Diffusion profiles of Na+-fluorescein in frog ventricular muscle

Frog ventricular muscle strips were placed in a single sucrose-gap chamber to measure the interdiffusion of solutes across the sucrose-Ringer's solution partition. Steady-state diffusion profiles of fluorescein sodium developed along the axis of the muscle in the physiological node by continuously perfusing the sucrose pool with 210-mM sucrose plus fluorescein (5-10 mM). Fluorescein was found to diffuse freely through the extracellular space of the ventricular muscle without binding to the tissue. The fluorescence of Na+-fluorescein in the muscle (measured at 530 +/- 30 nm) varied linearly with the dye concentration in the sucrose perfusate. The diffusion profiles of dye in the test node depended on the tightness or snaring of the muscle strip by the latex diaphragms, the diameter of the muscle strip, and changes in hydrostatic pressure between the sucrose and Ringer's solution pools. Fluorescein concentration in the cross section of test node closest to the latex partition (sucrose-Ringer's solution interface) ranged between 4-13% of the dye concentration in the sucrose pool. These values are more than five times smaller than those estimated theoretically, assuming free diffusion. The experimental findings indicate that the presence of a physical barrier, such as a rubber diaphragm, limits free interdiffusion of solutes across the sucrose gap. The presence of such a barrier thus prevents large concentration gradients from occurring in the extracellular spaces along the physiological node.