Pharmacological Modulation of Calcium Currents

Drugs were applied by dissolving the drugs at known concentrations in the extracellular recording solutions. Ni²⁺ and Dithiothreitol (DTT) were prepared as 100 mM stocks in water, 5,5′-Dithio-bis(2-nitrobenzoic acid) (DTNB) as 100 mM stock and nimodipine as 10 mM stock in Dimethyl sulfoxide (DMSO). DMSO (0.5%) had no effects on Ca²⁺ currents when tested alone in isolated retinal bipolar cells or HEK cells transfected with Ca²⁺ channel subunit cDNAs. To test the relative permeability of Ba²⁺ versus Ca²⁺, CaCl₂ (10 mM) was replaced by equal molar concentration of BaCl₂. In some recordings, 1 μM tetrodotoxin (TTX) was included in the extracellualr recording solutions to block possible voltage-gated Na⁺ currents in cone bipolar cells. Mibefradil was a kind gift from F. Hoffmann (La Roche, Basel, Switzerland). All other chemicals were purchased from Sigma (St. Louis, MO).