Sodium channels near end-plates and nuclei of snake skeletal muscle

1. The spatial distribution of the two predominant types of voltage-gated channels in the sarcolemma, Na+ channels and delayed K+ channels, was studied in skeletal muscle fibres of garter snakes (Thamnophis sirtalis) using loose-seal patch recordings. 2. The average Na+-current density was five times larger in the perijunctional sarcolemma (within 25 microns of the visible edge of the end-plate) than at distant locations. K+ currents were not larger near end-plates. The apparent membrane capacitance, 3-6 microF/cm2, was the same in perijunctional and extrajunctional regions, indicating that differences in Na+-current density reflect differences in the number of Na+ channels per unit membrane area. 3. Perijunctional Na+ channels had the same voltage dependence, gating kinetics and sensitivity to tetrodotoxin as extrajunctional Na+ channels, suggesting that these cells express a single type of Na+ channel. 4. Myonuclei were found to cluster near end-plates and to avoid regions where a nerve branch or blood vessel crossed a fibre's surface. 5. Na+-current density in the sarcolemma above a nucleus was no larger than away from nuclei, indicating that functional Na+ channels are probably not inserted near nuclei. 6. Maps spanning several millimetres of fibre length showed up to sixfold differences in current density between widely separated patches. Differences between patches separated by 50 or 100 microns were much smaller.