(A)(a) Double pulse protocol from a hyperpolarized prepulse potential V₀ to activating voltage V₁ followed by further depolarization to fixed depolarized voltage V₂, respectively assessing (b) Na⁺ current activation and subsequent inactivation produced by the voltage step to V₁. (B) 2–10 mM caffeine increases integrated background aequorin Ca²⁺ signal (upper trace) and twitch force (lower trace) in rat fast twitch muscle at 25°C over timecourses dependent upon caffeine concentration. Arrows denote periods of caffeine exposure. (C) Families of loose-patch clamp membrane currents in response to the double pulse protocol before (a, b) and at successive intervals ((i)-(iv)) following introduction (c, d) of caffeine (0.5 mM) before (a, c) and following (b, d) addition of dantrolene (10 µM). Currents expressed as current densities (pA/µm²) through 28–32 µm pipette diameters.((A) from Figure 2 by permission (Fryer & Neering [47]); (B) from Figure 3 by permission (Sarbjit-Singh et al. [48]).