Sodium channels near end‐plates and nuclei of snake skeletal muscle.

Abstract

SUMMARY1. The spatial distribution ofthe 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 (Thamnophi8 8irtali8) using loose-seal patch recordings.2. The average Na+-current density was five times larger in the perijunctional sarcolemma (within 25,m 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 ,F/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 ,um were much smaller.

Expression and distribution of sodium channels in short‐ and long‐term denervated rodent skeletal muscles.

“…2). Endplate and extrajunctional Na+ channel density Na+ channels are concentrated in the postjunctional membrane of mammalian skeletal muscles, where a Na+ current density of about 100 mA cm-2 can be measured (Beam et al 1985;Caldwell et al 1986;Roberts, 1987). In order to examine the long-term stability of this synaptic specialization, we recorded Na+ currents at endplates of muscle fibres denervated for as long as 6 weeks (Fig.…”

Blood vessels and desmin control the positioning of nuclei in skeletal muscle fibers

“…Nuclei of garter snake muscles are excluded from the zone immediately below nerve branches or BV but appear denser in adjacent regions (Roberts, 1987) and rat soleus (SOL) muscle nuclei are not longitudinally positioned in areas rich in BV (Ploug et al, 1998). Thus, BV may contribute to the non-uniform distribution of muscle nuclei but there has been, so far, no thorough examination of their potential contribution to the intracellular organization of the muscle fibers.…”

Na+ current densities and voltage dependence in human intercostal muscle fibres.

“…The technical details of the loose-patch voltage-clamp technique used to measure INa were previously described (Ruff, 1986;Simoncini & Stiihmer, 1987 Rp was calculated as previously described (Roberts, 1987). The pipettes were bent near the tip as previously described (Caldwell et al 1986;Roberts, 1987), coated with silicone elastomer (Sylgard, Dow Corning, Midland MI, USA) to reduce capacitative coupling of the pipette to the bath (Roberts, 1987) and filled with filtered Tyrode solution.…”

“…The pipettes were bent near the tip as previously described (Caldwell et al 1986;Roberts, 1987), coated with silicone elastomer (Sylgard, Dow Corning, Midland MI, USA) to reduce capacitative coupling of the pipette to the bath (Roberts, 1987) and filled with filtered Tyrode solution. Before loose-patch recordings were performed, the dissected muscle was vigorously perfused with filtered, gassed bathing solution to remove any residual collagenase.…”