Figure 7. Low (0.2 mm) but not high (1.0–5.0 mm) concentrations of caffeine reverse the action of 2 mm tetracaine and restore the steady-state q_γ charge.

Charge-voltage curves obtained from fibres that were studied in the absence of caffeine or tetracaine (○), and fibres studied in the presence of 2.0 mm tetracaine alone (•) are included as controls. These are compared with charge-voltage curves obtained from fibres that were studied in the presence of 2.0 mm tetracaine with the further addition of 0.2 mm (□), 1.0 mm (▿) and 5.0 mm caffeine (▵). All points represented as means ± s.e.m. The continuous line represents the two-state Boltzmann function drawn through the points obtained in the presence of 0.2 mm caffeine and 2.0 mm tetracaine characterized by: Q_max = 20.5 nC μF⁻¹, V* = −37.3 mV and k = 9.3 mV. The dotted line drawn through the points obtained in the presence of 5.0 mm caffeine and 2.0 mm tetracaine is described by: Q_max = 8.3 nC μF⁻¹, V* = −46.8 mV and k = 11.3 mV. Four fibres studied in the presence of 0.2 mm caffeine and 2.0 mm tetracaine: temperature = 4.8 ± 0.27 °C, R_i = 400.3 ± 3.42 Ω cm. Initial cable constants: λ = 2.2 ± 0.52 mm, r_i = 9078 ± 1795 kΩ cm⁻¹, d = 82.8 ± 10.68 μm, r_m = 330.7 ± 68.1 kΩ cm, R_m = 9.57 ± 3.28 kΩ cm², C_m = 9.6 ± 0.85 μF cm⁻². Final cable constants: λ = 1.8 ± 0.22 mm, r_i = 8351 ± 1282 kΩ cm⁻¹, d = 81.3 ± 5.76 μm, r_m = 243.4 ± 33.06 kΩ cm, R_m = 6.31 ± 1.13 kΩ cm², C_m = 10.1 ± 0.58 μF cm⁻². Eleven fibres studied in the presence of 1 mm caffeine and 2 mm tetracaine: temperature = 4.5 ± 0.20 °C, R_i = 403.7 ± 2.48 Ω cm. Initial cable constants: λ = 1.5 ± 0.17 mm, r_i = 15809 ± 2326 kΩ cm⁻¹, d = 62.1 ± 5.45 μm, r_m = 340.8 ± 42.56 kΩ cm, R_m = 6.36 ± 0.86 kΩ cm², C_m = 8.9 ± 0.92 μF cm⁻². Final cable constants: λ = 1.42 ± 0.15 mm, r_i = 14807 ± 2831 kΩ cm⁻¹, d = 69.9 ± 10.01 μm, r_m = 264.7 ± 45.17 kΩ cm, R_m = 5.05 ± 0.635 kΩ cm², C_m = 9.3 ± 0.77 μF cm⁻². Four fibres studied in the presence of 5 mm caffeine and 2 mm tetracaine: temperature = 5.1 ± 0.22 °C, R_i = 395.3 ± 2.62 Ω cm. Initial cable constants: λ = 1.8 ± 0.13 mm, r_i = 8609 ± 1386 kΩ cm⁻¹, d = 79.1 ± 5.59 μm, r_m = 261.6 ± 20.1 kΩ cm, R_m = 6.46 ± 0.60 kΩ cm², C_m = 9.6 ± 0.65 μF cm⁻². Final cable constants: λ = 1.6 ± 0.08 mm, r_i = 6827 ± 948 kΩ cm⁻¹, d = 88.2 ± 5.59 μm, r_m = 173.5 ± 18.8 kΩ cm, R_m = 4.71 ± 0.36 kΩ cm², C_m = 11.3 ± 0.91 μF cm⁻².