SUMMARY1. Membrane currents were recorded from voltage-clamped, EGTA-loaded muscle fibres under conditions where currents through ordinary Na+, K+ and Cl-channels were prevented by drugs or by absence of permeant ions (K+ and Cl-).2. At 10 mM-external [Ca2+], substitution of Na+ for the large and presumably impermeant organic cations tetramethyl-(TMA+) or tetraethylammonium (TEA+) failed to increase peak inward current. Hence the Ca2+ channel was not significantly permeable to Na+ under these conditions.

Sodium currents were studied under voltage clamp in the presence of neutral, amine, and quaternary local anesthetic compounds. Use-dependent block was observed as a cumulative depression of INa seen with repetitive depolarizing test pulses applied at frequencies of 2-10s-1. With quaternary QX-314, the time constant of use dependence was long, and with neutral benzocaine, very short. With lidocaine and procaine, increasing external pH (pHo) changed the time constant from long to short, but alterations of internal pH have no effect. Inactivation in Na channels was measured by the influence of prepulses on peak INa during test pulses. Single-stimulus inactivation curves were shifted more with lidocaine at high pHo than at low pHo, but inactivation curves measured during pulse trains with any of the drugs and at any pHo were strongly shifted. All measurements show that the drug-receptor reaction was slow for amine drugs at low pHo, as for quaternary drugs at any pHo, and fast for amine drugs at high pHo, as for neutral drugs at any pHo. The major effect of low pHo on amine drugs was to reduce the concentration of drugs in the fiber and to protonate drug molecules on the receptor, thus trapping them in the blocking position for a longer time. Direct effects of pH on the receptor seemed minimal.

SUMMARY1. Ca2+ currents in frog skeletal muscle fibres were studied with a voltage-clamp technique. Under membrane depolarization maintained for several seconds, Ca2+ current was found to decline with time constants of 0-2-2 see when LCa2+]o = 10 mM.2. Ca2+ currents are diminished by nifedipine, D-600, tetracaine and Ni2+.3. When peak current is diminished by making the membrane potential positive, by block with drugs or by substituting the relatively less permeant Mn2+ for Ca2+, then the rate ofdecline is diminished also. When peak current is increased by recording at relatively negative membrane potentials or by substituting for Ca2+ the more permeant ions Ba2+ or Sr2+, then the rate of decline is increased in proportion. Evidently, the size of the current determines the rate of decline.4. Decline of current is greatly slowed in isotonic Ca2+ saline or when the [Ca2+]o is buffered by the organic anion malate. These findings indicate that the decline of current arises from Ca2+ depletion in an extracellular compartment, most probably the transverse tubules. On this basis, an analysis of Ca2+ current decline and recovery leads to the following conclusions.5. Ca2+ current flows almost entirely across the membranes of the transverse tubules.6. After allowing for the tortuosity of the tubular network, the apparent diffusion coefficient for Ca2+ in the transverse tubules is about 2-6 x 10-6 cm2/sec, three times less than the diffusion coefficient for K+ in the transverse tubules and about three times less than the diffusion coefficient for Ca2+ in free solution.7. The transverse tubule lumen does not appear to have a large Ca2+-buffering capacity in the millimolar range. At [Ca2+]j = 10 mM, the tubule lumen binds less than 0-6 dissociable Ca2+ ions for every free ion.

A B S T R A C T In muscle fibers from the rat diaphragm, 85% of the resting membrane ion conductance is attributable to Cl-. At 37°C and pH 7.0, Go averages 2.11 mmho/cm 2 while residual conductance largely due to K + averages 0.34 mmho/cm ~. The resting Go exhibits a biphasic temperature dependence with a Q10 of 1.6 between 6°C and 25°C and a Q10 of nearly 1 between 25°C and 40°C. Decreasing external pH reversibly reduced Go; the apparent pK for groups mediating this decrease is 5.5. Increasing pH up to 10.0 had no effect on Go. Anion conductance sequence and permeability sequence were both determined to be CI-> Br--> I-> CHaSO4-. Lowering the pH below 5.5 reduced the magnitude of the measured conductance to all anions but did not alter the conductance sequence. The permeability sequence was likewise unchanged at low pH. Experiments with varying molar ratios of C1-and I-indicated a marked interaction between these ions in their transmembrane movement. Similar but less striking interaction was seen between CI-and Br-. Current-voltage relationships for Gcl measured at early time-points in the presence of Rb + were linear, but showed marked rectification with longer hyperpolarizing pulses (>50 ms) due to a slow time-and voltage-dependent change in membrane conductance to CI-. This nonlinear behavior appeared to depend on the concentration of CI-present but cannot be attributed to tubular ion accumulation, Tubular disruption with glycerol lowers apparent Go but not GK, suggesting that the transverse tubule (T-tubule) system is permeable to CI-in this species. Quantitative estimates indicate that up to 80% of Gcl may be associated with the T tubules.