Voltage sensitive calcium entry in frog motoneurones.

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Abstract
The electrical properties of motoneuron membrane were investigated in the isolated and hemisected spinal cord of frogs, using intracellular recording techniques. TTX [tetrodotoxin] (1 .times. 10-6 g/ml) blocked action potentials produced by intracellular depolarizing current pulses or ventral root stimuli. Voltage-current relations from these cells showed a diminishing slope for depolarizing current pulses of increasing intensity. If TEA [tetraethylamonium] (5-10 mM) was added to the media containing TTX, intracellular depolarizing pulses elicited prolonged regenerative depolarizations characterized by a peak of variable amplitude and a repolarizing phase preceded by a prolonged plateau of variable duration. During the plateau of the response, the membrane conductance was increased above its resting value. The response was shortened during repetitive stimulation and was curtailed by applying a hyperpolarizing pulse during the plateau. The response depended on the presence of external Ca2+ and increased in size and duration with increasing Ca2+ concentration. Sr2+ substituted effectively for Ca2+. Sr2+-dependent responses were considerably longer than the Ca2+-dependent ones. Ca2+ or Sr2+ dependent responses persisted in Na+-free media containing isotonic TEA, and were abolished by addition of Co2+. Ca2+ or Sr2+-dependent regenerative responses were followed by a hyperpolarization which could last several seconds. The current responsible for this after-hyperpolarization was TTX- and TEA-resistant. The TTX-resistant regenerative response is probably generated in the soma-dendritic membrane, and is due to influx of Ca2+ or Sr2+ through voltage sensitive channels different to those through which Na+ permeates during generation of normal action potentials. The hyperpolarization following Ca spikes, which may be due to an increase in K+ conductance is triggered by Sr2+.