Ratio of transverse to longitudinal resistivities of isolated cardiac muscle fiber bundles

The ratio of transverse to longitudinal resistivities (рt/р1) of isolated cat cardiac muscle bundles (papillary muscles and ventricular trabeculae) composed of parallel fibers was measured and compared with frog sartorius muscles. Similar measurements made on a physical model representing such bundles of parallel fibers, consisting of layers of glass rods orderly packed with Ringer solution filling the interstices, gave a рt/р1 of 7. This ratio averaged 11 for cardiac muscle and 21 for skeletal muscle, indicating that in both muscles the transverse conductivity and effective diffusion coefficient are much less than the longitudinal values. Equilibration of cardiac muscle in ten-fold diluted Ringer (sucrose to isotonicity) to raise the resistance of the interstitial fluid (ISF) 8.5-fold raised р1 6.3-fold, but hardly affected р1, so that рt/р1 was markedly reduced to 2.0. In sartorius, the high-resistance solution had much less of an effect on р1 or on рt/р1. Equilibration in hypertonic Ringer (2 X isotonicity with sucrose) raised р1 and lowered рt slightly in both types of muscles, resulting in a small decrease in рt/р1. Calculations of the ratio of the resistance of the cellular pathway to that of the extracellular pathway averaged 20 for cardiac muscle compared to only 0.82 for skeletal muscle. The data indicate that the longitudinal current path through the myocardial cells is high in resistance compared to that through the ISF, suggesting that the intercalated disks are high-resistance membranes. The resistivity of the disk membranes averages 576 Ω-cm², assuming a 6.4-fold increase in surface area due to convolutions; this value approaches that of the sarcolemma, and is much higher than the maximum (2 Ω-cm²) allowable for significant DC electrical coupling to occur between contiguous cells. The fact that the extracellular fluid of a parallel-fibered muscle bundle itself acts as a cable could account for the observation that measurements of the tissue length constant by extracellular application of current give large values; thus, such measurements cannot be used to determine the resistance of the cell junctions in muscles composed of short cells.