Calcium channel and its pharmacological properties in motor nerve terminals of snakes

The characteristics of Ca²⁺ channels in snake motor nerve terminals of the transverses abdominis muscle of snakes (Elaphe dione) were studied by using subendothelial recording. The results are as follows. (1) In medium containing 300 μmol/L 3,4-diaminopyridine (3,4-DAP) or 10 mmol/L tetraethylammonium (TEA), Ca²⁺ current (I(Ca)) was a sharp positive peak and relatively insensitive to stimulation frequency (< 1 Hz). A slow component of I(Ca), could be revealed by addition of both TEA and 3,4-DAP, which was more sensitive to stimulation frequency, i.e. 10 Hz stimulation would suppress almost all I(Ca). (2) I(Ca) was far more sensitive to Cd²⁺ than to Ni²⁺ or Co²⁺. Verapamil (40 μmol/L), nifedipine (10 μmol/L) or BayK 8644 (10 μmol/L) had no effect. However, all terminal currents including Na⁺ current (I(Na)) could be blocked by 80 μmol/L nifedipine. (3) 2 μmol/L ω-conotoxin GVIA irreversibly blocked I(Ca). (4) 0.5 mmol/L Co²⁺ eliminated endplate potential (EPP) in 5 min. The effect was reversible. Cd²⁺ (100 μmol/L), Ni²⁺ (0.5 mmol/L) or ω-conotoxin GVIA (1 μmol/L) also blocked EPP, whereas verapamil (40 μmol/L), nifedipine (20 μmol/L) or BayK 8644 (20 μmol/L) had no effect. The above results indicate that Ca²⁺ channels in snake motor nerve terminals were mainly composed of one type of Ca²⁺ channel, with pharmacological characteristics pertaining to N type channel.