A theoretical study on the sucrose gap technique as applied to multicellular muscle preparations. III. Methodical errors in the determination of inward currents.

The analysis of errors associated with saline-sucrose interdiffusion in sucrose gap experiments on multicellular muscle preparations described in two previous papers (Lammel, E., 1981, Biophys. J., 36:533-553, 555-573) is extended to the determination of current-voltage relations that contain an activated inward current component. The membrane current-voltage (i(t)-V(m)) relation used in the computations was N-shaped and consisted of two components, an outward (background) current (i(bg)) with properties of anomalous (inward-going) membrane rectification, and an inward current (i(s)) resembling the slow inward current of cardiac muscle. Reconstruction of current-voltage relations, which simulate those determined experimentally, indicates that in the potential range in which the total membrane current (i(t)) is outward, i(t) is measured too high, whereas it is measured too low in the range of net inward current. Reversal potentials of the inward and outward components are both shifted to more negative values, that of the inward current being more affected. Simulation of the experimental approach to evaluate i(s) as the difference between i(t) and i(bg) shows that errors that produce values too high for i(bg) are partly compensated by errors that lead to values of the net inward component that are too low. The basic features of the distorting effects analyzed are independent of different assumptions made on the selectivity of the slow inward current channels. They are related to currents emerging from the sucrose compartment (local circuit as well as externally applied currents).