Inverse relation between input resistance and threshold current in canine cardiac syncytium.

Introduction: The present study was designed to examine the extent to which differences in input resistance contribute to the heterogeneity of excitability in canine endocardium.
Methods and Results: The experiments were performed in isolated canine right anterior papillary muscles. The preparations were superfused with oxygenated Tyrode's solution at 36 degrees to 37 degrees C. Conventional methods were used for recording and stimulation. To obtain pertinent data, two microelectrodes were inserted into the same cell or into two contiguous cells. One microelectrode injected intracellular hyperpolarizing current to polarize the membrane; the second microelectrode recorded the changes in transmembrane potential. The two micreoelectrodes were separated by 8 to 10 microm. The procedure used provided input resistance and threshold current values for the doubly impaled cell or the two contiguous cell units. The same procedure was repeated at different sites of the preparation. The plot of input resistance versus threshold current showed a good fit (R2 = 0.97) between the experimental data and the curve for the rectangular hyperbola XY = 30 mV; therefore, the input resistance and the threshold current are inversely related.
Conclusion: The results indicate that the canine cardiac syncytium is nonhomogeneous with respect to input resistance and that input resistance is inversely related to the minimal current needed to reach threshold. Accordingly, the electrical excitability of the cells studied also is inhomogeneous.