Reexamination of the double sucrose gap technique for the study of lobster giant axons. Theory and experiments.

The double sucrose gap technique for the study of lobster giant axons has been reexamined. The leakage behavior of the system cannot be successfully modeled by conventional sucrose gap theory, but is accounted for by the McGuigan-Tsien model that takes into account the cable properties of membrane under sucrose. The facts of high-leakage conductance and the ability to maintain large resting potentials in the face of low sucrose gap resistance lead to a hypothesis that membrane resistance under sucrose is very low because of a large negative surface potential. Computer simulations of the leakage behavior of the conventional gap model and the McGuigan-Tsien model were compared with experimental measurements on lobster axons using normal sucrose or sucrose doped with Na+, Ca2+ or La3+ ions. As the concentration of doping ion increased, the leakage rose, but the species of doping ion had more influence on leakage than gap resistance. At equal gap resistance, leakage decreased with an increase in valence of the doping species. Leakage was even lower in La-doped sucrose at 20 M omega gap resistance than in normal sucrose at 200 M omega gap resistance. Resting potentials decreased with decreasing gap resistance and increasing valence of the doping species. Resting potential behavior was successfully simulated with a hybrid model consisting of a point node flanked by infinite cables and a shunt between ground and the voltage-measuring pool. The data support the hypothesis that the membrane resistance under sucrose is low and that it can be raised by doping the sucrose with multivalent cations, with La3+ being particularly effective. Both the leak conductance and resting potential are influenced more by membrane under sucrose than membrane in the node. The experiments also demonstrate that doping with La3+ vastly improves the stability and longevity properties of the lobster axon preparation.