Figure 1. Caffeine conserves both the steady-state distribution and the quantity of the intramembrane charge through a wide range of applied concentrations.

Curve a, the steady-state dependence of the overall intramembrane charge upon test potential in voltage-clamped fibres studied at a −90 mV holding potential at 0.0 mm (controls: ○), 0.2 mm (□), 1.0 mm (▿) and 5.0 mm caffeine (▵). These are compared with the charge-voltage curve obtained following the addition of 2.0 mm tetracaine to abolish selectively the q_γ charge (•; curve b). All points represented as means ± s.e.m. The continuous line represents a two-state Boltzmann function drawn through the points, defined by the parameters: Q_max = 9.2 nC μF⁻¹, V* = −37.1 mV and k = 14.1 mV. Curve a, seven control fibres studied in the absence of either perchlorate or tetracaine (controls: ○): temperature = 2.9 ± 0.31 °C, R_i = 407 ± 5.7 Ω cm. Initial cable constants: λ = 1.9 ± 0.35 mm, r_i = 12265 ± 2077 kΩ cm⁻¹, d = 71 ± 6.8 μm, r_m = 386.3 ± 72.5 kΩ cm, R_m = 8.82 ± 2.62 kΩ cm², C_m = 6.8 ± 0.39 μF cm⁻². Final cable constants: λ = 1.8 ± 0.33 mm, r_i = 11617 ± 2142 kΩ cm⁻¹, d = 72.0 ± 6.7 μm, r_m = 323.8 ± 65.6 kΩ cm, R_m = 8.13 ± 2.41 kΩ cm², C_m = 7.9 ± 0.60 μF cm⁻². Six fibres studied in 0.2 mm caffeine (□): temperature = 6.2 ± 0.14 °C, R_i = 382.7 ± 1.56 Ω cm. Initial cable constants: λ = 2.0 ± 0.15 mm, r_i = 9113.5 ± 714.5 kΩ cm⁻¹, d = 74.2 ± 3.13 μm, r_m = 363.1 ± 39.79 kΩ cm, R_m = 8.49 ± 1.08 kΩ cm², C_m = 6.3 ± 0.46 μF cm⁻². Final cable constants: λ = 1.9 ± 0.23 mm, r_i = 9040.0 ± 709.1 kΩ cm⁻¹, d = 74.6 ± 3.33 μm, r_m = 347.3 ± 43.78 kΩ cm, R_m = 8.23 ± 1.28 kΩ cm², C_m = 6.7 ± 0.45 μF cm⁻². Six fibres in 1.0 mm caffeine (▿): temperature = 5.9 ± 0.27 °C, R_i = 385.0 ± 2.62 Ω cm. Initial cable constants: λ = 2.1 ± 0.16 mm, r_i = 9405.0 ± 1199.9 kΩ cm⁻¹, d = 75.6 ± 4.55 μm, r_m = 362.0 ± 20.62 kΩ cm, R_m = 8.72 ± 0.92 kΩ cm², C_m = 9.9 ± 1.02 μF cm⁻². Final cable constants: λ = 1.8 ± 0.12 mm, r_i = 10587 ± 1424 kΩ cm⁻¹, d = 71.3 ± 4.34 μm, r_m = 312.6 ± 10.28 kΩ cm, R_m = 7.07 ± 0.59 kΩ cm², C_m = 10.5 ± 1.15 μF cm⁻². Six fibres in 5.0 mm caffeine (▵): temperature = 5.5 ± 0.21 °C, R_i = 391.0 ± 2.55 Ω cm. Initial cable constants: λ = 1.9 ± 0.13 mm, r_i = 06582.0 ± 1031.0 kΩ cm⁻¹, d = 91.0 ± 6.56 μm, r_m = 224.4 ± 24.81 kΩ cm, R_m = 6.32 ± 0.59 kΩ cm², C_m = 9.52 ± 1.56 μF cm⁻². Final cable constants: λ = 1.9 ± 0.16 mm, r_i = 6787.0 ± 1070.9 kΩ cm⁻¹, d = 91.8 ± 7.67 μm, r_m = 214.0 ± 23.14 kΩ cm, R_m = 6.05 ± 0.71 kΩ cm², C_m = 10.18 ± 1.90 μF cm⁻². Curve b, six fibres studied in the presence of 2.0 mm tetracaine (•): temperature = 5.4 ± 0.1 °C, R_i = 367 ± 1.2 Ω cm. Initial cable constants: λ = 2.39 ± 0.24 mm, r_i = 6899 ± 1566 kΩ cm⁻¹, d = 93.8 ± 12.3 μm, r_m = 334.7 ± 46.8 kΩ cm, R_m = 9.14 ± 0.79 kΩ cm², C_m = 7.13 ± 1.3 μF cm⁻². Final cable constants: λ = 2.1 ± 0.25 mm, r_i = 7691 ± 1746 kΩ cm⁻¹, d = 90.0 ± 13.0 μm, r_m = 268.7 ± 40.2 kΩ cm, R_m = 6.88 ± 0.45 kΩ cm², C_m = 7.94 ± 1.67 μF cm⁻².