Properties of action potentials carried by divalent cations in identified leech neurons

1. Properties of divalent cation potentials carried by either Sr²⁺ or Ca²⁺ ions in Na⁺-free, TEA-Ringer solution were characterized in identified neurons of two species of leeches (Macrobdella andHaementeria). 2. InMacrobdella, the overshoot of the potentials varied logarithmically with [Sr²⁺]₀ (28.5 mV per 10-fold change). The overshoot,\(\dot V_{\max } \), and duration of the potentials increased with increasing divalent cation concentration and saturated at about 20 to 30 mM [Sr²⁺]₀. 3. The\(\dot V_{\max } \), amplitude, and duration of the potentials were reversibly blocked by Co²⁺ and Mn²⁺. The block by Mn²⁺ could be well-fitted by a reverse Langmuir-curve with an apparentK_I of 100 micromolar. 4. The local anesthetic procaine also reversibly inhibited the\(\dot V_{\max } \) and duration of the potentials. The inhibition was greater at alkaline pH suggesting that procaine blocks the calcium channel from inside the membrane. 5. The identified leech neurons examined inMacrobdella varied considerably in their ability to sustain somatic divalent cation potentials. Stimulation of T cells and most motoneurons produced no or only weak potentials, whereas stimulation of Retzius, N, Nut, and AP cells evoked overshooting potentials of several seconds' duration. 6. Stimulation of the ALG cell ofHaementeria in normal Ringer solution evoked a slowly-rising, purely Ca²⁺-dependent potential of approximately 100 ms duration. This response was TTX-resistant, unaffected by complete removal of Na⁺ from the Ringer solution, and abolished by 1 mM Mn²⁺. The overshoot varied logarithmically with a slope of 28 mV/decade change in [Ca²⁺]₀.