 1.1 × 10⁻⁷ cm²/s in cardiomyocytes and, again, similar for lactalbumin. D _l and D _r for ovalbumin are 0.5 × 10⁻⁷ cm²/s. In the case of myoglobin, both D _l and D _r at 37°C are about 80% higher than at 22°C. We conclude that intracellular diffusivity of myoglobin and other proteins (i) is very low in striated muscle cells, ≈1/10 of the value in dilute protein solution, (ii) is not markedly different in longitudinal and radial direction, and (iii) is identical in heart and skeletal muscle. A Krogh cylinder model calculation holding for steady-state tissue oxygenation predicts that, based on these myoglobin diffusivities, myoglobin-facilitated oxygen diffusion contributes 4% to the overall intracellular oxygen transport of maximally exercising skeletal muscle and less than 2% to that of heart under conditions of high work load.