Nimodipine does not reduce the [Ca²⁺] increase evoked by hyperpolarization in voltage-clamped single colonic myocytes. Hyperpolarization (−130 mV; F) from the resting potential (−70 mV; F) increased [Ca²⁺]_PM (B) and [Ca²⁺]_c (C). The 60-mV hyperpolarization (to −130 mV) increases the electrochemical driving force on Ca²⁺ by an amount equivalent to a 100-fold increase in extracellular [Ca²⁺]; i.e., an increase to 300 mM. In control (B–F, left panels), approximately reproducible rises were evoked by two hyperpolarizations separated by ∼60 s. In the next part of the experiment, the hyperpolarizations were repeated, but 1 µM nimodipine (D), an inhibitor of voltage-dependent Ca²⁺ channels, was applied by pressure ejection from a puffer pipette (D) before the second hyperpolarization. Nimodipine reduced neither the hyperpolarization-evoked [Ca²⁺]_c nor [Ca²⁺]_PM rise. The slight increase in [Ca²⁺] (B and C), which occurred just before nimodipine application (D), arose from a small change in holding current (E). (A; left panel) A bright field image of the cell; see also whole cell electrode (right side) and 1 µM nimodipine–containing puffer pipette (left side). Changes in the fluorescence ratio with time (B and C) are derived from the region encompassing that part of the cell in TIRF (A; right panel).